The formula
How to calculate bMR
Basal metabolic rate is the energy your body uses doing nothing at all — breathing, circulating blood, maintaining temperature and repairing tissue. It typically accounts for 60–70% of daily calorie use.
The Mifflin-St Jeor equation, published in 1990, is the one most dietitians use. It replaced the older Harris-Benedict formula because it proved more accurate across modern populations, typically within about 10% for people of average body composition.
Fill in the following:
- Weight (kg)
- Height (cm)
- Age (years)
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
None of the fields above need specialist equipment to fill in — the inputs are the kind of measurements that can be taken at home with a tape measure or scale, which is exactly why this style of calculation is widely used as a first, approximate check rather than a diagnostic one.
Why bMR matters
This kind of calculation gets used both out of general interest and ahead of a specific conversation — with a trainer, a midwife or a GP — where having an actual number ready, rather than a description, makes that conversation more concrete and specific.
This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.
It is also worth being honest about what a formula like this is actually built from. Most of the equations behind calculators like this one were derived from population studies — averages taken across large groups of people — and applied here as a general-purpose estimate rather than a measurement tailored to any one individual. That is a reasonable trade-off for a quick, free estimate, and it is also exactly why a result that looks unusual is worth discussing with a professional rather than acted on alone.
A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.
Worked example
Here is the calculation with the starting values:
- Weight: 75 kg
- Height: 175 cm
- Age: 35 years
That gives:
- BMR, male equation: 1,674 kcal/day
- BMR, female equation: 1,508 kcal/day
- Per hour, male equation: 70 kcal
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
BMR is not what you can eat. It is the floor: total daily energy use adds movement, digestion and exercise on top, usually 20–70% more depending on how active you are.
Where this goes wrong. The equation does not know your body composition. Muscle burns more at rest than fat does, so BMR is underestimated for very muscular people and overestimated at high body fat. This is an estimate for general guidance and not medical advice.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
BMR is measured under strict conditions — fully rested, fasted, at a comfortable temperature. RMR is measured under less rigid conditions and comes out roughly 10% higher. In everyday use the two terms are often mixed up.
Somewhat. Adding muscle raises it modestly, since lean tissue is more metabolically active than fat. Prolonged, aggressive dieting lowers it, which is part of why weight regain after a crash diet is so common.
The headline figure is bMR, male equation. With 75 kg weight, 175 cm height and 35 years age, that comes to 1,674 kcal/day. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.