IIFYM Calculator (If It Fits Your Macros)

By Joanna Michałowska, PhD candidate and Rita Rain
Last updated: Sep 22, 2020

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:

  • 1 gram of fat = 9 kcal
  • 1 gram of proteins or carbs = 4 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 fits 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:

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

Before starting following IIFYM diet, have a look at the pros and cons presented in the table below:

Pros Cons
Flexible - you don't follow strict meal plans May involve consuming a lot of unhealthy foods, which can be detrimantal for your health!
Less restrictions - you can eat what you want, as long as it fits your macros Ignores the recommendations for micronutrients and vitamins
More accessible - you only need to know how to calculate macros in the foods you eat 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. The result depends on your sex, height, weight, and age.

    WOMEN: BMR (kcal/day) = 10 * weight (kg) + 6.25 * height (cm) – 5 * age (years) – 161

    MEN: BMR (kcal/day) = 10 * weight (kg) + 6.25 * height (cm) – 5 * age (years) + 5

  2. Next, the calculator estimates your total daily energy expenditure (TDEE), based on your BMR and physical activity level.

  3. 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 kcal a day to lose one pound or 0.5 kg a week; and
  • 1000 kcal a day to lose two pounds or 1 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. 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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