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Formula
Mifflin-St Jeor: BMR = 10W + 6.25H − 5A + 5 (men) or − 161 (women); Harris-Benedict (revised): men 88.362 + 13.397W + 4.799H − 5.677A, women 447.593 + 9.247W + 3.098H − 4.330A; Katch-McArdle: BMR = 370 + 21.6 × LBM
- BMR
- basal (resting) metabolic rate, in kcal per day
- W
- body weight in kilograms
- H
- height in centimeters
- A
- age in years
- LBM
- lean body mass in kilograms, W × (1 − body fat % ÷ 100)
What basal metabolic rate means
Your basal metabolic rate (BMR) is the energy your body uses in a day just to stay alive at rest: breathing, circulating blood, keeping body temperature steady, running the brain, liver and kidneys, and renewing cells. For most adults it is the largest part of daily energy use, typically around 60–70% of the total.
Strictly, BMR is measured lying still after a night’s sleep and a 12-hour fast, in a warm room. Resting metabolic rate (RMR) or resting energy expenditure (REE) is measured under slightly looser conditions and runs a little higher. The equations below were fitted to resting measurements, so in practice BMR, RMR and REE are used interchangeably for estimates.
How to use the BMR calculator
- Choose a formula. Mifflin-St Jeor is selected by default and suits most people.
- Choose your sex and enter your age (18 or over).
- Enter weight and height in US or metric units. In feet, put the remaining inches in Extra inches.
- If you know your body fat percentage, enter it. Katch-McArdle requires it; the other two do not.
The result shows your BMR in kcal per day, per hour and in kilojoules, plus the result of each other formula you have data for, so you can compare them side by side.
Which BMR formula is most accurate
Mifflin-St Jeor (1990). Derived from 498 healthy men and women aged 19 to 78, both normal-weight and obese, with resting energy measured by indirect calorimetry. A 2005 systematic review for the American Dietetic Association found it predicted resting metabolic rate within 10% of the measured value in more people than other common equations, so it is the usual default.
Harris-Benedict, revised (1984). The original 1919 Harris-Benedict equations were refitted by Roza and Shizgal using the original data plus Benedict’s later subjects. They tend to give results a few percent higher than Mifflin-St Jeor, and the authors put their precision at about ±14%.
Katch-McArdle. BMR = 370 + 21.6 × lean body mass, the general equation proposed by Cunningham in 1991. Because it is based on lean mass rather than total weight, it can suit people whose body fat is well above or below average, as long as the body fat figure is reliable. The BMR calculator shows a Katch-McArdle result only when you enter body fat.
How to calculate BMR by hand
Take a 35-year-old man who weighs 80 kg and is 178 cm tall, with 22% body fat.
Mifflin-St Jeor: 10 × 80 + 6.25 × 178 − 5 × 35 + 5 = 800 + 1,112.5 − 175 + 5 = 1,742.5 kcal/day.
Harris-Benedict (revised): 88.362 + 13.397 × 80 + 4.799 × 178 − 5.677 × 35 = 88.362 + 1,071.76 + 854.222 − 198.695 = 1,815.6 kcal/day.
Katch-McArdle: lean mass = 80 × (1 − 0.22) = 62.4 kg, so BMR = 370 + 21.6 × 62.4 = 370 + 1,347.84 = 1,717.8 kcal/day.
The three answers span about 100 kcal, which is typical. For a woman, Mifflin-St Jeor subtracts 161 instead of adding 5, so the same measurements give 166 kcal less.
BMR by age: reference table
BMR falls with age in every equation, because lean mass tends to decline. Values below are for a woman 5 ft 4 in and 150 lb and a man 5 ft 10 in and 180 lb, rounded to the nearest calorie.
| Age | Woman, Mifflin-St Jeor | Woman, Harris-Benedict | Man, Mifflin-St Jeor | Man, Harris-Benedict |
|---|---|---|---|---|
| 20 | 1,435 | 1,494 | 1,833 | 1,922 |
| 30 | 1,385 | 1,450 | 1,783 | 1,865 |
| 40 | 1,335 | 1,407 | 1,733 | 1,808 |
| 50 | 1,285 | 1,364 | 1,683 | 1,752 |
| 60 | 1,235 | 1,321 | 1,633 | 1,695 |
| 70 | 1,185 | 1,277 | 1,583 | 1,638 |
Mifflin-St Jeor drops by exactly 50 kcal per decade at a fixed weight and height. Harris-Benedict drops by about 43 kcal for women and 57 kcal for men.
What changes your BMR
- Lean mass. Muscle and organs use more energy than fat, which is why lean mass predicts BMR best.
- Body size. Larger bodies burn more in absolute terms.
- Age and sex. Both shift lean mass on average, which the equations capture with an age term and a sex constant.
- Weight loss. Losing weight lowers BMR, and the body can adapt a little further. Recalculate after a meaningful change in weight.
- Health and medications. Thyroid conditions, fever, pregnancy and some medicines change metabolic rate in ways no equation can see.
From BMR to daily calories
BMR is only the resting part of energy use. To estimate everything you burn in a day, multiply BMR by an activity factor with the TDEE calculator. To turn that into a daily target for losing, keeping or gaining weight, use the calorie calculator, and the macro calculator can split the target into protein, carbs and fat. For a Katch-McArdle estimate, the body fat calculator gives a starting body fat figure.
Common mistakes
- Eating only your BMR. BMR excludes all activity, so eating just that much is a large deficit for most people.
- Guessing body fat. An optimistic guess makes Katch-McArdle overstate your BMR. Use a measured value or stay with Mifflin-St Jeor.
- Treating the result as exact. A BMR calculator gives a starting point; use it and adjust based on how your weight actually changes.
Worked examples
Mifflin-St Jeor, US units
A 42-year-old woman, 5 ft 5 in, 150 lb, using the default Mifflin-St Jeor equation.
Inputs: Formula = Mifflin-St Jeor (recommended); Sex = Female; Age = 42 years; Weight = 150; Weight unit = Pounds; Height = 5; Height unit = Feet; Extra inches = 5 in
150 lb = 68.04 kg; 5 ft 5 in = 165.1 cm Mifflin-St Jeor: 10 × 68.04 + 6.25 × 165.1 − 5 × 42 − 161 = 1,341 kcal/day
Answer: Estimated BMR (Mifflin-St Jeor): 1,341 kcal/day for a 42-year-old female, 150 lb, 5 ft 5 in. Estimate, not medical advice.
Harris-Benedict (revised), metric units
A 28-year-old man, 183 cm, 82 kg, using the Roza and Shizgal revision of Harris-Benedict.
Inputs: Formula = Harris-Benedict (revised 1984); Sex = Male; Age = 28 years; Weight = 82; Weight unit = Kilograms; Height = 183; Height unit = Centimeters
Harris-Benedict (revised): 88.362 + 13.397 × 82 + 4.799 × 183 − 5.677 × 28 = 1,906 kcal/day
Answer: Estimated BMR (Harris-Benedict (revised)): 1,906 kcal/day for a 28-year-old male, 82 kg, 183 cm. Estimate, not medical advice.
Katch-McArdle with a measured body-fat percentage
A 50-year-old man, 6 ft, 200 lb with 28% body fat. Only lean mass enters the equation.
Inputs: Formula = Katch-McArdle (needs body fat %); Sex = Male; Age = 50 years; Weight = 200; Weight unit = Pounds; Height = 6; Height unit = Feet; Body fat (optional) = 28 %
200 lb = 90.72 kg; 6 ft 0 in = 182.9 cm Lean mass: 90.72 × (1 − 28 ÷ 100) = 65.32 kg; Katch-McArdle: 370 + 21.6 × 65.32 = 1,781 kcal/day
Answer: Estimated BMR (Katch-McArdle): 1,781 kcal/day for a 50-year-old male, 200 lb, 6 ft 0 in. Estimate, not medical advice.
Assumptions and limitations
- Results are an estimate, not medical advice. Equations predict the average for people like you; an individual's measured rate can differ by 10% or more.
- Mifflin-St Jeor (1990) is the default. Harris-Benedict uses the 1984 revision by Roza and Shizgal. Katch-McArdle uses 370 + 21.6 × lean mass (the Cunningham 1991 equation) and ignores sex and age.
- The equations were developed in healthy adults, so the calculator accepts ages 18 to 100. Mifflin-St Jeor was derived from people aged 19 to 78.
- BMR here means resting energy expenditure measured after an overnight fast at rest, as in the source studies; it does not include activity or digestion.
- Weight must be between 20 and 400 kg and height between 100 and 250 cm. Body fat, needed only for Katch-McArdle, must be 3% to 70%.
- If your BMI is under 18.5 or 40 or more, the result adds a note to check with a professional, because the equations are less reliable there.
Frequently asked questions
How do you calculate BMR?
Use a published equation. With Mifflin-St Jeor, BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age, then add 5 for men or subtract 161 for women. A 60-year-old woman weighing 70 kg at 160 cm: 700 + 1,000 − 300 − 161 = 1,239 kcal/day.
Which BMR formula is most accurate?
For most adults, Mifflin-St Jeor. A 2005 systematic review found it estimated resting metabolic rate within 10% of the measured value in more people than Harris-Benedict and other common equations. Katch-McArdle can do better if your body fat percentage is measured accurately.
What is the BMR formula for women?
Mifflin-St Jeor for women is 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161. The revised Harris-Benedict equation for women is 447.593 + 9.247 × weight + 3.098 × height − 4.330 × age.
How does a BMR calculator with body fat percentage work?
It uses the Katch-McArdle equation, which works from lean body mass rather than total weight: BMR = 370 + 21.6 × lean mass in kg. At 90 kg and 30% body fat, lean mass is 63 kg, giving 370 + 1,360.8 = 1,730.8 kcal/day.
How accurate is a BMR calculator?
Equations predict an average for people of your size, age and sex. Individuals commonly differ from the prediction by 10% or more. Only indirect calorimetry, a breathing test done in some clinics, measures your actual rate.
Should I eat my BMR to lose weight?
Not as a rule. BMR covers only the energy used at complete rest; daily activity adds to it. Your maintenance calories are BMR times an activity factor, and a weight-loss target is set below that, not at BMR.
Is BMR the same as RMR?
Nearly. BMR is measured under strict conditions after an overnight fast and sleep; RMR is measured under looser rest conditions and is usually slightly higher. The equations here estimate resting energy expenditure, and the terms are used interchangeably for estimates.
Sources
- A new predictive equation for resting energy expenditure in healthy individuals (Mifflin MD, St Jeor ST et al., Am J Clin Nutr 1990;51:241–7) — American Journal of Clinical Nutrition, via PubMed (NIH). Checked October 10, 2026.
- The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass (Roza AM, Shizgal HM, Am J Clin Nutr 1984;40:168–82) — American Journal of Clinical Nutrition, via PubMed (NIH). Checked October 10, 2026.
- Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation (Cunningham JJ, Am J Clin Nutr 1991;54:963–9) — American Journal of Clinical Nutrition, via PubMed (NIH). Checked October 10, 2026.
- Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review (Frankenfield D et al., J Am Diet Assoc 2005;105:775–89) — Journal of the American Dietetic Association, via PubMed (NIH). Checked October 10, 2026.
- Human energy requirements: report of a joint FAO/WHO/UNU expert consultation (2004) — Food and Agriculture Organization of the United Nations / World Health Organization. Checked October 10, 2026.