BMR Calculator

This estimates your basal metabolic rate, the energy your body uses at complete rest in a comfortable room with nothing left to digest, and then scales it up to a daily calorie figure for six activity levels. You can switch between the three standard equations and see what each one says for you.

What BMR actually is

BMR is what it costs to keep you alive doing nothing. It is roughly the fuel an idling car burns while parked. In that state the energy goes to the heart, brain, kidneys, liver, lungs, nervous system, intestines, muscles and skin, all of which run continuously whether or not you get out of bed.

It is also the largest part of your day. For most people around 70% of daily energy goes on this upkeep. Physical activity accounts for roughly 20%, and about 10% goes on digesting food itself, a cost known as the thermic effect of food. That ratio surprises people who assume exercise dominates the equation. On the defaults here, a sedentary day comes to 1,926 calories of which 1,605 is BMR, the moving around adds 321.

A true BMR measurement is taken under strict conditions: awake, completely rested, with the sympathetic nervous system quiet and roughly twelve hours since the last meal. Multiply the result by a factor between 1.2 and 1.9 depending on how active you are and you get your daily calorie need.

The three equations

Outside a laboratory, BMR is estimated from equations fitted to measured data. The calculator offers the three in general use, and the second chart shows all three side by side for the details you entered, because the gap between them matters.

Mifflin-St Jeor, published in 1990, is the default and is generally the most accurate of the three for the general population:

Men: BMR = 10W + 6.25H − 5A + 5

Women: BMR = 10W + 6.25H − 5A − 161

Harris-Benedict was one of the earliest, dating from 1919 and revised in 1984. It was the standard until Mifflin-St Jeor replaced it, and it tends to read slightly high:

Men: BMR = 13.397W + 4.799H − 5.677A + 88.362

Women: BMR = 9.247W + 3.098H − 4.330A + 447.593

Katch-McArdle works differently. It ignores height, age and sex entirely and uses lean body mass, which makes it the better choice for lean people who know their body fat percentage:

BMR = 370 + 21.6 × (1 − F) × W

W is weight in kilograms, H is height in centimetres, A is age in years and F is body fat as a decimal fraction.

Run the defaults through all three and Mifflin-St Jeor gives 1,605, Harris-Benedict gives 1,614, and Katch-McArdle at 20% body fat gives 1,407. A spread of 207 calories on the same person is the honest measure of how precise any of this is. Katch-McArdle sits lowest here because 20% body fat on a 60 kg frame leaves relatively little lean mass; on a leaner person it moves the other way.

Turning BMR into a daily calorie target

The table and the first chart multiply your BMR by an activity factor:

• ×1.2: sedentary, little or no exercise
• ×1.375: exercise 1 to 3 times a week
• ×1.465: exercise 4 to 5 times a week
• ×1.55: daily exercise, or intense exercise 3 to 4 times a week
• ×1.725: intense exercise 6 to 7 times a week
• ×1.9: very intense exercise daily, or a physically demanding job

Exercise here means 15 to 30 minutes with your heart rate raised. Intense means 45 to 120 minutes. Very intense means two hours or more. Most people overestimate which band they belong to, and a desk job with three gym sessions a week is nearer 1.375 than 1.55. Choosing one band too high adds a few hundred calories a day, enough to stall weight loss entirely.

Note too that the multiplier covers everything above resting, including the energy spent digesting food and the fidgeting, walking and standing that never appears in a workout log. That last category, non-exercise activity thermogenesis, varies by hundreds of calories a day between people of similar size and is one reason two people on identical diets diverge.

What moves your BMR

Muscle mass. Aerobic work like running or cycling has no direct effect on BMR. Resistance training does, indirectly, by adding muscle, which costs more to maintain than fat. The effect is real but modest: a pound of muscle burns roughly 6 calories a day at rest against about 2 for a pound of fat, so the often-repeated claim that a pound of muscle burns 50 calories a day is wrong by an order of magnitude. Muscle is still worth building, just not on that arithmetic.

Age. BMR falls as you get older, largely because muscle mass declines. Large-scale work published in 2021 refined the familiar picture: metabolism per kilogram is highest in infancy, settles from about age 20, then holds remarkably steady until roughly 60 before declining. So the "metabolism slows in your thirties" story is mostly a story about losing muscle and moving less, not about the metabolism itself.

Body size and composition. Bigger bodies have more tissue to maintain, which is why weight appears in every equation. It also means BMR falls as you lose weight, so a target set at the start of a diet is too high by the end.

Genetics. Inherited traits shift BMR between people of otherwise identical build.

Temperature. Cold environments raise BMR because holding body temperature steady costs energy, and so does excessive heat, since cooling is also work. BMR rises roughly 7% for every 1.36°F increase in internal body temperature, which is why a fever burns through calories.

Hormones. Thyroid hormone is the main regulator: an underactive thyroid lowers BMR and an overactive one raises it, sometimes substantially. Persistent unexplained changes in weight or energy are worth investigating rather than adjusting a calorie target around.

Diet. Prolonged severe restriction lowers BMR, by as much as 30% in extreme cases. The body behaves like a phone dropping into power-saving mode, trading energy levels, mood and maintenance for survival. This adaptive component is a large part of why aggressive dieting stops working and why lost weight returns.

Pregnancy. Supporting a fetus raises BMR, which is why appetite increases. Menopause can move it in either direction depending on the hormonal changes involved.

Stimulants. Caffeine and some other compounds raise BMR slightly. The effect is small, temporary, and tends to shrink as tolerance builds.

BMR, RMR and TDEE

Three terms get used interchangeably and are not the same thing.

BMR is measured under total physiological equilibrium: fasted, fully rested, temperate room. RMR, resting metabolic rate, sometimes called resting energy expenditure, is measured under looser conditions and typically comes out a few per cent higher. Because the strict conditions are hard to achieve, most figures quoted as BMR are really RMR, and the equations above were fitted to measurements of both. TDEE, total daily energy expenditure, is the whole day including activity and digestion, the numbers in the table.

How accurate is any of this?

Less than the precision of the output suggests. A 2005 meta-analysis found that after controlling for fat-free mass, fat mass, age and circulating thyroxine, about 26% of the variation in BMR between people remained unexplained. Two people matched on every measurable factor can still differ by a few hundred calories a day for reasons that are not understood.

So treat the figure as a starting point rather than a prescription. Equations are estimates; even direct measurement with a metabolic cart, the handheld calorimetry devices available at some gyms, clinics and weight-loss centres, has its own error margin.

The practical approach is to use the number to set an opening target, then correct from what actually happens. Track weight and intake for two to three weeks, take the average rather than any single day, and adjust. If weight is stable, your real maintenance intake is whatever you have been eating, which beats any equation. That feedback loop is what makes progress; the calculator just saves you guessing at the first number.

Our Calorie Calculator works from BMR toward a weight goal, the TDEE Calculator covers total daily expenditure, and the Body Fat Calculator gives the percentage Katch-McArdle needs.