BMR Calculator

How to use this calculator

1. Select biological sex (the published equation has sex-specific constants).
2. Enter weight, height, and age in either unit system.
3. The result is an estimate of resting energy expenditure in kcal/day.

The formula

kg

Weight in kilograms

cm

Height in centimeters

age

Age in years

How it works

Basal metabolic rate is the energy your body needs for essential functions — breathing, circulation, cell repair — before any movement.

The Mifflin–St Jeor equation (1990) is the formula most often recommended by the Academy of Nutrition and Dietetics for estimating resting metabolism in healthy adults, and was shown to predict measured values within about 10% for most people.

Why Mifflin-St Jeor

Published in 1990, the Mifflin-St Jeor equation replaced the older Harris-Benedict formula in most clinical guidance because it predicts measured resting energy expenditure more closely across modern populations. It uses weight, height, age, and sex, each of which independently affects metabolic rate.

All predictive equations carry error, typically around ten percent for any given individual. They estimate the average person with your measurements, not you specifically. Indirect calorimetry is the only way to measure BMR precisely.

What raises and lowers your BMR

Lean mass is the strongest driver: muscle is metabolically active tissue, so two people of identical weight can differ substantially in BMR if their body composition differs. This is a key reason resistance training supports long-term weight management.

BMR declines gradually with age, partly through muscle loss and partly through cellular changes. Prolonged aggressive calorie restriction also lowers it through adaptive thermogenesis, which is one mechanism behind weight-loss plateaus. Thyroid function, illness, fever, pregnancy, and some medications also shift the number.

From BMR to a daily target

BMR is not a calorie target. Multiply it by an activity factor to get total daily energy expenditure, which is what you actually burn. Sedentary work with little exercise sits near 1.2; moderate exercise three to five days a week is around 1.55; heavy physical work or daily training reaches 1.7 and above.

For weight change, adjust from TDEE rather than BMR. A deficit of roughly 500 calories a day corresponds to about half a kilogram or one pound per week. Eating below BMR for extended periods is not recommended without medical supervision.

Limitations of predictive BMR formulas

Every predictive equation, including Mifflin-St Jeor, is built by fitting a curve to measurements taken from groups of people and then applying that curve to everyone else. It cannot see your actual muscle mass, organ size, genetics, or hormonal status; it only sees height, weight, age, and sex. This means two individuals with identical inputs receive an identical BMR estimate even though their true resting metabolism could differ by several hundred calories a day.

Formulas also assume a broadly typical body composition for a given weight and height. People with unusually high muscle mass, unusually low muscle mass, or significant amounts of fluid retention will see larger errors, because the equation cannot separate these components. This is why the result is best treated as a reasonable starting estimate rather than a measured fact about your body.

How BMR differs across populations

BMR per kilogram of body weight tends to be higher in children and adolescents, whose bodies are actively growing, and lower in older adults, who typically carry less muscle mass relative to total weight. This is one reason calorie needs decline gradually through adulthood even if activity levels stay constant.

Sex differences are built into the Mifflin-St Jeor formula itself, since men on average carry more lean mass at a given height and weight, giving them a modestly higher resting metabolic rate. Body size also matters: taller, heavier individuals generally have higher absolute BMR simply because they have more tissue to maintain, even though their BMR relative to body weight may be similar to a smaller person's.

Common mistakes people make with BMR

A common error is trying to eat at or below BMR in the belief that it will accelerate fat loss; in reality this ignores the energy used by daily movement and exercise, risks nutrient deficiency, and can trigger the body's adaptive slowdown in metabolism. Another mistake is recalculating BMR too frequently — daily weight fluctuations from water and food intake do not reflect real changes in metabolism, so weekly or monthly recalculation is more meaningful.

People also sometimes confuse BMR with TDEE and set calorie targets directly from the BMR figure without applying an activity multiplier, which results in a plan that is far too low for anyone who moves during the day. Ignoring the roughly ten percent margin of error and treating the number as exact is another frequent misstep.

Using BMR practically

The most useful way to apply BMR is as the base layer of a calorie calculation: multiply it by an honest activity factor to reach TDEE, then set your intake target relative to that figure rather than to BMR itself. Recalculate every few kilograms of weight change, since a smaller or larger body needs a different amount of energy to maintain itself.

If you are trying to build muscle, preserve it during weight loss, or manage a medical condition that affects metabolism, tracking BMR alongside actual weight trends over several weeks tells you whether the estimate matches your real-world response, and lets you adjust intake gradually rather than making large, disruptive changes.

Related metrics and when to get help

Resting metabolic rate (RMR) is closely related to BMR but measured under slightly less strict conditions and is often what indirect calorimetry devices in gyms and clinics actually report; the two numbers are usually close. Total daily energy expenditure (TDEE) builds on BMR by adding activity, exercise, and the thermic effect of food, and is the figure that should guide any calorie target.

If you suspect your metabolism is unusually slow or fast — for example, due to thyroid disease, a hormonal condition, a restrictive diet history, or unexplained weight change — a doctor or registered dietitian can arrange metabolic testing or blood work rather than relying on a predictive formula alone.

Why estimates differ between people

Published BMR equations are averages fitted to study populations, and an individual can sit ten percent either side of the predicted value without anything being wrong.

Lean mass is the strongest driver, which is why two people of identical weight can have noticeably different resting needs.

Thyroid function, medication, age, and recent dieting history all shift resting expenditure, sometimes substantially.

Use the estimate as a starting point, then adjust based on how weight actually responds over two to three weeks.

Adaptation during weight change

Resting expenditure falls as body mass falls, partly because there is less tissue to maintain and partly through metabolic adaptation.

This is why a deficit that worked initially stops producing change after a couple of months without recalculation.

Preserving muscle through resistance training and adequate protein limits how far resting needs decline.

Recalculate the estimate after every few kilograms of change rather than holding the original figure for the whole journey.

Using BMR to plan intake

BMR alone is never an intake target; it describes the energy your body uses at complete rest, before any movement, digestion, or exercise is counted.

Eating at or below BMR for long periods is difficult to sustain, tends to cost lean tissue, and often makes adherence collapse before the goal is reached.

Multiply the figure by an honest activity factor to reach total daily expenditure, then set a modest surplus or deficit against that number instead.

Recheck the figure whenever body weight, training volume, or daily routine changes meaningfully, since all three move the estimate.

Measurement conditions that matter

A laboratory resting measurement is taken after an overnight fast, at rest, in a neutral temperature, because food and cold both raise energy use temporarily.

Digestion accounts for roughly ten percent of daily expenditure, which is one reason an estimate taken after eating reads higher than true BMR.

Illness, poor sleep, and stress hormones all shift the figure over short periods, so a single unusual day tells you very little.

Consistency in how and when you weigh and measure yourself matters more than the precision of any single reading.

When this calculator is useful

• A baseline for calorie planning
• Understanding how age, weight, and height affect metabolism
• Input for TDEE (total daily energy expenditure) estimates