How to Calculate BMR: Step by Step With Worked Examples

5 November 2026 · 15 min read · AI Guides

Introduction

Basal metabolic rate, usually shortened to BMR, is the number of calories your body burns in a day if you do nothing at all. Not a lazy day, not a rest day, but a day spent lying still in a comfortable room, awake, having eaten nothing for hours. It is the energy cost of keeping you alive: breathing, circulating blood, keeping your brain running, maintaining body temperature, and renewing cells.

Knowing your BMR matters because almost every calorie target starts from it. A weight loss plan, a muscle building plan, and a maintenance plan all begin with the same question: how much energy does this body need before any activity is added? Answer that, add activity honestly, and the rest of the maths is straightforward. Skip it, and you are guessing from a generic 2,000 calorie figure that was never meant to describe you.

This guide shows you how to calculate BMR yourself, with a calculator-free method you can do on paper, three worked examples you can check, the activity multipliers that turn BMR into daily needs, and the common mistakes that quietly break the result. The free BMR Calculator on this site does the same arithmetic instantly, but working through it once by hand means you will trust the number, and know when it does not apply.

What BMR actually measures, and what it does not

BMR is a laboratory concept with a strict definition. It is measured after a full night of sleep, in the morning, in a thermoneutral room, with the person lying down, awake, and completely at rest, having fasted for about 12 hours. Those conditions exist to remove every variable except the body's baseline cost. A measurement taken after walking to the clinic, after coffee, or after a meal is not BMR, it is resting metabolic rate, or RMR, which is close but not identical, usually a little higher because it is measured under less strict conditions.

In practice, nobody lives a BMR day. The moment you sit up, walk to the kitchen, digest food, or think hard about a problem, you burn more. That is why BMR is a starting point, not a daily target. Eating exactly your BMR every day while living a normal active life would put you in a deficit, because activity, digestion, and even fidgeting all sit on top of it. The distinction matters most for people who find a BMR figure online and treat it as their eating target: it is not. It is the floor under the target, and the target is built by adding activity on top.

Three related terms get confused with BMR, and keeping them apart prevents most misunderstandings. Resting metabolic rate, RMR, is the same idea measured under looser conditions, and the two figures are often within 10 percent of each other. Total daily energy expenditure, TDEE, is BMR plus everything else: activity, exercise, the thermic effect of food, and non-exercise movement like walking, standing, and fidgeting. Maintenance calories are the TDEE figure in practical terms: eat that amount, keep your weight stable, and your activity unchanged. BMR is the foundation, TDEE is the house built on it.

The Mifflin-St Jeor equation, the standard way to calculate BMR

The equation used by almost every serious calculator today is the Mifflin-St Jeor equation, published in 1990 by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition, based on resting energy expenditure measured by indirect calorimetry in 498 healthy adults. It replaced the older Harris-Benedict equation as the default because validation studies found it predicted measured resting energy expenditure within about 10 percent for most healthy, non-obese adults, more reliably than its predecessor. The Mifflin-St Jeor equation explained gives the full background, and the Mifflin-St Jeor equation on Wikipedia summarises its history and variants.

For men, BMR equals 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age in years, plus 5. For women, the same formula applies with minus 161 instead of plus 5. The result is in kilocalories per day, the unit food labels call calories. Weight contributes the most, which makes sense: a larger body has more tissue to maintain. Height adds a smaller amount, age subtracts a little each year, reflecting the slow decline in metabolic rate with age, and the final constant adjusts for average differences in body composition between men and women at the same weight and height.

The units are the first trap. The equation needs kilograms and centimetres. Pounds must be divided by 2.205 to become kilograms, and inches must be multiplied by 2.54 to become centimetres. A person who enters pounds as if they were kilograms will get a wildly inflated figure, and a person who enters feet and inches as a single number will get nonsense. Convert first, calculate second, and double check the converted figures against what you know: a 75 kilogram person weighs about 165 pounds, and a 175 centimetre person is about 5 feet 9 inches.

Step by step: calculate your BMR by hand

Three worked examples you can check

Example one: a 28 year old woman weighing 62 kilograms at 168 centimetres. Weight term: 620. Height term: 168 times 6.25 equals 1,050. Age term: 28 times 5 equals 140. Combine: 620 plus 1,050 minus 140 minus 161 equals 1,369 calories per day. That is her BMR, before any activity.

Example two: a 52 year old man weighing 88 kilograms at 180 centimetres. Weight term: 880. Height term: 180 times 6.25 equals 1,125. Age term: 52 times 5 equals 260. Combine: 880 plus 1,125 minus 260 plus 5 equals 1,750 calories per day. Notice that despite being older, his larger weight and height give him a higher BMR than the younger, smaller woman in example one. Weight dominates the equation, which is why two people of the same age can have very different figures.

Example three: a 40 year old woman weighing 70 kilograms at 160 centimetres. Weight term: 700. Height term: 160 times 6.25 equals 1,000. Age term: 40 times 5 equals 200. Combine: 700 plus 1,000 minus 200 minus 161 equals 1,339 calories per day. If any of these examples surprises you, recheck the units first: kilograms and centimetres, not pounds and inches, and the female constant of minus 161, not plus 5.

From BMR to daily calories: activity multipliers

BMR alone tells you what a motionless day costs. Daily needs, or TDEE, multiply BMR by an activity factor. The standard factors are 1.2 for sedentary, meaning a desk job with little or no exercise; 1.375 for lightly active, meaning light exercise one to three days a week; 1.55 for moderately active, meaning moderate exercise three to five days a week; 1.725 for very active, meaning hard exercise six to seven days a week; and 1.9 for extra active, meaning a physical job or twice-daily training. Our 35 year old man with a BMR of 1,674 needs about 2,009 calories sedentary, 2,302 lightly active, 2,595 moderately active, 2,888 very active, and 3,181 extra active.

Choosing the factor honestly is where most estimates go wrong, and the error is almost always upward. People count a gym session three times a week and call themselves very active, forgetting that the other 21 hours of those days are spent sitting. A desk worker who trains for an hour three times a week is moderately active at most. A nurse, builder, or warehouse worker on their feet all day is genuinely active even without a gym. If you are unsure, pick the lower factor for two weeks, watch your weight trend, and adjust: the scale over a fortnight is a better activity meter than any questionnaire.

The thermic effect of food, the energy spent digesting what you eat, is already folded into these multipliers in practice, at roughly 10 percent of intake, and so is non-exercise activity thermogenesis, or NEAT, the calories burned by fidgeting, standing, walking to the bus, and all the small movements that are not exercise. That is why the multipliers look generous: they are not just exercise, they are the whole of a lived day. A step counter helps here: someone averaging 3,000 steps a day is sedentary whatever their gym habit, and someone averaging 12,000 steps is active even without structured workouts.

What BMR cannot tell you

The equation uses only four inputs, so it cannot see body composition. Two people weighing 85 kilograms can have very different BMRs if one is mostly muscle and the other mostly fat, because muscle tissue burns more energy at rest than fat tissue. The equation gives the average for their weight, height, age, and sex, and muscular people will tend to sit above it while people with very low muscle mass sit below it. Equations that use lean body mass, such as Katch-McArdle and Cunningham, exist for exactly this reason, but they need a body fat percentage, which most people do not know accurately, so for most users Mifflin-St Jeor remains the better trade-off.

The equation also struggles at the extremes. It was validated mainly on healthy adults, and its accuracy drops for people with obesity, for very elderly people, for children and teenagers, for pregnant people, and for people with thyroid disorders, fever, or other conditions that change metabolic rate. Children and teenagers should not use adult BMR equations at all: their energy needs include growth, and paediatric equations exist for them. Anyone with a medical condition that affects metabolism, anyone pregnant, and anyone under 18 should get a personal figure from a clinician or dietitian rather than a calculator.

Finally, BMR is an estimate with a margin of error, typically around 10 percent, which is 130 to 170 calories a day for most adults. That sounds small until you multiply it by months: a 150 calorie daily error is over a kilogram of body weight a month in energy terms. The correct response is not to chase decimal places, it is to treat the calculated figure as a starting hypothesis, eat to it for two to three weeks, watch the trend on the scale, and adjust by 100 to 200 calories based on what actually happens. The equation starts the conversation; your body's response finishes it.

Mistakes that break the calculation

The commonest mistake is units: pounds entered as kilograms, or feet entered as centimetres. The second is using a stale weight, sometimes years old, sometimes a goal weight rather than a current weight. The equation describes the body you have, not the body you want, and entering a goal weight gives you the maintenance calories of a smaller person, which is a diet plan disguised as a calculation. The third is the wrong sex constant, plus 5 for men and minus 161 for women, a 166 calorie swing if swapped. The fourth is treating BMR as the eating target instead of the floor, which creates an unnecessarily large deficit. The fifth is double counting exercise: adding gym calories on top of an activity multiplier that already includes them.

A subtler mistake is recalculating too often. BMR changes slowly, with weight, age, and body composition, not day to day. Weighing yourself daily and recalculating BMR daily adds noise, not information. Recalculate when your weight has changed by about 5 kilograms, when you cross a birthday that changes your age term by a meaningful amount over years, or when your activity pattern changes for a sustained period, such as starting or leaving a physical job. Otherwise, leave the figure alone and let the weight trend guide small adjustments.

BMR, weight loss, and weight gain in practice

For weight loss, the standard approach is to eat below TDEE, not below BMR. A deficit of about 500 calories a day from TDEE typically produces around half a kilogram of loss a week, which is sustainable for most people. Eating below BMR is sometimes done under supervision for short periods, but as a self-directed plan it risks hunger, fatigue, muscle loss, and a rebound, because the body adapts to very low intake by reducing spontaneous movement and, over time, metabolic rate itself. A floor of roughly 1,200 calories a day for women and 1,500 for men is often quoted as a general minimum for self-directed diets, and anything below that deserves professional guidance.

For weight gain, the mirror image applies: eat above TDEE by a modest surplus, around 300 to 500 calories a day, with enough protein and resistance training to direct the gain toward muscle rather than fat. Simply eating far above TDEE produces mostly fat gain, and the extra food does not become muscle without the training stimulus. For maintenance, eat at TDEE and watch the trend: a stable weight over a month confirms the figure better than any calculator could.

For athletes and very active people, BMR is still the base, but the activity factor matters more than the equation choice. A marathon runner's TDEE can be double their BMR, and small errors in the multiplier dwarf small errors in the equation. Athletes should also remember that BMR equations assume a normal body composition range: a heavily muscled athlete will sit above the estimate, and a body fat measurement plus a lean-mass equation may serve them better, ideally interpreted by a sports dietitian.

Frequently asked questions

The Bottom Line

Calculate BMR with Mifflin-St Jeor, in kilograms and centimetres, with the right constant for your sex, then multiply by an honest activity factor to get your daily needs. Treat the result as a starting hypothesis, check it against two weeks of real weight trend, and adjust in small steps. That loop, equation plus evidence, is more reliable than any single calculator result on its own.