How to Calculate Your BMR (Basal Metabolic Rate)
By Jesse · Last updated August 20, 2026
Your basal metabolic rate is what your body spends staying alive with no movement at all: breathing, circulating blood, holding its temperature, replacing cells. It is the largest single component of what you burn in a day and the starting point for every calorie target. This guide is about the arithmetic — the two formulas, what each input is worth, and what the number does and does not include. How much to trust the result, and how to correct it with your own data, is covered in why TDEE calculators are estimates.
The two formulas
Two equations dominate practical use. Both take the same four inputs — sex, age, height, weight — and neither reads body composition, which is the source of most of their error.
Mifflin-St Jeor (1990)
The default recommendation for healthy adults. It was derived from measurements on 498 people, and a 2005 review of predictive equations found it the most likely of the common formulas to land within 10% of measured resting energy expenditure.
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
Harris-Benedict (revised 1984)
The original equations date to 1919, from a population whose body composition and activity patterns were not those of adults today. Roza and Shizgal revised them in 1984 against a larger dataset. This version usually runs a little above Mifflin-St Jeor.
Men: BMR = 88.362 + 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age
Women: BMR = 447.593 + 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age
A worked example, term by term
Take a 35-year-old woman, 168 cm tall, weighing 68 kg:
Mifflin-St Jeor:
BMR = 10 × 68 + 6.25 × 168− 5 × 35− 161
BMR = 680 + 1050 − 175 − 161 = 1394 kcal/day
Notice the shape of it: weight contributes 680 kcal and height 1050 kcal, while age removes 175 and the sex constant removes another 161. Body size does almost all the work. The same body through Harris-Benedict gives 1445 kcal, a difference of 51 kcal.
What each input is worth
Because the formula is linear, the value of every input is fixed and can be read off exactly. Starting from the example above and changing one thing at a time:
| Change | Effect on BMR | Why |
|---|---|---|
| 10 kg heavier | +100 kcal | Directly proportional, at 10 kcal per kilogram. Weight is the strongest single input. |
| 10 cm taller | +63 kcal | A taller body of the same weight has more surface area to heat and more tissue to maintain. |
| 10 years older | -50 kcal | A straight-line adjustment, standing in for the muscle typically lost with age. |
| Same body, male instead of female | +166 kcal | The gap between the two sex constants, which is the only place sex enters the equation. |
Computed from the same function the BMR calculator uses, starting from a 35-year-old woman at 168 cm and 68 kg.
Two things are worth taking from this. Weight dominates, at 10 kcal per kilogram, which is why the estimate moves noticeably as you lose weight and why a calorie target set once goes stale. And the entire effect of sex is a single constant — the formula does not model anything else about it.
Both formulas, four bodies
The gap between the two equations is usually modest and not always in the same direction. Four representative adults, each run through both:
| Body | Mifflin-St Jeor | Harris-Benedict | Gap |
|---|---|---|---|
| Man, 30, 180 cm, 80 kg | 1,780 kcal | 1,854 kcal | +74 kcal (+4.1%) |
| Man, 55, 175 cm, 95 kg | 1,774 kcal | 1,889 kcal | +115 kcal (+6.5%) |
| Woman, 30, 165 cm, 65 kg | 1,370 kcal | 1,430 kcal | +60 kcal (+4.4%) |
| Woman, 55, 160 cm, 75 kg | 1,314 kcal | 1,399 kcal | +85 kcal (+6.4%) |
Computed from the site's own BMR functions. The gap is Harris-Benedict minus Mifflin-St Jeor.
We ran the same comparison over a systematic grid of adult bodies rather than four cases. The short version is that the choice of formula matters less than almost anyone expects, and much less than the activity level you pick afterwards. The full result is in how much BMR formulas actually disagree. The BMR calculator reports both and their average, which is a reasonable way to avoid having to choose.
BMR, RMR, and what the formulas actually predict
A distinction worth knowing, because it explains part of the gap between a calculator and a lab. Basal metabolic rate is measured under strict conditions: awake but fully rested, fasted overnight, at a neutral temperature, typically before getting out of bed. Resting metabolic rate is measured under looser conditions and comes out slightly higher.
The equations above were fitted to measured resting energy expenditure. So a field labelled "BMR" in any calculator, including ours, is really a prediction of RMR. The difference is small and does not change how you use the number, but it is a reason not to treat the output as a precise physiological constant.
Where BMR sits inside your daily burn
Your total daily energy expenditure is BMR plus three other components: the energy spent digesting food, the movement you do without thinking of it as exercise, and deliberate training. Calculators bundle all three into one activity multiplier, which means BMR's share of your maintenance calories is fixed by whichever multiplier you pick:
| Activity level | Multiplier | BMR share of maintenance |
|---|---|---|
| Sedentary (little or no exercise) | x 1.2 | 83% |
| Lightly active (1–3 days/week) | x 1.375 | 73% |
| Moderately active (3–5 days/week) | x 1.55 | 65% |
| Very active (6–7 days/week) | x 1.725 | 58% |
| Athlete (twice-daily training) | x 1.9 | 53% |
A property of the multiplier model rather than a measurement: the share is simply 1 divided by the multiplier.
For a sedentary adult, resting metabolism accounts for around 83% of the day's calories; for someone training twice a day, around 53%. That is why BMR is the right place to start and the wrong place to stop. Choosing the multiplier is the step where most of the error enters, and it is the subject of the TDEE guide; the TDEE calculator does both steps at once.
What moves your BMR, and what does not
- Body size, most of all. More tissue costs more to maintain. This is why the estimate falls as you lose weight, and why a target set at the start of a diet is wrong by the end of it.
- Body composition, which the formula cannot see. Muscle is more expensive at rest than fat. Two people with identical inputs and different compositions get identical predictions and have different true rates — the single largest structural weakness of every weight-based equation.
- Age, gradually. The formula subtracts a fixed amount per year. Most of what it is standing in for is the muscle people tend to lose with age rather than age itself, which is why staying trained blunts the decline.
- Sustained calorie restriction. Resting expenditure falls further than the weight loss alone predicts. This is metabolic adaptation, and it is a real reason plateaus happen on an unchanged intake.
- Not much else you can buy. Caffeine and cold exposure move resting expenditure slightly and briefly. Nothing available without a prescription changes it in a way that competes with the effect of carrying more muscle or weighing more.
How to use the number
BMR is an input, not a target. Turn it into maintenance calories first, then set intake relative to that:
- Weight loss: below maintenance, typically 300 to 700 kcal a day less. See how many calories to eat to lose weight for choosing the size of the deficit.
- Maintenance: at maintenance, and re-check it after any substantial change in weight or training.
- Weight gain: above maintenance, typically 200 to 500 kcal a day for a lean gain.
Never set a target at or below your BMR on the strength of a formula. It is an estimate of the floor, with a margin of error, and eating there means eating far below what your body actually spends.
Common questions
How do I calculate my BMR by hand?
Use Mifflin-St Jeor: multiply your weight in kilograms by 10, add 6.25 times your height in centimetres, subtract 5 times your age in years, then add 5 if you are male or subtract 161 if you are female. For a 35-year-old woman at 168 cm and 68 kg that gives 1394 kcal a day.
Which BMR formula is most accurate?
Mifflin-St Jeor is the usual recommendation for healthy adults, and a 2005 review of predictive equations found it the most likely of the common formulas to land within 10% of measured resting energy expenditure. Accurate here means accurate on average across a group; for any one person the error can be larger in either direction.
What is the difference between BMR and RMR?
Basal metabolic rate is measured under strict conditions: awake, fasted, fully rested, at a comfortable temperature, usually first thing in the morning. Resting metabolic rate is measured under looser conditions and comes out slightly higher. The equations in common use, including the ones on this page, were fitted to measured resting energy expenditure, so a calculator labelled BMR is really predicting RMR.
Should I eat my BMR calories?
No. BMR is what your body needs at complete rest, before any movement, digestion, or exercise is counted. Eating at your BMR means eating well below maintenance for almost everyone. BMR is an input to a calorie target, not the target itself.
Does BMR slow down when you diet?
Yes, and by more than the weight lost alone accounts for. Sustained restriction lowers resting expenditure through metabolic adaptation, which is one reason a fixed calorie target produces less loss over time. Resistance training and adequate protein reduce the effect but do not eliminate it.
Why do two people the same weight have different BMRs?
Because the formulas read weight, not composition. Muscle costs more energy to maintain at rest than fat does, so a muscular person and a less muscular person of identical height, weight, age, and sex get the same prediction and have different true rates. This is the largest structural weakness of every weight-based equation.
These are estimates for healthy adults, not medical advice. BMR formulas have a margin of error of roughly 10% for most people and wider margins for those with unusual body compositions. See our methodology for the sources behind these calculations.