What is BMR, and how do you calculate it?
Basal metabolic rate is what your body costs to run while doing nothing at all: breathing, circulating blood, maintaining temperature, repairing tissue, keeping your brain lit. For most people it is the single biggest line in the daily energy budget — usually somewhere around 60–70% of the total.
Which means the ring you close at the gym is the smaller half of the story.
BMR, RMR and TDEE
Three terms get used interchangeably and should not be:
- BMR — measured at complete rest, fasted, in a thermally neutral room. A laboratory figure.
- RMR (resting metabolic rate) — the same idea under realistic conditions. Typically runs a few percent above BMR, and is what most devices report.
- TDEE (total daily energy expenditure) — everything: BMR, plus digestion, plus incidental movement, plus deliberate exercise.
A deficit is TDEE minus intake. BMR is the foundation you build TDEE on.
The Mifflin–St Jeor equation
Published in 1990, this is the formula most clinicians and apps reach for. It displaced the older Harris–Benedict equation by being more accurate for modern body compositions.
Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161The equation is metric, so imperial measurements convert first:
kg = lb ÷ 2.205 and cm = inches × 2.54.
Worked example
A 35-year-old man, 82 kg (181 lb), 178 cm (5’10”):
10 × 82 = 820
6.25 × 178 = 1,112.5
5 × 35 = 175
820 + 1,112.5 − 175 + 5 = 1,762.5 kcal/dayAbout 1,760 kcal before he gets out of bed. Add ordinary daily movement and a couple of sessions a week and his TDEE might sit near 2,300–2,500.
How accurate is it?
Mifflin–St Jeor lands within roughly 10% of measured BMR for most people. That sounds tight until you apply it: 10% of 1,760 is about 176 kcal a day, which is a third of a typical deficit.
It is least reliable at the edges. The equation knows your weight but not what that weight is made of, and muscle is metabolically more expensive than fat. Two people at identical height, weight, age and sex can differ meaningfully if one is considerably leaner. At very high or very low body fat the estimate degrades further.
So: a good starting point, not a measurement.
Using it without being misled
Start with the estimate, then correct with evidence. Run your plan for four weeks and compare actual weight change against what your tracked deficit predicted. If you ran what you believed was a 500 kcal daily deficit and lost clearly less than ~2 kg (4.4 lb), your real TDEE is lower than estimated. Adjust the input.
Recalculate as you lose. Weight is in the formula. Drop 8 kg (18 lb) and your BMR falls by roughly 80 kcal/day — a deficit that quietly shrinks if you never revisit it.
Prefer measured data when you have it. If a Watch is supplying real resting energy to Apple Health, that beats a formula. Caldown uses Mifflin–St Jeor to set your starting figure and to keep the countdown ticking between syncs, but defers to Health’s recorded resting energy whenever it exists.
Why it matters for a deficit
If you only count active calories, you will think a 400 kcal workout is your whole burn and build a plan around a number that is roughly a fifth of reality. Counting BMR puts the workout in proportion: useful, but a supporting player next to the energy you spend simply existing.
How accurate is Apple Watch calorie tracking? covers the measured side of the same question.
General information, not medical advice. Estimated figures are not a substitute for clinical assessment.