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BMR Calculator

The calories your body burns doing absolutely nothing — computed three different ways so you can see the spread.

Runs in your browser Free, no sign-up Updated 2026-08-21

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Three formulas, three answers

Mifflin–St Jeor (1990) — clinical standardMen: 10w + 6.25h − 5a + 5 Women: 10w + 6.25h − 5a − 161 Harris–Benedict (revised 1984) — older, still commonMen: 88.362 + 13.397w + 4.799h − 5.677a Women: 447.593 + 9.247w + 3.098h − 4.330a Katch–McArdle — best if you know body fatBMR = 370 + 21.6 × lean body mass (kg)

The first two work from height, weight, age and sex. They cannot see body composition, so they treat 80 kg of muscle and 80 kg of fat identically — even though muscle is metabolically far more expensive to maintain. Katch–McArdle sidesteps the problem by using lean mass alone, which makes it the most accurate of the three for lean or very muscular people, provided your body-fat estimate is good.

What the number is for

BMR on its own is not an eating target. It is the floor — the energy your body spends before you have stood up, walked anywhere or lifted anything. Almost nobody should eat at their BMR.

How BMR becomes a real daily numberTDEE = BMR × activity factor ×1.2 desk job, little exercise ×1.375 1–3 sessions a week ×1.55 3–5 sessions a week ×1.725 6–7 sessions a week ×1.9 twice-daily training or heavy manual work

That multiplier covers three separate things: deliberate exercise, everyday movement (fidgeting, walking, stairs — collectively called NEAT), and the energy cost of digesting food, which runs at roughly 10% of intake.

NEAT is the wildly variable one. Between two people of identical size, it can differ by 800 kcal a day, and it drops silently when you diet. That is a large part of why weight loss slows even when the eating has not changed.

Worked example

A 30-year-old man, 178 cm and 80 kg, has a BMR of about 1,768 kcal. With a desk job that is roughly 2,120 kcal a day; training four times a week it is about 2,740. The difference between those two lifestyles is over 600 kcal daily — more than most people cut when dieting.

What raises and lowers BMR

  • Lean mass — the largest controllable factor. More muscle means a higher floor.
  • Body size — bigger bodies cost more to run, which is why BMR falls as you lose weight.
  • Age — resting metabolism declines gradually in adulthood, largely because muscle mass declines.
  • Sex — men average higher BMR at the same height and weight, mostly through greater lean mass.
  • Thyroid function — hypothyroidism and hyperthyroidism can shift BMR substantially; this is medical territory.
  • Prolonged dieting — sustained deficits suppress resting expenditure beyond what the weight loss alone predicts.

Things that barely matter, despite the marketing: green tea, spicy food, eating six small meals instead of three, and almost every supplement sold as a "metabolism booster". Their combined effect is a rounding error next to lean mass and total activity.

Frequently asked questions

Does eating more often speed up my metabolism?

No. The thermic effect of food is roughly 10% of what you eat, so it scales with total intake rather than with how many sittings you split it into. Six small meals and three larger ones with identical calories produce essentially identical energy expenditure. Choose whichever pattern keeps you fuller.

Why did my metabolism slow down after dieting?

Partly because you are smaller and a smaller body costs less to run, and partly through adaptive thermogenesis — the body becomes somewhat more efficient and you move less without noticing. The effect is real but usually modest, and it is why recalculating your BMR every few kilograms matters more than pushing calories ever lower.

Do men and women have different BMRs?

Yes, on average, and the main reason is body composition rather than sex itself. Men typically carry more lean mass, which is metabolically more expensive to maintain. The Katch–McArdle formula largely removes the difference because it works from lean mass directly.

Is a slow metabolism why I cannot lose weight?

Very rarely. When researchers measure resting expenditure directly, people who report a slow metabolism usually have a normal one and are underestimating intake — a finding replicated many times under controlled conditions. Genuine metabolic disorders such as hypothyroidism exist and are worth testing for, but they are the exception rather than the explanation.

What is the difference between BMR and RMR?

BMR is measured under strict conditions — fully rested, fasted, in a thermoneutral room, immediately after waking. RMR is measured under looser conditions and comes out about 10% higher. In everyday use the two terms are often swapped, and the difference rarely changes a decision.

Which BMR formula is most accurate?

Katch–McArdle when you have a reliable body-fat percentage, because it works from lean mass. Otherwise Mifflin–St Jeor, which validation studies show outperforms Harris–Benedict for most modern adults.

Should I eat at my BMR to lose weight?

No. BMR is your at-rest floor and eating at that level while also moving around creates a very aggressive deficit. Subtract 250–500 kcal from your TDEE — your total daily expenditure — instead.

Can I increase my BMR?

Meaningfully, yes, but slowly: build and keep muscle through resistance training and adequate protein, avoid prolonged severe dieting, and stay generally active. Expect single-digit percentage changes over months, not transformations.

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