Rapid Fat Loss Diet Calculator

If you’re looking to shed fat fast, you’ve probably come across the concept of rapid fat loss. Whether you’re an athlete cutting for a competition, a fitness enthusiast aiming for a lean physique, or simply someone who wants to lose weight quickly, rapid fat loss can be an effective approach. But how do you know how much to eat, what to track, and whether this approach is right for you?

Enter the Rapid Fat Loss Diet Calculator, a tool designed to help you determine your calorie intake and macronutrient targets to lose fat rapidly while maintaining as much muscle as possible.

Triage Rapid Fat Loss Diet Calculator 

Rapid Fat Loss

Our rapid fat loss diet calculator is simply a tool that helps determine the optimal calorie and macronutrient intake for rapid fat loss while minimising muscle loss. Unlike standard weight loss calculators that suggest a moderate caloric deficit, our rapid fat loss calculator calculates your calories and macronutrients so that you lose ~1.5% of your body weight per week.

This is an extremely aggressive rate of weight loss, that is likely to be unsustainable for most people. So, before I get stuck into explaining how the calculator actually works and how to use it effectively, I just want to discuss who should and shouldn’t use a rapid fat loss approach.

Who Should Use A Rapid Fat Loss Diet Calculator?

Rapid fat loss isn’t for everyone. It’s best suited for: 

• Experienced Dieters: Those who have successfully followed calorie tracking and macro-based eating plans in the past will have an easier time adhering to the strict deficits required.
• Athletes Needing to Make Weight: Fighters, bodybuilders, and other competitive athletes may need to drop weight quickly before an event without losing muscle mass.
• Individuals with a Deadline: If you have an upcoming photoshoot, wedding, or vacation and need to get lean in a short time frame, this approach can be effective (but needs to be done under control).
• Those with a Solid Muscle Base: Individuals who have already built a significant amount of lean muscle and want to achieve better muscle definition and a shredded appearance.
• People with a High Degree of Self-Discipline: Rapid fat loss requires strict adherence to diet and exercise protocols, making it best suited for those who can stay consistent despite challenges.

If you don’t fit into some (or at least 1) of these categories, you likely shouldn’t be thinking about rapid fat loss.

Who Should NOT Use A Rapid Fat Loss Diet Calculator?

While rapid fat loss can be effective, it’s not suitable for:

• Beginners: An aggressive calorie deficit can be overwhelming and unsustainable for those new to dieting. Starting with a more moderate deficit helps create lasting habits.
• Individuals Who Have Never Successfully Dieted: If you have a history of unsuccessful fat loss diets, it is incredibly unlikely that a rapid fat loss diet will be the one that actually works. Putting the diet on hard mode is unlikely to make it more likely that you will succeed with sticking to the diet when you couldn’t do it on easier modes.
• Individuals With A History of Disordered Eating: Extreme calorie deficits can trigger unhealthy eating behaviours and increase the risk of relapse into harmful patterns.
• People With High Stress Levels: A severe calorie deficit can elevate cortisol (stress hormone) levels, negatively impacting sleep, recovery, and mental well-being.
• Anyone Seeking Long-Term Sustainability: Rapid fat loss is intended as a short-term strategy, not a long-term lifestyle. Those looking for gradual, sustainable weight loss should opt for a moderate deficit.
• Women With Irregular Menstrual Cycles or Low Energy Availability: Extreme caloric restriction can disrupt menstrual cycles and overall hormonal balance, leading to fatigue and potential long-term health consequences.
• Individuals who are pregnant, dealing with disease, or managing an injury, should absolutely not engage with a rapid fat loss diet.

So, in reality, maybe 1-2% of the population should engage in a rapid fat loss diet. I know that isn’t what you want to hear, but it is the truth. Most of you would be much better served to diet much more slowly.

Pros and Cons of Rapid Fat Loss

Now, if you do fall into a category that could benefit from a rapid fat loss diet,

Pros:

• Fast Results: A steep calorie deficit leads to rapid fat loss, which can be highly motivating.
• Short-Term Commitment: Instead of dieting for months, rapid fat loss allows for a faster and more intense cut.
• Effective for Specific Goals: It can be ideal for people who need to lose fat quickly for a time-sensitive event or competition.
• Potential Psychological Boost: Seeing quick results may reinforce motivation and adherence, encouraging long-term healthy habits (in some people).

Cons:

• Hunger & Cravings: A steep calorie deficit increases hunger and cravings, making adherence very challenging.
• Muscle Loss Risk: Without adequate protein intake and resistance training, muscle breakdown can occur when on a rapid fat loss phase. But even if you do everything right, you are still risking muscle loss.
• Energy & Mood Swings: Low energy availability can cause fatigue, irritability, and brain fog, affecting daily life and workouts. It can also lead to hormonal disruptions, which may have long(er) term effects.
• Not Sustainable: This is a short-term strategy; prolonged aggressive dieting can lead to burnout and weight regain.
• Potential Metabolic Slowdown: A drastic calorie deficit can slow down metabolism and negatively affect certain hormone levels, making it harder to maintain weight loss in the long term.
• Risk of Nutrient Deficiencies: Cutting calories too aggressively may lead to inadequate intake of essential vitamins and minerals, negatively impacting overall health.

I really do implore you to weigh up the pros and cons of whether a rapid fat loss phase is for you.

Rapid fat loss is a powerful but incredibly demanding approach to cutting fat quickly. It requires discipline, careful tracking, and a strong foundation in nutrition. While it’s not suitable for everyone, those who approach it with knowledge and structure can achieve impressive results in a short period.

Now, with all of that in mind, we can get stuck into actually discussing how to use the calculator and how to understand the calculator more fully.

How to Use Our Rapid Fat Loss Diet Calculator

Using the rapid fat loss diet calculator is simple enough:

1. Enter Your Stats: Input your weight, height, age, and sex.
2. Set Your Activity Level: Choose from sedentary, light, moderate, very, or extra active. (I will elaborate on this in a moment, as this is the step that most people get wrong)
3. Review The Calorie and Macronutrient Targets: The calculator will provide calorie, protein, fat, and carb targets to help you set up your rapid fat loss diet. There will also be a column for your maintenance calories (this is important, and I will explain this more in a moment). I would suggest reviewing these numbers to see if they actually make sense (for example, if you know your maintenance is 2000 calories and the calculator says your maintenance is 3000, then in all likelihood, you have entered some data incorrectly).
4. Monitor & Adjust: Once you have your numbers, track your intake with an app (such as MyFitnessPal) and monitor your weight, strength levels, and energy. Adjust as needed based on your results.

It really is that simple. However, actually putting this into practice is more challenging, and the only way I can show you how to use this rapid fat loss calculator to actually succeed with this kind of diet is if I discuss some foundational information about nutrition and how to set up a rapid fat loss diet correctly.

Rapid Fat Loss Diet Foundational Information

Before you really get stuck into using the calculator to set up a rapid fat loss diet, you generally should have some foundational knowledge in place, so you can both use the calculator properly and understand the results it gives. A lot of the foundational nutrition information is covered more thoroughly in our foundational nutrition article (and the articles linked in it), so we really do recommend reading that if you want a more comprehensive explanation.

To understand how to use the rapid fat loss calculator, you do actually have to have some basic understanding of nutrition first and foremost. The easiest place to start is building out a basic understanding of calories. 

Calories

Calories are simply a unit of energy. They represent the amount of energy that our body gets from the foods we eat, and then this energy is used to power essential bodily functions, ranging from basic metabolic processes to physical activities. So when we talk about calories, we are generally talking about the energy we consume from food, and the energy we expend doing everything that we do in a day.

The reason we need to eat calories is so we can fuel the activities we do each day, along with sustaining our baseline functions (such as your heart beating and lungs breathing). So, even if you are just lying in bed all day, you do actually still require calories. However, if you are more active, then you will require more calories to fuel that activity.

The body has the capacity to store calories, and this is mainly in the form of body fat (although some of it is also in the form of glycogen stored in your organs, and you could argue that the protein that makes up your body is also a form of stored energy). This is important to understand because it informs how we actually use a calorie and macronutrient calculator to help us get the results we desire.

You see, if you consume the same amount of calories as you expend each day, you are going to maintain your body weight. If you eat fewer calories than you burn each day, then you are going to tap into your stored energy (body fat) and you are going to lose weight. If you eat more calories than you expend each day, you are going to gain weight (this can be in the form of fat gain, or muscle gain depending on other variables in your diet and lifestyle).

This is often called the calories in, calories out (CICO) equation, and you can use it to create more tailored dietary recommendations. Our rapid fat loss diet calculator will calculate your maintenance calories, and then it will also calculate your calories for rapid fat loss (fat loss at a rate of 1.5% per week). We will discuss this in more depth in a moment.

To find out your daily maintenance calorie requirements, you are generally better off using the average and adjust method we describe in our article: How Many Calories Should You Eat? However, I do understand that many people just want to find out roughly how many calories they should be eating, rather than spend time tweaking their diet until they find the right amount, especially when it comes to rapid fat loss.

So using a calculator can be quite helpful as a starting point, but it isn’t as scientific as you would initially think. It is just based on averages, and depending on what you input, you can get an output that isn’t really aligned with your actual needs. This is especially the case when you have either high or low amounts of muscle mass, as muscle is more calorie-demanding than body fat.

For example, if two people put in their weight as 80kg but one of them is 5% body fat and the other is 30% body fat, the calculator will tell them that they have the same calorie requirements. However, this is not the case in the real world, as the person with more muscle will generally have a higher calorie requirement.

This is further compounded by the fact that we also have to account for activity levels. As we noted previously, your calorie requirements are based on the energy your body needs to not only carry out baseline functions, but also to fuel the requirements of the activities you engage in daily.

This is accounted for in our rapid fat loss diet calculator by applying an activity multiplier. The calculator first calculates your calorie needs as if you were basically not moving all day, and then it multiplies that figure based on your activity level.

These activity multipliers are as follows:

• Sedentary 1.2
• Lightly active 1.37
• Moderately active 1.55
• Very active 1.75
• Extra active 1.9

These activity multipliers are meant to capture all of your daily activity, and in doing so, give you a more accurate reflection of your actual calorie needs.

However, this is where most of you will go wrong. Most people are simply terrible at accurately assessing their activity levels, and thus they don’t select the correct multiplier. Most people are simply unaware of how inactive they actually are, especially as they fail to take into account non-exercise activity thermogenesis (NEAT)(you can use daily step count as a rough proxy for NEAT).

Most people will do maybe 3 gym sessions per week, and as a result, they will select “moderately active” or potentially even “very active”.

Now, relative to their peers, they may actually feel like they are way more active than the average, and thus they think they should select “moderately active” at the very least. However, someone who goes to the gym maybe 3 times a week, with little activity outside of that is probably somewhere between sedentary and lightly active, rather than moderately active or above.

This is where most people go wrong when using an online calorie and macronutrient calculator. They simply select the wrong activity level and thus, they get recommended a much higher calorie amount than they actually require.

Most people are generally way less active than they think they are, and most people overestimate how many calories they burn with purposeful exercise (exercise activity thermogenesis, EAT) and underestimate how many calories are burned via NEAT (daily step count). So someone who goes to the gym 3 times per week will think they are way more active than they are, while someone who gets 10,000 steps or more per day will generally underestimate how active they are.

It is important to keep all of this in mind when using the calculator, as the activity multiplier is the place where most people go wrong.

In general, I tend to discuss the activity multipliers with the following guidelines in mind:

These aren’t rock solid, and there are gradations with them. Various combinations of gym training, cardio training, and NEAT can all add up to a specific activity level, and they aren’t always as clear as you would ideally like.

For example, you could do no cardio but train in the gym 6 days per week for 2 hours each time, while also having a low daily step count, and still only end up with a calorie expenditure that is the equivalent of a moderately active multiplier (this is especially true if you take long rest periods and stay seated in between your sets).

Alternatively, you could do 3 hours in the gym per week, and have a job that has you doing low level activity all day, and even though you don’t feel like you are exercising excessively, you could actually end up being in the “very active” to “extra active” category (for example, I have had clients who work as nurses, surgeons, postmen, warehouse workers, retail workers and a variety of other jobs that you wouldn’t necessarily think have a super high-calorie requirement, but these people very often do end up with higher calorie output by virtue of the accumulation of the low-level activities they do all day).

Using myself as an example, I currently get about 7500 steps per day, do about 270 minutes of hard resistance training and 270 minutes of hard BJJ training per week, and outside of that, I pretty much sit at a desk all day.

Now, you may think that is quite an active lifestyle, and yet my actual maintenance calorie requirements are the equivalent of someone who selected moderately active.

I have previously worked in a gym where I taught some spin classes, did lots of walking around and helping people out on the gym floor, did my own training and cycled to and from the gym each day, and my calorie requirements were over double what they are now. Even if I were to select the “extra active” multiplier, it wouldn’t give me enough calories to even maintain my weight (it underestimated my calorie requirements by over 1,000 calories).

Realistically, this could be solved with an additional activity multiplier for those who are extremely active, but this is much rarer than people think and generally only applies to athletes or those who do lots of very hard manual labour. This is why it is so important to really pay attention to what activity multiplier you choose, and then how you choose to actually interpret the results of the calculator.

This is why we generally recommend most people be a bit more conservative with the activity multiplier you choose if you are doing a normal amount of activity per week, and if you are at either end of the extreme (either lots of movement or very little movement), you should be a little bit cautious in how you interpret the calculated figures.

This is why we generally prefer the average and adjust method for figuring out where your calorie requirements actually are, as it lets your body do the calculations and you just need to interpret the outcome (i.e. assess your body weight change and energy levels). If you do choose to base your diet purely on the figures given by the calculator, please do realise that these are just a starting point and you will have to adjust them based on your real-world results. If you need help with this, then we would generally recommend getting coaching, as it can take a while to really figure out your exact requirements.

Now, remember, this is all just related to maintenance calories. We will discuss the calories for rapid fat loss in a moment. We will also need these maintenance calories too, to set up an effective rapid fat loss diet. So, bear with me here.

Our calculator doesn’t just give you calorie targets, it also gives you macronutrient targets. 

Protein

The first macronutrient we tend to discuss is protein, as it is the thing that most people likely under-consume, and the macronutrient you generally need to think more about when trying to get enough in your daily diet.

Protein is an important component of your diet, as it is the fundamental building block for your body, and you need to consume it to help with building and repairing tissues. Protein is composed of amino acids, and some protein sources have a better amino acid profile than others (i.e. they contain more of the amino acids that humans need).

Protein intake is important for baseline health, as it is important for immune function, and the production of enzymes and hormones. Protein is also important because it contributes to muscle development. Sources of protein include meat, fish, dairy products, beans, and nuts.

The dietary recommendations for protein are generally set quite low, as they are based on older research. Newer, more accurate research suggests that higher protein intakes are required, and this is especially the case for individuals who exercise regularly.

Our rapid fat loss diet calculator uses the figure of 2g of protein per kilogram of body weight per day, as the target, although the range we generally recommend is between 1.5-2.5g of protein per kilogram of body weight per day, depending on the specific individual and their goals. As protein contains ~4 calories per gram, we can then calculate the rest of the macronutrients based on whatever calories we have left.

Now, when on a rapid fat loss diet, some of you may actually need to eat even more protein. This is both to keep hunger at bay, and to actually stave off muscle loss. Some people do tend to lose more muscle when dieting aggressively, even when they set up things correctly and resistance train, and consuming more protein is the only thing that helps (aside from simply not dieting so aggressively).

If you want to dig into this more, we recommend reading: How Much Protein Should You Eat?

Fats

Dietary fats are the next macronutrient that our calculator calculates. Fats are an essential component of the diet as it is required to aid in the assimilation of fat-soluble vitamins, and all of your cells are actually encased in a lipid (fat) layer. Dietary fat has been vilified in many media outlets, but you do actually need to consume dietary fat for optimal health.

While the thought process that eating fat will lead to fat gain seems to be self-evident (i.e. eating fat leads to fat gain), it isn’t actually accurate. Dietary fat provides calories, and a gram of fat contains 9 calories, which is more than twice the amount contained in protein or carbs. So it is very easy to overconsume calories when you are eating a lot of fat, but dietary fat isn’t inherently fat-promoting.

There are a variety of fat sub-types (such as saturated, monounsaturated and polyunsaturated), and generally, we want to try and keep saturated fat intake to less than 10% of calories, while consuming a mix of poly and monounsaturated fats. This means you should aim to consume your fats from stuff like avocados, nuts, olive oil, and leaner cuts of meat, rather than getting your fats from processed foods, fried foods or excessive quantities of butter.

Our rapid fat loss diet calculator sets your fat target as 0.6 grams of fat per kg of body weight, with a minimum intake of 20g per day. Some people may want to consume more than this, and this is more of a minimum target, rather than being set in stone. Some people do enjoy eating a diet that is higher in fats, and that can be quite health-promoting, once calories are accounted for and carbohydrates aren’t excessively low. But when on a rapid fat loss diet, you are more than likely going to want to keep fats low, while still consuming at least the minimum necessary for health.

Our calculator also gives you a target for saturated fats, so you know how much you should be consuming (it will give you a target of eating less than or equal to 10% of calories).

While the calculator doesn’t give you a recommendation for this, our general recommendation for omega-3s is to try and get somewhere between 1-5g of omega-3 per day. This can either be consumed as part of the diet by virtue of eating fatty fish, or it can be supplemented in the form of fish, krill or algae oil.

Carbohydrates

Our calculator then uses the remaining calories (once protein and fats have been set) to calculate your carbohydrate targets. Carbohydrates are the primary source of energy for the body, and they provide the fuel the body ideally wants to use to power our organs and muscles.

We generally recommend that people base their carbohydrate intake around complex carbohydrates (such as grains, tubers, pulses, and fibre-rich fruits and vegetables) rather than simple carbohydrates (such as sugars and many refined products).

On a rapid fat loss diet, it is almost certain that you will have to eat lower carbohydrates than you would generally otherwise prefer. If calories are low, and you have to hit specific protein and fat targets, naturally enough, there aren’t going to be too many calories left for carbs.

Fibre

Finally, our calorie and macronutrient calculator calculates fibre recommendations. Many people simply ignore fibre as a macronutrient, but ensuring your fibre intake is adequate does tend to lead to better health outcomes across the board. It is also vital that you eat enough fibre on a rapid fat loss diet, because fibre will keep you fuller for longer, and hunger can be an issue when on a rapid fat loss diet.

So understanding how much you need to consume is actually quite helpful, as adequate fibre is associated with a host of positive health outcomes and most people simply don’t consume enough. The recommendations for fibre are to consume 10-15g of fibre for every 1000 calories consumed. Our calculator calculates the fibre target as 15g of fibre for every 1000 calories consumed, however, our calculator also sets the minimum target of 25g of fibre per day. This is generally seen as the lowest intake that should be eaten by adults. Given that a lot of you will be eating less than 2000 calories per day on a rapid fat loss diet, it is important to ensure the minimum target is at least met.

Calorie and Macronutrient Targets

So, in summary, our calorie and macronutrient calculator is going to calculate your baseline diet as follows:

While you may feel that specific macronutrient ratios would be better for you, and your goals, these macronutrient ranges are generally what we recommend people start with for their diet. You do have a degree of flexibility with regard to your fat and carbohydrate targets, so you can adjust the targets yourself so that you can eat a bit more fats, and thus eat less carbohydrates.

However, when you are on a rapid fat loss diet and calories are quite low, there just isn’t a huge degree of wiggle room with the targets. You do have a degree of flexibility, but not a lot.

Rapid Fat Loss Diet

Now, so far, we have just discussed the rapid fat loss calculator’s ability to work out maintenance calories and macronutrient targets. However, many of you are not simply looking to work out your calorie and macronutrient needs so you can set up a baseline diet, you are working them out so you can set up a rapid fat loss diet. This is where the final column comes into play.

However, if you are going to be using the calculator as your main method of setting up the diet for rapid fat loss, what we generally recommend is that you set your goal as maintenance, and you eat like that for at least 2 weeks first. This will allow you to validate that these are actually your maintenance calories (i.e. you won’t see much if any change in your body weight), and it will allow you to set up good dietary habits and practices that allow you to actually eat in accordance with your targets.

However, I know many of you will have come to this calculator so you can set up the rapid fat loss diet right now. You don’t have 2 weeks to wait.

So, we need to discuss how the calculator calculates your rapid fat loss calories.

While it isn’t a precise science, we can come up with very rough estimates for calories based on specific desired outcomes. This is because the body is governed by the laws of thermodynamics, and we have a rough idea of how many calories it takes to burn a specific amount of fat.

Rapid Fat Loss

While most calorie and macronutrient calculators will usually just provide specific absolute calorie deficits (i.e. they will set the calorie deficit by fixed calorie amounts, such as a 300-calorie deficit) or fixed percentages, we tend to use a more tailored approach to this.

We generally use reverse engineered percentage-based recommendations for weight loss, rather than absolute calorie deficit or specific hard outcomes (such as 1kg loss per week). This tends to result in much better results, because it is actually tailored to your unique circumstances and needs. A 300-calorie deficit for someone who has a 3000-calorie maintenance requirement is much different than a 300-calorie deficit for someone who has a 1400-calorie maintenance requirement.

Similarly, a rate of 1kg weight loss per week is very achievable for someone who weighs 100kg, as it is only 1% of their body weight, but incredibly aggressive weight loss for someone who weighs 55kg. So, reverse engineered percentage-based recommendations tend to work best.

You see, we can reverse engineer the required deficit based on the rough approximation that 1 kilogram of body fat is equivalent to ~7,700 calories.

Now, you may be thinking that surely 1kg of body fat should actually be 9,000 calories, as we said earlier that 1g of dietary fat is equal to 9 calories. And you would be right in your thinking, however, body fat, or rather what we collectively refer to as body fat, isn’t actually entirely stored fatty acids. It is a combination of fat, water, proteins and various other chemicals.

So 7,700 calories per kg of fat loss seems to be relatively accurate, although there is some variation here, and there are a whole host of other variables that can confound that figure being perfectly accurate. However, as a rough approximation, it works quite well.

Using this knowledge, we can then do a bit of reverse engineering with the diet, where we can set a percentage-based rate of weight loss per week, and then roughly calculate what the required deficit would be to accomplish that rate of loss.

I am roughly 100kg, so for me to lose 1% of my body weight (i.e. 1kg) as body fat per week, I would need to eat a roughly 7,700 calorie deficit across the week. This is roughly a 1,100 calorie deficit per day, spread across the week.

If I were someone who weighed 80kg, to lose 1% of my body weight per week, I would need to eat a deficit of roughly 6,160 calories per week (~880 calorie deficit per day).

Now, you don’t need to work this out yourself, as the calculator will do the maths for you. These calculations are never absolutely perfect, and as you begin dieting, the first week will generally see you losing weight more rapidly (due to water weight loss, stored glycogen being used up, reduced quantity of food in your digestive system and other factors), but overall, the calculations do give a very good approximation of how weight loss will go (assuming you keep your activity levels stable, and don’t start exercising less or generally moving less (i.e. your NEAT doesn’t lower)).

For most people, an appropriate rate of weight loss is somewhere between 0.5-1% weight loss per week. If you have good dietary and lifestyle habits in place, a rate of 1% weight loss can be very manageable and it is a nice balance between getting faster results (and thus not having to eat in a deficit for so long) and actually being able to stick to the diet (i.e. it isn’t so aggressive that you feel excessively restricted). However, for most people who have not had success in dieting before, or who are new to weight loss, then we generally recommend choosing a slower rate of weight loss (i.e. 0.5% weight loss per week).

However, I know you guys are here for rapid fat loss. The calculator will calculate an aggressive deficit that will have you losing weight at around 1.5% body weight per week. This will allow you to set up your rapid fat loss diet.

But I can’t stress this enough, for most people, this is an inappropriate goal to have, and while you may think you want to lose weight as fast as possible, in reality, you likely want to actually lose weight and keep it off long term. You will likely fail to lose the weight if you are too aggressive (as you won’t be able to stick to the diet) and you will likely fail in keeping it off long term even if you do manage to stick to the diet for any length of time (as you will likely have a worsened relationship with food, and you won’t have really solid, long term dietary habits in place).

While we have been heavily focusing on calorie intake so far, it is also important to understand that macronutrients are also important. The calculator will calculate your specific targets based on the previously discussed targets, and they will be adjusted to what you will need to eat for a rapid fat loss diet.

However, it is important to understand the order of priorities of the diet, especially when we are discussing a rapid fat loss diet.

While calories will dictate changes in your body weight, the diet composition (and your exercise habits) will dictate the look you actually achieve at the end of the diet. Most people aren’t dieting to simply become a smaller version of themselves, they are dieting because they want to create a specific look.

As a result, your exercise habits and your protein intake are very important. Exercise habits are outside of the scope of this article, but it is important to bear in mind that the calculator will give you a protein recommendation, and if you are trying to create a specific look with your body, getting sufficient protein will help you facilitate this. Protein is also very satiating and will keep you fuller for longer, and this will dramatically increase the likelihood of you actually sticking to your diet long-term. Low protein intakes are one of the major reasons people struggle to stick to diets and to actually get the results from diets. So protein is very important.

The fibre target is also important, especially for fat loss, as getting sufficient fibre will allow you to feel more satiated, and this dramatically increases the likelihood of you actually sticking to the diet. Carbohydrates and fats can be thought of as energy sources, and you can actually play around with the exact figures here (although the calculator will set the lower limit of fat intake to 20g per day, and carbs to 50g per day, and on a rapid fat loss diet, you more than likely won’t have a lot of wiggle room).

There is a situation where the calculator will prioritise fats and carbs over protein, even though this is generally the reverse of what we recommend. That situation is where you have a smaller individual who is sedentary, yet wants aggressive fat loss. This is a situation which we tend not to run into quite frequently, as we generally don’t recommend people being sedentary or chasing aggressive weight loss (and especially not both at the same time).

If you are a smaller individual who is sedentary and you want to do a rapid fat loss diet, you simply won’t have enough calories to get sufficient protein while also ensuring your fat intake is at least 20g and carbs are at least 50g. As a result, the calculator will keep the minimum fat and carb targets, and remove calories from protein to allow for that. This is a less-than-optimal situation, and again, it really is not something we recommend.

Rapid Fat Loss Diet Planning

It is important to realise that rapid fat loss is not something you do for months. It is something you do for 2-8 weeks (3-12% body weight drop). You will almost certainly fail if you try to do this for longer. Even 8 weeks of this is quite a stretch.

We generally recommend a much more gradual reduction in weight over a few weeks/months, and then have people stay within a relatively narrow band of weight (i.e. they maintain the weight loss).

However, with rapid fat loss, the goal is to lose weight very quickly. The biggest mistake most people make with this is not having a plan for what they are going to do after the rapid fat loss phase. This is where the maintenance calories we worked out earlier come in handy.

While your maintenance calories will change if you lose significant amounts of weight, we can use the calculated maintenance calories and macros for 2 purposes. We can use them to set up the post rapid fat loss phase, and we can use them to set up refeeds.

Most people fail with these kinds of rapid fat loss diets, because they have no plan for after the diet phase. If you have no plan, and you have just spent 2-8 weeks aggressively dieting, you are more than likely going to be hungry. And as a result, for most people, this means they end up binge eating.

So it is vital that you have a plan. The plan can be as simple as setting your targets as the targets the calculator gives you for maintenance. Of course, if you were my client, I would go into much greater detail on how to set this up specifically for your needs, but there is only so much I can include in this article without making it even longer than it already is. So, have a plan for after the diet ends.

If you lose significant amounts of weight 5-10% (or more) of your body weight, you may want to recalculate your maintenance calories, but I would argue that it is probably a good thing to ever so slightly overshoot your maintenance calories, at least for the first week or two after a rapid fat loss phase. This will allow a lot of the metabolic and hormonal changes to normalise, and if you only slightly overshot maintenance, you won’t gain back significant weight (outside of water and muscle glycogen).

The other thing you can use the maintenance calories for is refeeds. This isn’t as relevant if you are only dieting for a very short period of time (i.e. less than 2 weeks), but if your rapid fat loss phase is to go longer than this, refeeds can make sense.

A refeed is a planned increase in calorie intake, usually from carbohydrates, after a period of caloric restriction or dieting. It is commonly used in bodybuilding circles to temporarily boost metabolism, replenish glycogen stores, and improve hormonal balance, particularly leptin levels. However, to really impact the hormonal system, you would typically need at least ~3 days of eating more.

However, when aggressively dieting, having a refeed planned for every 5-10 days, can significantly improve adherence to the diet and your overall energy levels. You can simply use the maintenance calories/macros as the targets for this refeed, during the rapid fat loss diet. You would generally eat more than maintenance if you really wanted to impact the metabolic/hormonal systems, but this does also drag the diet out for longer (and in my experience, people doing this stuff on their own tend to freak out when they see the scale jump up from the extra stored water and glycogen). So setting the refeed targets at maintenance makes a lot of sense.

And that is your rapid fat loss diet! Of course, to really explain all the ins and outs. of rapid fat loss, we would need about 10,000 more words, and I know the vast majority of you are purely here to just use the calculator. On that note, I do just want to touch on a few potential issues with the rapid fat loss diet calculator.

Potential Issues With The Rapid Fat Loss Diet Calculator

There are a few situations where the rapid fat loss diet calculator may be inaccurate. We touched on a few of these already, but I know some people just skimmed the previous content.

The calculator may be inaccurate because you selected the incorrect activity multiplier for your current lifestyle. This is generally due to overestimating how active you really are, and in general, we tend to recommend people be more cautious in what activity multiplier they select.

The calculator may also give inaccurate calorie recommendations if you are very lean or very fat. Muscle is more metabolically active than fat, so someone who is 80kg but 6% body fat will have a higher calorie requirement than someone who is 80kg but 30% body fat. For most, this simply won’t be a major issue, but it may mean that the calorie (and protein) recommendations will be slightly off for people who are at the extremes of the body fat range.

Protein recommendations are set as 2g per kilogram of body weight, but this may be inaccurate for people who are very lean, have high levels of body fat, or engage in lots of very intensive exercise. This generally shouldn’t be an issue, because our protein recommendations are in the middle of the recommended range (1.5-2.5g of protein per kilogram of body weight per day) and thus, you will still likely fall into a generally good ballpark figure for protein intake. However, as rapid fat loss requires an aggressive calorie deficit, some of you may actually need to eat more than this to stave off muscle loss.

Smaller individuals who are sedentary, and want to lose weight aggressively, will also generally find that their protein requirements come out slightly differently than we would generally recommend. This is because we generally don’t recommend people be sedentary, and if they are, we definitely don’t recommend they try to chase aggressive fat loss. As the calculator sets minimum fat intake to 20g and minimum carb intake to 50g, the closer your calorie target is to ~380 calories per day, the less protein it will recommend. This is very much a less than ideal situation, and we really do not recommend rapid fat loss if you are a smaller individual who is sedentary (in general, we don’t recommend most people select aggressive fat loss as the goal).

Metabolism Is Adaptive

This is less of a problem with the calculator and more of a feature of metabolism, but the recommendations that the calculator provides are not static recommendations. You are going to need to use real-world outcomes to adjust the diet. You see, as you increase or decrease calories, your metabolism is going to adapt to these changes over time.

Some of this will be due to actually trending towards your goal (i.e. if you have lost 5kg, then the calorie deficit that you worked out initially for someone who is 5kg heavier isn’t going to be as accurate), but a lot of it will be due to changes in activity (the “calories out” side of the CICO equation).

In response to underfeeding and/or overfeeding, you will generally see peoples’ energy expenditure vary quite dramatically. Some people will fidget way more when they eat more, and fidget way less when they eat less. Some will fidget more when they eat more, but not fidget less when they eat less. Some will not fidget more when they eat more, but fidget less when they eat less. And some will even fidget less when they eat more and fidget more when they eat less.

This stuff isn’t a precise science, especially when we are talking about this to a larger cohort of people, rather than a specific person (it is much easier to dial things in for an individual, as you can discuss and account for many of the variables).

I wish there was an easy way around this, and while I can give some blanket recommendations, the reality is that you will just have to try to monitor your calorie intake and calorie output as best you can, and then adjust things methodically to keep things moving in the direction you want. This generally means having relatively fixed calorie intake targets that you actually stick to, and a relatively consistent exercise routine and daily step target. NEAT is the area that people tend to notice the most metabolic adaptations, so keeping your daily step count relatively standardised really does go a long way towards accounting for adaptations.

People often wonder how often they need to recalculate their calorie and macro targets and the reality is that you don’t actually need to do it frequently at all. You certainly can come back and recalculate your targets if you have lost/gained a significant amount of weight, but in reality, you should just view the recommendations that the calculator gives as a very rough starting point for the diet. You are going to have to adjust the diet based on your real-world results. I know this can be frustrating for a lot of people, especially if you are just beginning with all of this health and fitness stuff, as you just want to be given an exact plan of action, but unfortunately, it doesn’t work like that. Instead, you will just have to make small adjustments to the diet, to ensure things keep moving in the direction that you want.

Tips for Success with Rapid Fat Loss

Rapid fat loss is quite difficult. So, if you decide to follow an aggressive fat loss approach, here are some key strategies to maximise success:

• Prioritise Protein: Aim for at least 2g of protein per kilogram of lean body mass to maintain muscle mass and satiety.
• Resistance Train Consistently: Resistance training helps preserve muscle and keep metabolism high.
• Actively Manage NEAT: Your non-exercise activity thermogenesis is likely to drop when aggressively dieting. So set a step goal and track it religiously.
• Eat High-Volume, Low-Calorie Foods: Vegetables, lean proteins, and fibre-rich carbs help with fullness while keeping calories low.
• Stay Hydrated: Drinking at least 3-4 litres of water per day can help manage hunger and maintain energy levels.
• Use Diet Breaks Strategically: If adherence becomes too challenging, a short maintenance phase can help reset hormones, energy levels, and motivation.
• Optimise Sleep Quality: Poor sleep increases hunger and reduces willpower, making aggressive dieting much harder.
• Monitor Progress Closely: Weigh yourself multiple times per week, track body measurements, and assess energy levels to ensure you’re losing fat efficiently without severe side effects.
• Manage Stress Effectively: Incorporate relaxation techniques like meditation, light walking, or yoga to counteract stress-related challenges.

By following these strategies, you can make aggressive fat loss more effective while minimising negative side effects. However, always listen to your body and be prepared to adjust based on your individual response. Rapid fat loss is no walk in the park.

Rapid Fat Loss Diet Calculator Conclusion

Hopefully, this rapid fat loss calculator has given you the information you need to better inform your rapid fat loss diet set up. No calorie and macronutrient calculator is going to be perfect, but based on our coaching experience, this one does work quite well. There are some situations where you really do have to pay a bit more attention to the output data, but overall, this calculator will give you information that you can use to help you better set up your rapid fat loss diet. It definitely should not be seen as prescriptive, and you will generally be better off getting professional help to get your diet set up correctly and fully tailored to your needs, but I do understand that getting professional help isn’t always possible. Some of you are going to want to play around with the specific macronutrient ratios, and for those of you who do, we also have a macronutrient percentage calculator.

If you would like more help with your diet (or training), we do have online coaching spaces available. if you are interested in learning more about the diet and how to coach people to a better diet pattern, we also have an Online Nutrition Coaching Certification program that may be of interest to you. We also recommend reading our foundational nutrition articles, along with our foundational articles on sleep and stress management, if you really want to learn more about how to optimise your lifestyle. You can also sign up to our newsletter and YouTube channel if you wish to stay up to date with our content. You can also subscribe to our podcast on Spotify.

We have a number of articles that will really help you to understand all of this stuff, and make much better decisions around your diet:

• How Many Calories Should You Eat?
• How Much Protein Should You Eat?
• How Much Fat Should You Eat?
• How Much Carbohydrate Should You Eat?
• How Much Fibre Should You Eat?
• How Much Water Should You Drink?
• Dealing With Alcohol In The Diet
• Overview of Diet Quantity
• Understanding Diet Quality
• Putting Healthy Eating Guidelines Into Practice
• Understanding Food Selection
• Diet Quality: Micronutrients and Nutrients Of Concern
• Diet Quality Overview
• Longer Term Diet Planning
• How To Set Up Your Diet Overview

If you don’t find the answer to your questions in these articles, then you may find the answer on our Nutrition Content Hub page.