What it does
This calculator estimates how many calories your body uses in a day. It starts with your basal metabolic rate (BMR), the energy used at rest, from the Mifflin-St Jeor equation published in 1990. It multiplies that by an activity factor to get the calories that keep your weight steady, then adds or takes away a little for a weight goal and splits the result into protein, fat and carbs.
It is an estimate, not medical advice, and everything runs in your browser. To check your weight range first, use the BMI calculator; to count a walk, try steps to distance.
How to use it
- Pick metric or imperial, then type your sex, age, weight and height.
- Choose how active you are. You can fine-tune the factor.
- Pick a goal, then read the daily target and the macro table.
What each setting does
- Sex: Mifflin-St Jeor adds 5 for men and takes 161 for women.
- Activity factor: 1.2 for little exercise up to 1.9 for very hard training or a physical job.
- Goal: 0.25 or 0.5 kg a week up or down. Each kg is counted as about 7,700 kcal, an approximate figure.
- Lowest daily target: a warning shows below this; it starts at 1,500 kcal for men and 1,200 for women, common minimums without medical supervision.
- Protein: grams per kg (the default 1.6 sits in the 1.4 to 2.0 g/kg range for active people) or a percent.
- Fat: percent of calories; 20 to 35% is the US adult range. Carbs get the rest.
Worked example
A man aged 30, 80 kg and 180 cm tall:
BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 1,780 kcal. Moderately active (× 1.55) gives 2,759 kcal to keep his weight. To lose 0.5 kg a week he takes off 0.5 × 7,700 ÷ 7 = 550 kcal, a target of 2,209 kcal.
At 2,759 kcal with 1.6 g/kg protein and 30% fat: protein 128 g, fat 92 g, carbs 355 g.
A woman aged 30, 60 kg and 165 cm: 600 + 1,031.25 − 150 − 161 = 1,320.25, shown as 1,320 kcal.
FAQs
How accurate is it? For most healthy adults Mifflin-St Jeor lands within about 10% of a measured value, but people vary.
Is anything saved? No. Nothing is stored.
Where the numbers come from
- Mifflin MD, St Jeor ST et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 51(2): 241–247 (the BMR formula).
- Activity factors 1.2 to 1.9 and about 7,700 kcal per kg of body weight are common rules of thumb; both are editable.
- Jäger R et al. (2017). ISSN position stand: protein and exercise. J Int Soc Sports Nutr 14: 20 (1.4 to 2.0 g/kg for active people). Institute of Medicine (2005), Dietary Reference Intakes for Energy and Macronutrients (fat 20 to 35% of energy).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Read more on the Blog
- Daily calorie needs explained: BMR, activity, goals and macros(the full guide)
Last updated 2026-11-10.