BMR calculator (three formulas)

Your BMR (basal metabolic rate) is the number of calories your body burns in a day at complete rest, just to keep your heart beating, lungs breathing, brain working and body warm. A 30-year-old man at 175 cm and 70 kg has a BMR of about 1,650 kcal by the Mifflin-St Jeor equation, and his full daily need is higher once you add movement.

What BMR actually measures

Strictly, BMR is measured lying still in a lab, usually first thing in the morning after an overnight fast, using a machine that tracks the oxygen you breathe in and carbon dioxide you breathe out (indirect calorimetry). Most of us estimate it instead from height, weight, age and sex. Studies often call the same thing RMR or REE (resting metabolic rate or resting energy expenditure), measured under slightly looser conditions.

The three formulas

Mifflin-St Jeor (1990)

This is the main result in the calculator above. Mifflin and colleagues built it from measurements on 498 healthy adults aged 19 to 78, both normal-weight and obese.

• Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
• Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

Worked example for that 30-year-old man: 700 + 1,093.75 − 150 + 5 = 1,648.75 kcal.

Revised Harris-Benedict (1984)

The original Harris-Benedict equations date from 1919. Mifflin's team found they overestimated measured energy use by about 5% in their group. In 1984 Roza and Shizgal re-fitted the equations on the original data plus extra subjects, and reported a precision of about ±14% in normally nourished people. The revised coefficients are listed on the Harris-Benedict equation page:

• Men: BMR = 88.362 + 13.397 × weight (kg) + 4.799 × height (cm) − 5.677 × age
• Women: BMR = 447.593 + 9.247 × weight (kg) + 3.098 × height (cm) − 4.330 × age

Same man: about 1,696 kcal, so a little higher than Mifflin-St Jeor, which is typical.

Katch-McArdle

This one ignores sex, age and height and works only from lean body mass, meaning everything that isn't fat:

• Lean mass (kg) = weight × (1 − body fat % ÷ 100)
• BMR = 370 + 21.6 × lean mass (kg)

It comes from McArdle, Katch and Katch's textbook Exercise Physiology and is still used in current research. Muscle and organs burn far more energy than fat, so for someone very muscular, or someone carrying a lot of fat, it can be closer than a weight-based formula. It is only as good as your body fat number, though. A tape-measure estimate from the body fat calculator is reasonable; a cheap smart scale can be off by several percent.

Which one should you use?

Use Mifflin-St Jeor unless you have a good reason not to. A systematic review by Frankenfield and colleagues for the American Dietetic Association compared the common equations and found Mifflin-St Jeor predicted resting metabolic rate within 10% of the measured value in more non-obese and obese adults than any other, with the narrowest error range. The same review warned that errors for individuals can still be large, and that older adults and ethnic minorities were underrepresented in the studies. None of these formulas was built on Indian subjects, so treat your number as a starting point.

Switch to Katch-McArdle if you know your body fat fairly well and you are clearly not average: a regular lifter with a lot of muscle, for example. Harris-Benedict is shown mainly for comparison.

How BMR relates to TDEE

BMR is only what you burn lying still. Your TDEE (total daily energy expenditure) adds walking, work, chores, training and the energy used to digest food. The usual shortcut is to multiply BMR by an activity factor:

TDEE is the number you'd eat to keep your weight steady. Most people with an office job who train three or four times a week land between 1.375 and 1.55; if unsure, pick the lower one. The BMR vs TDEE guide explains the difference in more detail, and the calorie calculator turns TDEE into a target for losing or gaining with a protein, carb and fat split.

Why you shouldn't eat below your BMR for long

To lose fat you eat below TDEE, not below BMR. Eating under your BMR for weeks on end usually means a very large deficit. It makes it hard to get enough protein, iron and calcium, tends to cost muscle along with fat, and leaves you tired and hungry. Fast weight loss also raises the risk of gallstones, according to the US National Institute of Diabetes and Digestive and Kidney Diseases.

A deficit of 10 to 20% below TDEE is a sensible place to start, and the calorie deficit and maintenance calories guide shows how to set it. If a doctor has put you on a very low-calorie plan, follow their advice; that is a supervised medical diet, not a DIY one. For a ready-made menu, try the 1,500-calorie Indian diet plan; to gain, see the weight gain diet for skinny people.

Limits of any BMR formula

Genetics, thyroid function, some medicines, PCOS, pregnancy and illness all change energy needs, and no equation knows about them. If you have a thyroid condition, diabetes or kidney disease, are pregnant, or have a history of eating disorders, get your targets from a doctor or dietitian. The practical test is the scale: eat at your estimated TDEE for two to three weeks, watch your weekly average weight, and adjust by 100 to 200 kcal.

Useful links

• Mifflin et al. 1990, A new predictive equation for resting energy expenditure (AJCN)
• Roza and Shizgal 1984, The Harris Benedict equation reevaluated (AJCN)
• Frankenfield et al. 2005, Comparison of predictive equations for resting metabolic rate: a systematic review (JADA)
• Tang et al. 2025, Metabolic adaptation with different prediction equations, using Katch-McArdle (Frontiers in Nutrition)
• NIDDK, Dieting and gallstones

This calculator is for general information and isn't medical advice.