BMR Calculator

Basal metabolic rate by three formulas, side by side.

Calculate basal metabolic rate with the Mifflin-St Jeor, Harris-Benedict and Katch-McArdle equations, and see why they disagree.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Units

Sex

Age

years

Height

Weight

Body fat percentage

Optional. Supplying it unlocks Katch-McArdle, the most accurate of the three.

BMR (Mifflin-St Jeor)

1,649 kcal

at complete rest, before any activity

Harris-Benedict1,696 kcal

Katch-McArdleadd body fat %

Per kg of body weight23.6 kcal

Per hour69 kcal

BMR excludes all movement and digestion. Never use it as a calorie target, that comes from TDEE.

How the BMR Calculator works

Basal metabolic rate is what your body spends doing nothing at all: running your heart, brain, kidneys and cell repair while you lie still. It is 60 to 70 percent of most people's daily burn, and it is the floor every other calorie figure is built on.

Also known as: basal metabolic rate · calories burned at rest · Mifflin St Jeor calculator · bmr calculator with body fat percentage

What basal metabolic rate covers

Basal metabolic rate is the energy the body uses at complete rest, in a thermally neutral environment, at least twelve hours after eating. It funds breathing, circulation, cell maintenance, brain activity and organ function.

It accounts for roughly 60% to 70% of total daily energy expenditure in a sedentary adult, which makes it the largest single component by a wide margin.

Resting metabolic rate is a closely related term measured under less strict conditions and typically comes out a few percent higher. The two are used interchangeably in most contexts, and the distinction only matters if you are comparing a prediction against a laboratory measurement.

The equations

Mifflin-St Jeor, published in 1990, is the current default. For men: 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age, plus 5. For women the same with minus 161 instead of plus 5.

Harris-Benedict, from 1919 and revised in 1984, is older and tends to overestimate slightly in modern populations, probably because body composition has changed.

Katch-McArdle uses lean body mass instead of total weight: 370 plus 21.6 times lean mass in kilograms. It has no separate equation for men and women because lean mass already captures most of that difference, and it is the most accurate of the three for anyone with an unusual body composition, provided the lean mass figure is reliable.

How accurate any of them are

Mifflin-St Jeor predicts measured BMR within 10% for roughly 70% of people. That means three people in ten are outside 10%, and some are considerably outside.

For someone with a BMR around 1,600 calories, 10% is 160 calories a day. Over a month that is a difference of roughly half a kilogram of body weight, which is enough to make a diet appear to be failing when the plan was simply based on the wrong starting number.

The variation between individuals of identical height, weight, age and sex is real and comes from differences in organ size, lean mass, thyroid function and genetics. No equation using those four inputs can capture it.

What actually changes it

Lean mass is the largest modifiable factor. Muscle is metabolically more active than fat, and adding muscle raises BMR. The effect is smaller than commonly claimed: roughly 13 calories per kilogram of muscle per day, so adding two kilograms of muscle is worth around 26 calories, not the several hundred sometimes advertised.

Age reduces it, largely through loss of lean mass rather than through any separate ageing effect on metabolism. Resistance training substantially offsets this.

Prolonged energy restriction reduces it beyond what the weight loss alone predicts, an effect called adaptive thermogenesis. It is real, it is usually in the range of 5% to 15%, and it partly reverses when energy intake returns to maintenance.

Using the number sensibly

BMR is a starting point rather than a target. Nobody should eat at their BMR, since it excludes all activity and all digestion.

The practical approach is to treat the predicted figure as a hypothesis, apply an activity multiplier to get a maintenance estimate, eat at that level for two to three weeks, and adjust based on what the scale actually does.

Body weight fluctuates by a kilogram or more day to day with fluid, food volume and glycogen, so the assessment has to be made on a weekly average across at least a fortnight. Judging a plan on a single weigh-in is reading noise.

Where to go next

The BMR question rarely arrives on its own. These are the ones that usually come with it:

TDEE Calculator — Total daily energy expenditure and what each part contributes.
Calorie & BMR Calculator — Daily calorie needs for losing, holding, or gaining weight.
Macro Calculator — Protein, fat and carbs in grams for your goal.
BMI Calculator — Body mass index with category and healthy weight range.

Sources

Mifflin & St Jeor, Am J Clin Nutr 1990 (PubMed)

Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.

Frequently asked questions

Which BMR formula is most accurate?

Mifflin-St Jeor is the current standard and lands within about 10% for most adults. Harris-Benedict, published in 1919 and revised in 1984, tends to read high. Katch-McArdle is the most accurate of the three but needs your body fat percentage, because it works from lean mass rather than total weight.

Why do the three formulas give different answers?

They were derived from different populations at different times. Harris-Benedict came from a small early-20th-century sample; Mifflin-St Jeor from a larger, more modern and more varied one. Katch-McArdle sidesteps the problem by measuring what actually burns energy, lean tissue, rather than inferring it from height and weight.

Can I change my BMR?

Somewhat, and slowly. More lean mass raises it, since muscle is metabolically active at rest. Prolonged aggressive dieting lowers it, partly through lost tissue and partly through adaptation. The day-to-day swings people attribute to a fast or slow metabolism are usually differences in activity, not BMR.

Should I eat at my BMR to lose weight?

No. BMR excludes all movement, digestion and exercise, so eating at it creates a much larger deficit than the number suggests. Diet targets are set from TDEE, BMR multiplied by an activity factor, not from BMR itself.