BMR Calculator

Background.

This BMR calculator estimates your basal metabolic rate — the number of calories your body burns over a 24-hour period at complete physical, digestive, and thermal rest — using the Mifflin-St Jeor equation, the predictive formula that the Academy of Nutrition and Dietetics recommends as the most accurate non-calorimetric estimator for healthy non-obese and obese adults. Enter your sex, age, weight in kilograms, and height in centimetres, and the calculator returns a single number in calories per day. There is no submit button; the result updates as you type, nothing is sent off your device, and no account is required. The output is the foundation that every diet plan, every TDEE estimate, and every clinical nutrition prescription starts from, because everything else — activity, the thermic effect of food, exercise — is layered on top of the calories your body would burn if you spent the whole day lying motionless in a thermoneutral room.

Basal metabolic rate is a specific, technically defined quantity, and it is worth being careful about the words. The classical definition, inherited from the indirect-calorimetry work of Magnus-Levy in the 1890s and codified by Harris and Benedict in their 1919 Carnegie Institution monograph A Biometric Study of Basal Metabolism in Man, is the energy expenditure of an adult who is awake, lying supine, post-absorptive (no food for 12 or more hours), at complete physical rest, in a thermoneutral environment, and free of emotional stress or recent exercise. Measuring that condition strictly requires an overnight stay in a metabolic ward — which is why nobody actually measures BMR in routine practice. What clinicians, dietitians, and gyms measure instead is resting metabolic rate (RMR), a slightly more permissive condition that allows the subject to have arrived at the lab in the morning rather than spending the night there. RMR typically runs about 3–10% higher than true BMR because the subject is not fully post-absorptive and may carry residual sympathetic tone from the commute. For practical purposes the two terms are used almost interchangeably in the nutrition literature, and the Mifflin-St Jeor equation that this calculator implements was actually derived from indirect-calorimetry measurements of RMR, not strict BMR; we keep the label 'BMR' because that is the search idiom and the term the calculator's outputs are most often compared against.

The Mifflin-St Jeor equation was published in 1990 by Mark Mifflin, Sachiko St Jeor, and colleagues in the American Journal of Clinical Nutrition, based on indirect-calorimetry measurements of 498 healthy adults (251 women, 247 men) aged 19–78 across a wide BMI range from 17 to 42 kg/m². For men, BMR = 10·weight(kg) + 6.25·height(cm) − 5·age(years) + 5. For women, BMR = 10·weight(kg) + 6.25·height(cm) − 5·age(years) − 161. The −161 versus +5 sex constant reflects the average lean-mass difference between adult men and women at the same height and weight, and the −5 per year of age captures the gradual decline in fat-free mass and metabolically active tissue that accompanies normal ageing. The original Harris-Benedict equation, derived from 239 subjects in 1919 and revised by Roza and Shizgal in 1984, gives slightly higher numbers, particularly in obese and elderly subjects; the 2005 Frankenfield meta-analysis, which pooled validation studies that compared every common predictive equation against directly measured RMR by indirect calorimetry, concluded that Mifflin-St Jeor predicts within 10% of measured RMR in roughly 82% of non-obese and 70% of obese subjects, more accurately than Harris-Benedict, Owen, or the WHO/FAO/UNU equations. That is why every major nutrition body that issues practice guidelines — the Academy of Nutrition and Dietetics, ESPEN, ASPEN — now recommends Mifflin-St Jeor as the default when indirect calorimetry is unavailable, and it is the equation this calculator uses.

BMR is not a constant of your biology. It changes, and it changes predictably. The single largest predictor of BMR is fat-free mass: muscle, organs (particularly liver, brain, heart, and kidneys), and bone are far more metabolically active than adipose tissue, so two people with identical body weight but different body compositions will have meaningfully different BMRs. A 70-kg adult with 20% body fat carries about 56 kg of lean mass; a 70-kg adult with 35% body fat carries only about 45.5 kg. The high-lean-mass person will burn roughly 100–200 more calories per day at rest, even though the Mifflin-St Jeor equation cannot see that difference because it only takes total weight as an input. That is the equation's single biggest limitation: it averages across body compositions and so systematically underpredicts BMR in lean, muscular subjects and overpredicts it in subjects with very high body fat. If you have a recent body-composition measurement (DEXA, BodPod, multi-frequency BIA) the Katch-McArdle equation — BMR = 370 + 21.6·lean mass(kg) — uses lean mass directly and is more accurate for athletes; this calculator does not implement Katch-McArdle because most users do not have a measured lean-mass number to enter.

BMR also drops with age, by roughly 1–2% per decade after age 20, driven primarily by sarcopenia — the gradual loss of muscle tissue — rather than by any change in the metabolic rate of the remaining tissue per kilogram. This is the mathematical core of the well-documented 'slowing metabolism' people report from their thirties onward: not that each kilogram of you burns less, but that you carry fewer kilograms of metabolically expensive tissue. Strength training that preserves or builds muscle is the single most evidence-backed intervention against this drift.

BMR responds to acute energy state as well: prolonged caloric restriction triggers adaptive thermogenesis, a measurable downward adjustment of BMR beyond what fat-free-mass loss alone would predict, typically in the 5–15% range and slow to reverse. This is why aggressive crash diets backfire metabolically, and why nutrition guidelines for sustained weight loss recommend modest deficits (300–500 kcal/day) over rapid ones. Thyroid status, fever (each 1°C raises BMR by about 13%), pregnancy, lactation, certain medications (caffeine, nicotine, sympathomimetics, beta-blockers in the opposite direction), and ambient temperature all shift BMR as well, none of which the Mifflin-St Jeor equation accounts for. Treat the number this calculator gives you as a well-calibrated population estimate, accurate to within roughly 200 calories per day for most adults, and use it as the starting point for the more interesting calculations — total daily energy expenditure (TDEE), calorie targets for cutting or bulking, macronutrient splits — that depend on knowing how many calories your body would burn doing nothing at all.

What is bmr calculator?

Basal metabolic rate (BMR) is the rate at which your body expends energy at complete rest — awake, lying supine, post-absorptive, in a thermoneutral environment, with no recent exercise or emotional stress. It represents the calories required to keep your basic cellular and organ functions running: heart beating, lungs breathing, brain firing, kidneys filtering, ion gradients maintained, and core temperature held at roughly 37°C. For most sedentary adults BMR accounts for 60–75% of total daily energy expenditure, with the remainder coming from the thermic effect of food (TEF, about 10%) and physical activity (15–30%). It is not the same as resting metabolic rate (RMR), although the two are routinely conflated. True BMR is measured after an overnight stay in a metabolic ward; RMR is measured after the subject arrives at the lab in the morning, having had no food for 8–12 hours but having moved around enough to elevate sympathetic tone slightly. RMR typically reads 3–10% higher than BMR for the same individual. The Mifflin-St Jeor equation this calculator uses was actually derived from RMR measurements, but the result is conventionally reported as 'BMR' because that is the term most users search for and because the gap between the two is smaller than the predictive equation's own error margin (roughly ±10% in 82% of healthy adults). BMR is determined primarily by fat-free mass — muscle, organs, and bone — which is why men typically have higher BMR than women at the same age, height, and weight, and why BMR declines about 1–2% per decade after age 20 as muscle mass gradually drops. It is the foundation calorie number every diet plan starts from: a TDEE estimate is BMR multiplied by an activity factor; a weight-loss target is TDEE minus a deficit; a maintenance prescription is TDEE itself.

How to use this calculator.

1. 01Select your biological sex from the dropdown. The Mifflin-St Jeor equation uses a +5 constant for male and a −161 constant for female to account for average lean-mass differences.
2. 02Enter your age in years. The equation was validated on adults aged 19–78, so it is not appropriate for children, adolescents, or extreme outliers in age.
3. 03Enter your current weight in kilograms. If you have your weight in pounds, divide by 2.2046 to convert (for example 154 lb ÷ 2.2046 ≈ 69.85 kg).
4. 04Enter your height in centimetres. If you know your height in feet and inches, multiply total inches by 2.54 (for example 5'9" = 69 in × 2.54 = 175.26 cm).
5. 05Read the BMR number in the results panel. It is reported in calories per day and represents the energy your body would burn over 24 hours at complete rest.
6. 06Use the number as the foundation for further calculations. Multiply by an activity factor (1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 very active, 1.9 athlete) to estimate total daily energy expenditure (TDEE), then add or subtract from TDEE to set a calorie target for weight gain, maintenance, or loss.

The formula.

This calculator implements the Mifflin-St Jeor equation, published in 1990 in the American Journal of Clinical Nutrition. For men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5. For women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) − 161. Each coefficient was fit by linear regression against indirect-calorimetry measurements of 498 healthy adults (251 women, 247 men) aged 19–78 across BMIs from 17 to 42 kg/m². The 10 kcal per kilogram of body weight captures the metabolic cost of maintaining that tissue mass (averaged across body composition); the 6.25 kcal per centimetre of height captures the surface-area component of basal heat loss; the −5 kcal per year of age captures the average decline in metabolically active tissue with ageing; and the +5 (male) versus −161 (female) constants encode the average lean-mass difference between sexes at the same height and weight. The result is rounded to the nearest whole calorie because the equation itself carries roughly ±10% predictive error against directly measured RMR in 82% of healthy adults (Frankenfield 2005), so reporting decimals would imply precision the equation does not have. The older Harris-Benedict equation (1919, revised 1984) is structurally similar but uses different coefficients and tends to produce numbers 5–15% higher than Mifflin-St Jeor, particularly in obese subjects; the WHO/FAO/UNU equations stratify by age bracket and are slightly less accurate in pooled validation studies. Mifflin-St Jeor was selected as the engine for this calculator because the Academy of Nutrition and Dietetics evidence-analysis review designated it the most accurate predictive equation for healthy non-obese and obese adults when indirect calorimetry is not available.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO — A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 51(2):241–247 (1990). The original derivation of the equation this calculator implements, based on indirect-calorimetry measurements of 498 healthy adults aged 19–78.
2. [2]Harris JA, Benedict FG — A Biometric Study of Basal Metabolism in Man. Carnegie Institution of Washington Publication No. 279 (1919). The foundational early-twentieth-century work that defined basal metabolic rate operationally and produced the first widely used predictive equation.
3. [3]Frankenfield D, Roth-Yousey L, Compher C — Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association 105(5):775–789 (2005). The meta-analysis that established Mifflin-St Jeor as the most accurate predictive equation for both lean and obese adults, leading to its adoption by the Academy of Nutrition and Dietetics.
4. [4]Roza AM, Shizgal HM — The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition 40(1):168–182 (1984). The revised Harris-Benedict coefficients still widely cited in clinical practice and used as the comparator in most equation-validation studies.
5. [5]FAO/WHO/UNU Expert Consultation — Human energy requirements: Report of a Joint FAO/WHO/UNU Expert Consultation. Rome, 2001. FAO Food and Nutrition Technical Report Series No. 1. The international reference framework for basal metabolic rate, physical activity levels, and total energy requirements across age, sex, and body weight strata.
6. [6]National Institutes of Health, Office of Dietary Supplements — Dietary Reference Intakes (DRIs): Estimated Energy Requirements. Based on the Institute of Medicine 2005 Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, which standardised the activity-factor multipliers (1.0 / 1.12 / 1.27 / 1.45) used to translate BMR into total daily energy expenditure.
7. [7]Fothergill E, Guo J, Howard L, Kerns JC, Knuth ND, Brychta R, Chen KY, Skarulis MC, Walter M, Walter PJ, Hall KD — Persistent metabolic adaptation 6 years after 'The Biggest Loser' competition. Obesity 24(8):1612–1619 (2016). Landmark NIH-led study documenting durable suppression of resting metabolic rate beyond what fat-free-mass change predicts after rapid weight loss.