BMR Calculator
Estimate your basal metabolic rate — the energy your body uses at complete rest — with a choice of Mifflin–St Jeor, Harris–Benedict, or Katch–McArdle.
How to use this calculator
- 1Enter your age, sex, height and weight.
- 2Choose a formula — Mifflin–St Jeor is the best default; switch to Katch–McArdle if you have a reliable body fat percentage.
- 3Read the BMR figure as your baseline — the calories you would burn lying still all day — then use the activity table to find your actual daily requirement.
How the calculation works
Mifflin–St Jeor (male): BMR = 10w + 6.25h − 5a + 5
Mifflin–St Jeor (female): BMR = 10w + 6.25h − 5a − 161
Katch–McArdle: BMR = 370 + 21.6 × lean mass (kg)- w
- Weight in kilograms
- h
- Height in centimetres
- a
- Age in years
Mifflin–St Jeor, published in 1990, is recommended by the Academy of Nutrition and Dietetics as the most accurate of the common predictive formulas for people of unknown body composition.
The revised Harris–Benedict equation tends to run slightly higher, particularly at higher body weights.
Katch–McArdle drops age and sex entirely and works from lean body mass directly, which is why it can outperform the other two for people who are notably muscular or lean — but it is only as good as the body fat percentage it is given.
Worked example
A 30-year-old male, 175 cm, 75 kg
- 1.BMR = 10(75) + 6.25(175) − 5(30) + 5.
- 2.= 750 + 1,093.75 − 150 + 5.
- 3.= 1,699 kcal a day at complete rest.
- 4.At a moderate activity level (×1.55) the daily requirement is about 2,633 kcal.
Result: 1,699 kcal per day
Same person, Katch–McArdle at 15% body fat
- 1.Lean mass = 75 × (1 − 0.15) = 63.75 kg.
- 2.BMR = 370 + 21.6 × 63.75 = 370 + 1,377 = 1,747 kcal a day.
Result: 1,747 kcal per day
What basal metabolic rate actually measures
Basal metabolic rate is the energy the body spends purely to stay alive — keeping the heart beating, the lungs breathing, the brain signalling, and cells replacing themselves — measured under conditions of complete rest. Strictly defined, it requires waking undisturbed in a thermally neutral room after an overnight fast and lying still before any activity at all, which is rarely practical to measure directly. In practice almost everyone, including this calculator, estimates it instead from a predictive equation built from age, sex, height and weight.
BMR typically makes up 60–75% of total daily energy expenditure for a sedentary adult — more than movement, exercise and digestion combined — which is why it is the natural starting point for any calorie or weight-management calculation, including the TDEE Calculator’s activity-adjusted estimate built on top of it.
Why there is more than one equation
Every BMR formula is a regression fitted to a specific measured population, and each generation has tried to correct the last.
- Harris–Benedict (1919, revised 1984) — the original equation, built from indirect calorimetry on a relatively small, lean early-20th-century sample. The 1984 revision by Roza and Shizgal adjusted the constants to better fit later data, but the equation still tends to run a little high for people carrying more body fat.
- Mifflin–St Jeor (1990) — developed from a larger, more contemporary and more representative sample. It is the formula the Academy of Nutrition and Dietetics recommends as the best general-purpose default, and it is what this calculator uses unless another is selected.
- Katch–McArdle — discards age and sex entirely and predicts from lean body mass alone, on the reasoning that metabolically active tissue — not total weight — is what actually drives resting energy use. It can outperform the other two for people who are notably muscular or lean, but only as accurately as the body fat percentage fed into it.
What actually moves BMR up or down
Lean tissue is the biggest lever: muscle burns meaningfully more energy at rest than fat does, which is why two people of identical weight can have noticeably different BMRs depending on body composition, and why BMR gradually declines with age as muscle mass tends to fall. Genetics contributes a real but modest spread around any predicted value — the standard error on these equations is commonly cited at around 10%. Thyroid function, pregnancy, illness, and significant or prolonged calorie restriction can all shift measured BMR further from what a formula predicts, sometimes substantially.
From BMR to a real-world calorie number
BMR alone answers only how much energy would be used lying still all day, which is not a target anyone actually eats to. Multiplying it by an activity factor — the approach previewed in this calculator’s activity table — turns it into total daily energy expenditure, the figure that actually reflects how much food a body needs on an ordinary day.
What this assumes, and where it stops
Assumptions
- You are an adult. These equations are not validated for children or adolescents.
- Body composition is typical for your height and weight.
- No condition significantly affecting metabolic rate, such as untreated thyroid disease.
Limitations
- Predictive equations have a standard error of roughly 10%, so your true BMR could be 150–200 kcal either side of the estimate.
- Accuracy falls for people at the extremes of body composition — very muscular or with a BMI above about 40.
- Does not account for adaptive thermogenesis: prolonged calorie restriction lowers BMR below what the equation predicts.
Common questions
What is the difference between BMR and RMR?
BMR is measured under strict conditions — fully rested, fasted, in a thermally neutral room, immediately on waking. RMR (resting metabolic rate) is measured under less rigorous conditions and comes out around 10% higher. In everyday use the terms are often treated as interchangeable.
Should I eat only my BMR to lose weight?
No. BMR is the energy needed for basic bodily function at complete rest, before you get out of bed. Eating at or below it for a sustained period is generally too aggressive, can be unsafe, and often backfires by lowering metabolic rate. Calculate your TDEE and take a moderate deficit from that instead, ideally with professional guidance.
Why do the equations disagree?
They were fitted to different study populations using different inputs. Harris–Benedict dates from 1919 (revised in 1984) using a smaller, leaner sample; Mifflin–St Jeor from 1990 using a more representative modern group — where the two differ, Mifflin–St Jeor is generally the better estimate. Katch–McArdle instead skips age and sex and works from lean body mass, so it disagrees with both whenever your body composition is unusual for your weight.
Which formula should I use?
Mifflin–St Jeor unless you have a specific reason not to — it is the most broadly validated for people of typical, unknown body composition. Switch to Katch–McArdle only if you have a trustworthy body fat percentage (from a DEXA scan, calipers, or even the Body Fat Calculator's Navy method), since it can be more accurate for people who are notably muscular or lean, where weight-based formulas are least reliable.
Sources
- Mifflin MD et al., A new predictive equation for resting energy expenditure — American Journal of Clinical Nutrition (1990)
- Dietary reference intakes for energy — US Department of Agriculture
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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