BMR Calculator

How to use the bmr calculator

Pick your sex, then enter your age, weight, and height, and the tool returns a single number: your basal metabolic rate, in kilocalories per day. The fields come pre-filled with a representative profile — a 30-year-old at 70 kg and 175 cm — so you can see the calculation work before you swap in your own figures. The sex toggle matters because the Mifflin-St Jeor equation uses a different constant for male and female bodies, and that constant alone shifts the result by well over a hundred kilocalories.

Weight and height each carry a unit toggle. Enter weight in kilograms or pounds and height in centimetres or inches — whichever your scale and tape measure happen to use — and the tool converts to metric internally before doing the math, because the equation is defined in kilograms and centimetres. There is no need to convert anything by hand; set the toggle to match your numbers and type them in as you read them off. Age is just whole years.

The output is your BMR: the energy your body burns at complete rest, over a full twenty-four hours, simply to stay alive. That covers everything you do not think about — your heart pumping, your lungs breathing, your kidneys filtering, your cells repairing and replacing themselves. It assumes you are awake but lying still, not digesting a recent meal and not moving a muscle on purpose. It is the floor of your energy needs, the amount you would burn if you did nothing at all that day.

Read it as a baseline, not a budget. BMR is not the number of calories you can eat, because almost nobody spends a day at complete rest — you stand, walk, work, and exercise, and all of that burns energy on top of the baseline. To turn BMR into a realistic daily figure you multiply it by an activity factor, which gives your total daily energy expenditure, or TDEE. For that step, take this number straight into the tdee-calculator, which applies the activity multiplier for you.

Treat the result as a population-based estimate, useful for orientation rather than precision. The Mifflin-St Jeor equation is fitted to averages across many people, so it describes a typical body of your sex, age, weight, and height — not your body specifically. Two people who match on all four inputs can still have meaningfully different real metabolisms, because muscle mass, body composition, genetics, hormones, and other factors all move the true number. This tool offers general information only and is not medical or nutritional advice.

The formula

The Mifflin-St Jeor equation takes your weight in kilograms, your height in centimetres, and your age in years, then adds or subtracts a constant set by sex. The two forms share the same first three terms and differ only in that final constant:

male:   BMR = 10 × kg + 6.25 × cm − 5 × age + 5
female: BMR = 10 × kg + 6.25 × cm − 5 × age − 161

Worked example with the defaults — a 70 kg, 175 cm, 30-year-old. Male: 10 × 70 + 6.25 × 175 − 5 × 30 + 5 = 700 + 1,093.75 − 150 + 5 = 1,648.75, which rounds to about 1,649 kcal/day. Female, same body: 700 + 1,093.75 − 150 − 161 = 1,482.75, about 1,483 kcal/day. The only thing that changed between the two is the sex constant, +5 versus −161, and that is the whole 166-kilocalorie gap between them.

Mifflin-St Jeor is the modern standard for estimating BMR, and it is generally more accurate for today’s population than the older Harris-Benedict equation it largely replaced. Even so, it remains an estimate: the equation was fitted to the average of a study population, so it predicts the typical metabolism for a given sex, age, weight, and height rather than yours exactly. Real metabolic rate varies with muscle mass, body composition, genetics, hormones, and more — factors the four inputs cannot capture — so use the number as a starting point, not a precise measurement.

Frequently asked questions