BMR Calculator

Enter your sex, age, height and weight, and the calculator returns your basal metabolic rate — the calories your body burns each day at complete rest — using the Mifflin-St Jeor equation. The units toggle switches between metric and imperial; the result updates as you type and nothing leaves your browser. Read the number as a floor, the energy you would spend doing absolutely nothing, not as a target for what to eat.

What basal metabolic rate actually is

Basal metabolic rate is the energy a body spends simply staying alive: keeping the heart beating, the lungs moving, the kidneys filtering, the brain firing and body temperature steady. For most sedentary people it is the largest single slice of daily energy use, commonly around 60 to 70 percent of the total. A handful of organs dominate that spending: measured per kilogram, the heart and kidneys turn over about 440 kcal a day, the brain about 240 and the liver about 200, so those four organs account for well over half of resting energy use despite making up only a few percent of body weight. That imbalance is part of why two people of the same height and weight can still differ. The word "basal" describes a strict physical condition, not a rough idea of rest. A true reading requires the subject to be awake but fully relaxed, lying down in a comfortably warm room, having fasted for at least twelve hours and avoided exertion beforehand. Measured while asleep the figure drops slightly; measured after a meal it rises, because digesting food itself costs energy. Because those laboratory conditions are hard to reproduce, most everyday figures — including this one — are really resting metabolic rate, which reviews place on the order of 10 percent above the true basal value, and which here is estimated from an equation rather than measured breath by breath.

From Lavoisier's guinea pig to the metabolic ward

The idea that a resting body burns fuel like a slow fire is a little over two centuries old. In the winter of 1782 to 1783 Antoine Lavoisier and Pierre-Simon Laplace built an ice calorimeter in Paris and placed a live guinea pig inside it. By measuring the heat that melted the surrounding ice against the carbon dioxide the animal exhaled, they found the two tracked together and concluded, in Lavoisier's phrase, that respiration is a form of combustion. That experiment established the principle every metabolic calculation still rests on: the oxygen an animal consumes and the heat it gives off are two views of the same chemical burning.

Through the nineteenth century the principle became measurable. The German physiologist Max Rubner set out his surface hypothesis in 1883, observing that heat production scaled roughly with body surface area rather than with sheer mass, and in 1894 established that the conservation of energy holds inside a living animal. In 1895 another German physician, Adolf Magnus-Levy, published the measurements that tied the resting figure — Grundumsatz in German, basal metabolism in English — to the thyroid gland, showing that an overactive thyroid pushed it up and an underactive one dragged it down. In the United States, Wilbur Atwater worked with the physicist Edward Rosa and the chemist Francis Benedict to build a room-sized respiration calorimeter at Wesleyan University, beginning human experiments in it in 1896; it was large enough for a person to live inside for days while every exhalation was accounted for. Atwater's wider program there also fixed the energy value of food itself — the round figures of roughly 4 kilocalories per gram of carbohydrate or protein and 9 for fat that still sit behind every nutrition label.

That instrument set the stage for the number this tool estimates. Benedict went on to direct the Carnegie Institution's Nutrition Laboratory in Boston from 1907 until his retirement in 1937, and with the biometrician J. Arthur Harris published A Biometric Study of Basal Metabolism in Man there in 1919, deriving from indirect calorimetry on 239 adults the first prediction equation to enter wide use. For decades afterward a basal metabolism test was routine clinical medicine — the standard way to check thyroid function, since an overactive thyroid raised the resting rate and an underactive one lowered it. Patients arrived fasting, lay still, and breathed into a spirometer. Blood tests for thyroid hormones replaced the procedure from the 1950s onward, but the prediction equations it produced outlived it.

The equation behind the number

BMR = 10w + 6.25h − 5a + s

Here w is weight in kilograms, h is height in centimeters, a is age in years, and s is a sex constant: +5 for men, −161 for women. The coefficients are not physiological constants but the best-fit slopes from a regression: more mass and more height mean more living tissue to run, and both push the number up, while each additional year of age trims it slightly.

Run the defaults through it: a 34-year-old man, 175 cm, 72.5 kg. Weight contributes 10 × 72.5 = 725. Height contributes 6.25 × 175 = 1,093.75. Age removes 5 × 34 = 170. So 725 + 1,093.75 − 170 + 5 = 1,653.75, which displays as 1,654 kcal/day. Flip the sex to female and only the constant changes, giving 1,487.75, shown as 1,488. In imperial mode the calculator converts first — 1 in is exactly 2.54 cm and 1 lb is exactly 0.45359237 kg — so 5 ft 9 in and 160 lb become 175.26 cm and 72.57 kg before the formula runs.

Why this formula and not Harris-Benedict

The equation comes from a 1990 study by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition, built on indirect calorimetry measurements of 498 healthy adults — 251 men and 247 women — aged 19 to 78, roughly half of normal weight and half obese. That method reads energy use from the oxygen a resting subject consumes and the carbon dioxide breathed out rather than from heat directly, the practical successor to Benedict's sealed chamber. The older Harris-Benedict equation dates to that 1919 Carnegie monograph and reflects the leaner, more active population of its era; against the calorimetry in the Mifflin paper it overestimated resting energy expenditure by 5 percent. A 1984 reappraisal by Roza and Shizgal added subjects across a wider age range but arrived at regressions close to the originals, so the bias stayed. In 2005 the American Dietetic Association, renamed the Academy of Nutrition and Dietetics in 2012, systematically reviewed the four equations then most used in practice; Mifflin-St Jeor predicted measured resting metabolic rate within 10 percent for more people than any other and had the narrowest error range, which is why it is the standard choice in dietetics and in this calculator.

From BMR to a daily calorie target

BMR is a floor, not a food budget. To estimate total daily burn, multiply by an activity factor:

At the moderate level, the default example gives 1,654 × 1.55 ≈ 2,563 kcal of maintenance intake. Eating below that loses weight; above it gains. The multiplier folds in three things the resting figure leaves out: the roughly 10 percent of intake spent digesting food, deliberate exercise, and the fidgeting and posture changes physiologists call non-exercise activity thermogenesis.

What four numbers cannot capture

The formula sees height, weight, age and sex — nothing about body composition. Skeletal muscle burns around 13 kcal per kg per day at rest against about 4.5 for adipose tissue — figures set out by Marinos Elia in 1992 and since confirmed by MRI-based studies in both sexes — so a muscular person is underestimated and someone with high body fat overestimated, sometimes by 10 percent or more. The 1990 sample was adults aged 19 and over, so the equation is not suitable for children or during pregnancy. A note on labels by market: US nutrition panels report kilocalories under the heading Calories, EU and UK panels must carry both kilojoules and kilocalories, and Australian panels must carry kilojoules with calories optional. Multiply kcal by 4.184 — the default result of 1,654 kcal/day is about 6,920 kJ.

This estimate is a population average, not a measurement of your metabolism — talk to a doctor or registered dietitian before making major dietary changes. See the site disclaimer.

Frequently asked questions

What is a normal BMR for a man or a woman?

Most adult men land between 1,500 and 2,000 kcal per day and most adult women between 1,200 and 1,600, though body size drives the number far more than sex does. The defaults here — a 34-year-old man at 175 cm and 72.5 kg — come out at 1,654 kcal per day. A woman with identical stats would be at 1,488, because the equation subtracts a constant 166 kcal.

Is my BMR the number of calories I should eat to lose weight?

No. BMR is what you would burn lying motionless all day, and even sedentary people burn roughly 20 percent more than that just by living. Estimate maintenance by multiplying BMR by an activity factor between 1.2 and 1.9, then eat below that figure — a deficit of about 500 kcal per day corresponds to roughly 0.45 kg of fat loss per week.

Why is Mifflin-St Jeor better than Harris-Benedict?

Harris-Benedict was published in 1919 and overestimates modern resting metabolism by around 5 percent on average. Mifflin-St Jeor, published in 1990 from calorimetry measurements of 498 adults, predicted measured resting energy expenditure within 10 percent for more people than any competing formula in the Academy of Nutrition and Dietetics review. It has been the recommended default for healthy adults since.

Does BMR really drop as you get older?

In the equation, each year of age subtracts 5 kcal per day, so a decade costs 50 kcal at the same height and weight. Large doubly-labeled-water studies published in 2021 found metabolism is surprisingly stable from roughly age 20 to 60 once body composition is accounted for. Much of the real-world decline comes from losing muscle, not from age itself.

Can I raise my BMR?

Meaningfully, only by adding muscle. A kilogram of skeletal muscle burns roughly 13 kcal per day at rest compared with about 4.5 kcal for a kilogram of fat, so gaining 5 kg of muscle adds on the order of 65 kcal per day. Sustained crash dieting works the other way, pushing resting metabolism down through adaptive thermogenesis.