BMR Calculator

What this calculator does

This tool estimates your Basal Metabolic Rate (BMR) — the number of calories your body uses in a day while completely at rest. BMR covers the energy spent on involuntary functions like breathing, pumping blood, maintaining body temperature and repairing cells. It does not include any movement or exercise. You enter your sex, age, height and weight in metric or imperial units, and the calculator immediately returns a BMR figure in kilocalories per day, rounded to a whole number. The result recalculates live as you change any input, and you can copy it with one click.

Why you might need it

BMR is the foundation of almost every calorie estimate. If you have ever wondered roughly how much energy your body needs before you account for walking to work or going to the gym, BMR is that baseline. It is useful as a reference point when reading nutrition labels, when setting up a food-tracking app that asks for a starting figure, or simply when you want to understand the numbers behind daily energy expenditure. Knowing your BMR also makes a TDEE or calorie calculation meaningful, because those tools start from BMR and scale it. People comparing how factors like age affect resting metabolism also find it instructive to change one input and watch the figure move.

How to use it

1. Pick Metric or Imperial with the units toggle.
2. Select your sex — the equation has a different constant for men and women.
3. Enter your age in years.
4. Enter your height and weight. In metric that is centimetres and kilograms; in imperial it is feet plus inches, and pounds.
5. Read the BMR figure in kilocalories per day. Use Copy BMR to copy it, or Reset to restore the default values.

How it’s calculated

This calculator uses the Mifflin-St Jeor equation, published by M.D. Mifflin and S.T. St Jeor in 1990. It is the same formula in two variants:

• Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
• Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

When you work in imperial units the tool first converts your input to metric — height to centimetres and weight to kilograms — then applies the equation, so the result is identical whichever system you choose. An older alternative is the Harris-Benedict equation, originally from 1919 and revised in 1984, which uses the same four inputs but different coefficients and tends to estimate slightly higher. Mifflin-St Jeor is generally preferred today because validation studies have found it closer to measured resting metabolic rates for the contemporary population, which is why it is the basis used here.

Common pitfalls

A prediction equation is an average, not a personalised measurement. Two people with the same sex, age, height and weight can have genuinely different metabolic rates because BMR depends heavily on body composition — muscle burns more energy at rest than fat — and on genetics and hormones, none of which appear in the formula. So the figure is best read as a ballpark. Another common mistake is mixing units: entering a weight in pounds while the toggle still says metric produces a nonsensical result, so confirm the toggle first. Finally, do not confuse BMR with the calories you actually need in a day — that is TDEE, which is always higher because it adds the energy cost of movement.

Tips and related calculations

To turn a BMR into a full daily figure, use a calorie or TDEE calculator: it multiplies BMR by an activity factor that ranges from about 1.2 for a sedentary day to roughly 1.9 for very heavy physical work. If you are also interested in how your weight compares with your height, a BMI calculator answers that separate question. When you re-run this calculator over time, keep your weight measurements consistent — same scale, similar time of day — so that changes you see reflect real trends rather than normal daily fluctuation. Because the maths runs entirely in your browser, you can experiment with the inputs freely and nothing you enter is ever transmitted.