IIFYM Calculator (If It Fits Your Macros)

Created by Joanna Michałowska, PhD candidate and Rita Rain
Reviewed by Dominik Czernia, PhD and Jack Bowater
Last updated: Jun 05, 2023

The IIFYM calculator (If It Fits Your Macros calculator) computes the optimal amount of macronutrients (proteins, fats and carbohydrates) you should consume every day.

This tool can help you maintain your weight, or calculate macros for weight loss/weight gain.

What are my macros?

The foods we eat contain three main macronutrients:

  • Proteins - a major functional and structural component of every animal cell. Proteins are composed of amino acids, some of which humans cannot synthesize. This means they must be obtained from the diet.
  • Fats - the most energy-dense macronutrient. You need them for the construction and maintenance of cell membranes, to maintain a stable body temperature, and to supply fat-soluble vitamins.
  • Carbohydrates - the main source of energy in the human diet. They include sugars, starches, and fiber.

All three macronutrients may play different roles in our body, but they all provide energy:

  • 11 gram of fat = 9 kcal9\ \mathrm{kcal}; and
  • 11 gram of proteins or carbs = 4 kcal4\ \mathrm{kcal}.

If It Fits Your Macros diet

The IIFYM diet tracks macronutrients rather than individual foods. It is based on the assumption that you can eat whatever you want as long as it fits your macros plan, so, in theory, you are allowed to eat a donut as long as the amount of carbs, proteins, and fats consumed fit your macros plan.

Estimating your daily intake of each macronutrient depends on your required calorie intake and can be adjusted depending on your target weight. Once you know the total number of calories you should consume each day, you can calculate how many of them should come from proteins, carbohydrates, and fats.

According to USDA Dietary Guidelines, a healthy diet should have the following macronutrient distribution:

  • 10%10\% - 35%35\% of energy should come from proteins;
  • 45%45\% - 65%65\% of energy should come from carbohydrates; and
  • 20%20\% - 35%35\% of energy should come from fats.

Before starting following IIFYM diet, have a look at its pros and cons:


  1. Flexible - you don't follow strict meal plans.
  2. Fewer restrictions - you can eat what you want, as long as it fits your macros.
  3. More accessible - you only need to know how to calculate macros in the foods you eat.


  1. May involve consuming a lot of unhealthy foods, which can be detrimental to your health!
  2. Ignores the recommendations for micronutrients and vitamins.
  3. Still requires some kind of calculation and knowledge about the nutritional value of food products.

IIFYM calculator

If it fits your macros calculator estimates the amount of proteins, fats, and carbohydrates you should eat every day in a few steps:

  1. The tool calculates your basal metabolic rate (BMR) using the Mifflin - St Jeor equation.

🙋 If you want to learn more about the basal metabolic rate, visit our BMR calculator: you will find a detailed explanation and a handy tool for all your needs!

The result depends on your sex, height, weight, and age.

For women:

BMR[kcal/day]=10×weight [kg]+6.25×height [cm]5×age [y]161\begin{split} \footnotesize{\mathrm{BMR}}& \scriptsize{[\mathrm{kcal/day}]}\footnotesize{ = 10\times \mathrm{weight}}\ \scriptsize{[\mathrm{kg}]}\\ &\footnotesize{+ 6.25\times \mathrm{height}\ }\scriptsize{[\mathrm{cm}]}\footnotesize{ - 5\times \mathrm{age}\ }\scriptsize{[\mathrm{y}]}\\&\footnotesize{ - 161} \end{split}

And for men:

BMR[kcal/day]=10×weight [kg]+6.25×height [cm]5×age [y]+5\begin{split} \footnotesize{\mathrm{BMR}}& \scriptsize{[\mathrm{kcal/day}]}\footnotesize{ = 10\times \mathrm{weight}}\ \scriptsize{[\mathrm{kg}]}\\ &\footnotesize{+ 6.25\times \mathrm{height}\ }\scriptsize{[\mathrm{cm}]}\footnotesize{ - 5\times \mathrm{age}\ }\scriptsize{[\mathrm{y}]}\\&\footnotesize{ +5} \end{split}
  1. Next, the calculator estimates your total daily energy expenditure (TDEE) (learn more about this quantity at our TDEE calculator). The TDEE is based on your BMR and physical activity level.

  2. Finally, it calculates your recommended amounts of macronutrients. The results will depend on your TDEE, and whether you want to lose, maintain, or gain weight.

How to calculate macros for weight loss?

If you are planning to lose or gain weight, you will need to adjust the number of calories computed by the total energy expenditure formula. It's estimated that each kilogram of body fat stores approximately 7,000 kcal of energy (so one pound stores approximately 3500 kcal). This is why you need a calorie deficit of:

  • 500 kcal500\ \mathrm{kcal} a day to lose one pound or 0.5 kg0.5\ \mathrm{kg} a week; and
  • 1000 kcal1000\ \mathrm{kcal} a day to lose two pounds or 1 kg1\ \mathrm{kg} a week.

If you want to gain weight, you will need to add the same amount of food to your diet.

Important! Make sure that your target weight is within a healthy range. If you are not sure how much you should weigh, check our ideal weight calculator.

Calorie deficit larger than 7,000 kcal a week is not recommended as it may cause fatigue, nutrient deficiencies, and negatively influence weight loss maintenance. You can learn about the consequences of a high reduction in your intake with our calorie deficit calculator.

If you want to increase your chances of successful weight loss, make sure that you are meeting recommendations for physical activity. Do you want to know which activities burn the most calories? Check out our calories burned calculator!

Joanna Michałowska, PhD candidate and Rita Rain
Personal details
Activity level
Light exercise 1-2 times/week
Maintain weight
Calories to maintain weight
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