BMR Calculator

How to Use the BMR Calculator

Basal metabolic rate is the energy your body would use lying completely still for twenty-four hours — keeping your heart beating, your brain running and your temperature steady. It is the floor everything else is built on, and it is not a calorie target.

1. Enter your height, weight and age. All three matter: taller and heavier bodies have more tissue to maintain, and metabolic rate falls slowly with age at about 5 kcal a year on the Mifflin equation.
2. Select sex at birth. The equations use it because average body composition differs, and it is worth being clear that this is a statistical adjustment rather than a statement about any individual.
3. Pick a formula. Mifflin-St Jeor is the current default and the one most dietitians use. Harris-Benedict is older and reads a little higher. Katch-McArdle works from lean mass and needs a body fat figure.
4. Add a body fat percentage if you have one and want the Katch-McArdle result. At similar weights, a leaner body burns more at rest, and only that formula captures it.
5. Compare all three tiles. They disagree by around 80 kcal here, which is a useful reminder that every one of them is a population estimate rather than a measurement of you.

Measuring your actual resting rate requires indirect calorimetry in a clinic. Every formula on this page is regression from population data, and individuals vary from their predicted value by roughly 10% in either direction.

BMR Formulas

Three published equations, each used in its own original form.

Katch-McArdle is the only one of the three that ignores sex, height and age entirely. That is deliberate: if you know lean mass, it already contains most of the information the other variables were standing in for. Its weakness is that a body fat estimate carries its own error, which then propagates into the result.

Example

A 34-year-old man, 178 cm and 82 kg

1. Mifflin-St Jeor: 10 × 82 = 820, plus 6.25 × 178 = 1,112.50, minus 5 × 34 = 170, plus 5.
2. That gives 820 + 1,112.50 − 170 + 5 = 1,767.50 kcal a day.
3. Harris-Benedict: 88.362 + (13.397 × 82) + (4.799 × 178) − (5.677 × 34) = 1,848.12 kcal.
4. The two differ by 80.62 kcal, or 4.56% — Harris-Benedict reads higher, as it usually does.
5. At 18% body fat, lean mass is 67.24 kg, so Katch-McArdle gives 370 + 21.6 × 67.24 = 1,822.38 kcal.
6. Per kilogram, the Mifflin figure is 21.55 kcal — comfortably inside the 20 to 24 range most adults fall into.

What age costs

The Mifflin equation subtracts 5 kcal for every year of age, so the same body at 20 would have a BMR of 1,838 kcal and at 60 would have 1,638 kcal. That is a 200 kcal spread across forty years — real, but considerably smaller than the folk belief that metabolism collapses after thirty. Most of the age-related drop people experience comes from losing muscle and moving less, both of which are addressable.

The same figures for a woman

Change nothing but the sex adjustment and Mifflin gives 1,601.50 kcal — 166 kcal lower, which is the constant difference between the two versions of the equation. Harris-Benedict gives 1,610.07 for the same inputs. The gap between the sexes in these formulas reflects average differences in lean mass at a given height and weight.

What the Number Is Actually For

The same body at different ages, on both main formulas.

The two formulas run roughly parallel and about 4% to 5% apart across the whole range. That consistency matters more than the absolute difference: if you use one formula and track what happens, the systematic offset cancels out and only the trend remains.

What the number is actually for is the next calculation rather than this one. BMR on its own describes a coma. Multiplying it by an activity factor gives total daily energy expenditure, which is the figure any calorie target is built from — and the TDEE calculator is where that step happens.

Between 60% and 75% of most people's daily energy goes on basal metabolism, which is why the number is worth knowing at all. Exercise is a much smaller share than intuition suggests: an hour of moderate running burns perhaps 600 kcal, against the 1,768 this body uses simply existing.

Five Things That Make a Predicted BMR Wrong

Five things that make a predicted BMR wrong for an individual.

Body composition. Muscle is more metabolically active than fat, so two people at the same height, weight and age can differ by a hundred calories or more at rest. Only Katch-McArdle accounts for this, and only if the body fat figure feeding it is reasonable — the body fat calculator gives a tape-based estimate.

Thyroid function. An under- or overactive thyroid moves resting metabolic rate substantially, and no equation built on height and weight can detect it. Unexplained weight change alongside fatigue or heat intolerance is a reason to see a doctor rather than to adjust a spreadsheet.

Sustained dieting. Prolonged restriction lowers resting expenditure by more than the weight loss alone predicts — an adaptive response that typically runs 5% to 15% below the equation. It is one reason long diets stall.

Population fit. These equations were derived largely from European-descended adults and fit less well at the extremes of the range, particularly at very high body weights and in older adults.

Medication and illness. Several common drug classes affect metabolic rate, as does fever, which raises it by roughly 10% per degree Celsius. These are not adjustments a calculator can make for you.

There is one more reason to hold the figure loosely, and it is measurement rather than biology. The equations were fitted by indirect calorimetry — measuring oxygen consumption and carbon dioxide production under strict conditions, after an overnight fast, at rest, in a thermally neutral room. Almost nobody meets those conditions in ordinary life, and a resting rate measured casually reads higher than a true basal rate by something like 10%.

That distinction explains why two devices can give you different answers and both be reporting honestly. A fitness tracker is usually estimating resting metabolic rate, not basal metabolic rate, and the two are different quantities with similar names. Comparing a tracker figure with this page is comparing measurements taken under different rules.

Treat the output as a starting estimate with a margin of about 10%, use the same formula consistently, and let real-world weight change over three or four weeks correct it. That feedback loop is more accurate than any equation.