What the Macro Calculator does
This macro calculator estimates how many calories you need each day for your goal - losing fat, maintaining or building muscle - and splits them into grams of protein, carbohydrate and fat, per day and per meal. It uses the Mifflin-St Jeor equation for resting energy, a standard activity multiplier, a protein target in grams per kilogram of body weight, a minimum share for fat, and carbohydrate for the rest.
Every number is shown with the step that produced it, and every assumption can be changed: the size of the deficit or surplus, the protein per kilogram, the fat share and its floor, and the number of meals. The result is an estimate from population averages, not medical or dietary advice, and your details stay in your browser - they are only remembered if you tick the box to say so.
How to use it
- Choose kilograms and centimetres or pounds and feet, then enter your sex (the equation has a different constant for each), age, weight and height.
- Pick the activity level that matches a typical week, including your job, not just your best training week.
- Choose a goal. Lose fat starts from a 20% deficit, maintain from your estimated maintenance calories and build muscle from a 10% surplus.
- Read the daily calories at the top and the grams of protein, carbohydrate and fat below, with the per-meal split for the number of meals you eat.
- Open Change the assumptions to set your own deficit, protein target or fat share; the working list underneath updates so you can see exactly what changed.
Reading the results
Maintenance calories (often called TDEE, total daily energy expenditure) are an estimate of what you burn on an average day. Mifflin-St Jeor predicts resting energy within about 10% for most adults, and activity factors add further uncertainty, so treat the target as a starting point and adjust it from what your weight actually does over two to three weeks.
Protein is set per kilogram because need scales with body size. 1.6 g/kg is a common target for active people; higher intakes during a deficit help preserve muscle. If you carry a lot of body fat, basing protein on your goal weight rather than current weight is a reasonable choice - type the lower figure into the protein field.
Fat has a floor because very low-fat diets make it hard to absorb fat-soluble vitamins and get essential fatty acids. Carbohydrate takes the remaining calories; it is the most flexible macronutrient and the one to adjust first if you prefer more or less.
Warnings flag targets that sit outside common guidance - very low calories, carbohydrate below 130 g, a target below your resting energy - so you can make a deliberate choice rather than an accidental one.
Worked example: a 30-year-old man, 80 kg and 180 cm, moderately active, losing fat
Resting energy: 10 x 80 + 6.25 x 180 - 5 x 30 + 5 = 800 + 1,125 - 150 + 5 = 1,780 kcal. Moderately active (x 1.55) gives maintenance of 2,759 kcal.
A 20% deficit removes 551.8 kcal, leaving 2,207 kcal a day. Protein at 1.8 g per kg is 144 g, which is 576 kcal. Fat at 25% of 2,207 kcal is 551.8 kcal, or 61 g.
Carbohydrate gets the remaining 2,207.2 - 576 - 551.8 = 1,079.4 kcal, which is 270 g. Over three meals that is about 736 kcal, 48 g protein, 90 g carbohydrate and 20 g fat each. A 20% deficit like this typically means losing roughly 0.5% of body weight a week, but the scale over a few weeks is the real test.
Formulas and scoring rules
- Resting energy, men (Mifflin-St Jeor)
BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) + 5- Resting energy, women (Mifflin-St Jeor)
BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) - 161- Maintenance calories
TDEE = BMR x activity factor (1.2, 1.375, 1.55, 1.725 or 1.9)- Goal calories
target = TDEE x (1 + goal% / 100), or TDEE + a fixed changeDefaults: -20% lose, 0 maintain, +10% gain. Targets under 800 kcal are refused.- Protein
grams = g per kg x body weight; kcal = grams x 4- Fat
kcal = target x fat%, never below the floor%; grams = kcal / 9- Carbohydrate
kcal = target - protein kcal - fat kcal; grams = kcal / 4- Rounding
calories and grams shown to the nearest whole numberCalculations are done unrounded, so grams x 4/4/9 add back to the target.
Why Mifflin-St Jeor
Several equations predict resting energy from weight, height, age and sex. A 2005 systematic review for the American Dietetic Association compared them against measured values and found Mifflin-St Jeor the most likely to be within 10% for healthy adults of normal weight and with obesity, which is why most dietitians and calculators use it. It does not use body-fat percentage; very muscular people tend to burn more than it predicts.
The activity factors are the conventional ones paired with it. They are the larger source of error: most people overestimate how active they are, so if in doubt choose the lower level.
Protein ranges in plain numbers
The adult Recommended Dietary Allowance for protein is 0.8 g per kg of body weight a day - enough to prevent deficiency in sedentary people. Sports nutrition position statements, such as the International Society of Sports Nutrition's, suggest about 1.4 to 2.0 g/kg for people who exercise, and somewhat more during a calorie deficit to protect lean mass. The calculator accepts 0.8 to 3.3 g/kg and notes anything above 2.2.
Limitations: what the result does not prove
- It is an estimate for healthy adults. It is not suitable for pregnancy or breastfeeding, for children and teenagers who are still growing, or for anyone with kidney disease, diabetes, an eating disorder or another condition affecting diet - they need individual advice.
- The equation does not know your body composition. Two people with the same weight and height but different muscle mass burn different amounts.
- It says nothing about food quality, fibre, vitamins, minerals or meal timing. Hitting macro targets with mostly processed food is not the same as eating well.
- Weight change is not linear. Water, glycogen and digestive contents move the scale by a kilogram or more from day to day, so judge the targets over weeks.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- Mifflin MD, St Jeor ST et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990
- Frankenfield D et al. Comparison of predictive equations for resting metabolic rate. J Am Diet Assoc 2005
- Institute of Medicine - Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids
- Jager R et al. ISSN position stand: protein and exercise. J Int Soc Sports Nutr 2017
Frequently asked questions
How do I calculate macros for weight loss?
Estimate maintenance calories (resting energy x activity factor), subtract a deficit such as 20%, set protein in grams per kilogram (around 1.6-2.2 g/kg), give fat a share of at least 20% of calories, and fill the rest with carbohydrate. The calculator does each step and shows the arithmetic.
How much protein do I need to build muscle?
Most sports nutrition guidance suggests about 1.4 to 2.0 g per kilogram of body weight a day for people doing resistance training, spread across several meals. An 80 kg person would aim for roughly 110 to 160 g. More than that brings little extra benefit for most people.
Is the Mifflin-St Jeor equation accurate?
It is the most accurate of the common equations for healthy adults, predicting resting energy within about 10% for most people. Individuals can still differ by a few hundred calories, so use the result as a starting point and adjust it from your actual weight trend.
What does IIFYM mean?
"If it fits your macros" - an approach where you hit daily targets for protein, carbohydrate and fat rather than following a fixed meal plan. This calculator produces those targets. The approach works best when most of the food that fits is still nutritious.
What is a safe calorie deficit?
A moderate deficit of about 10-25% of maintenance is commonly recommended because it allows steady fat loss while keeping enough protein and nutrients. The calculator refuses targets under 800 kcal, which should only be followed under medical supervision, and warns below 1,200 kcal or below your resting energy.
Should I count macros or just calories?
Calories decide whether your weight goes down, up or stays the same; macros affect what that weight change is made of and how full and energetic you feel. Many people get most of the benefit by hitting calories and protein and letting carbohydrate and fat vary.
Why does the calculator ask for my sex?
Because the Mifflin-St Jeor equation has two versions with different constants: plus 5 for men and minus 161 for women, reflecting average differences in body composition. If neither describes you well, try both and use a figure between them, then adjust from real results.
Last reviewed by the A2Z.Tools team against the sources listed above.
