BMR calculator

What BMR actually measures

Basal metabolic rate is the energy your body spends doing nothing at all: keeping your heart beating, your lungs moving, your brain running, your body at temperature. Measured properly it means lying still, fasted, in a temperature-controlled room, awake but at complete rest.

It is the largest single component of most people’s daily burn — typically around 60–70% of total expenditure for someone who is not an athlete. That surprises people who assume exercise dominates. For the average desk worker, the gym session is a small slice next to the cost of simply existing.

This calculator uses the Mifflin-St Jeor equation, developed in 1990 from indirect calorimetry measurements in 498 healthy adults, and still the most reliable of the widely used prediction equations:

Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. Women: the same, but − 161 instead of + 5.

BMR vs RMR vs TDEE

Three terms get used interchangeably and should not be.

• BMR — the strict resting measurement described above, under laboratory conditions.
• RMR (resting metabolic rate) — the same idea measured under looser, more practical conditions. It usually comes out slightly higher than BMR, by a few percent. Most equations, including this one, are close enough that the distinction rarely matters outside a lab.
• TDEE — BMR plus digestion, plus daily movement, plus exercise. This is the number your calorie target should be based on.

A worked example

A 45-year-old man, 180 cm, 85 kg.

BMR = (10 × 85) + (6.25 × 180) − (5 × 45) + 5 = 850 + 1,125 − 225 + 5 = 1,755 calories a day.

That is what he would burn lying in bed for 24 hours. With a desk job and three gym sessions a week (×1.55), his TDEE is roughly 2,720 — nearly a thousand calories above his BMR, which is the gap that matters when setting a target.

Never eat at your BMR

This is the single most consequential misuse of a BMR number, and it comes from a reasonable-sounding but wrong intuition: that eating at BMR means the body covers everything else from fat.

In practice, setting intake at BMR usually produces a deficit far larger than intended — for the example above, eating 1,755 against a 2,720 burn is a 35% cut. That is aggressive enough that hunger, poor training, low energy and eventual abandonment are the likely outcome, and more of the loss comes from lean tissue.

BMR is an input to a calculation, not a target. Multiply it by an activity factor to get TDEE, then take a modest percentage off that.

What moves your BMR, and what does not

BMR is less malleable than the internet suggests, and the things that genuinely shift it are slow.

• Body size moves it most. More mass of any kind means a higher BMR, which is why it falls as you lose weight.
• Lean mass costs more to maintain than fat mass, so two people at the same weight with different body compositions differ — though the effect is smaller than commonly claimed, since muscle at rest is not especially expensive.
• Age lowers it gradually, which the equation reflects directly through the age term.
• Sustained dieting reduces it somewhat beyond what weight loss alone predicts. Real, and worth knowing about; not the metabolic catastrophe it gets described as.
• Meal timing, "metabolism-boosting" foods and green tea do essentially nothing at the scale that matters. Neither does eating little and often versus few and large.

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