BMR Calculator

Your Basal Metabolic Rate (BMR) is the number of calories your body burns in a day doing nothing at all - lying still, fully rested, not digesting a meal. It covers the constant background work of staying alive: breathing, circulating blood, keeping your temperature steady, and repairing cells. For most people BMR is 60-70% of the total calories they burn, which makes it the single biggest piece of your daily energy needs.

This calculator uses the Mifflin-St Jeor equation, published in 1990 and recommended by the Academy of Nutrition and Dietetics over the older Harris-Benedict formula it largely replaced. It needs only four inputs - weight, height, age, and gender:

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

The two equations are identical except for the last term, which reflects the average difference in lean mass between men and women. The worked example above this guide plugs real numbers into the men’s version step by step.

One alternative is worth knowing: the Katch-McArdle formula works from lean body mass instead of total weight, so it can be more accurate for very lean or very muscular people - but only if you have a reliable body fat measurement. Estimate yours with the Body Fat Calculator. For everyone else, Mifflin-St Jeor is the better default.

Three terms get used loosely and mean slightly different things:

• BMR - measured under strict conditions: fully rested, fasted for 12 hours, lying down in a temperature-neutral room. It is the true floor of your energy use.
• RMR (Resting Metabolic Rate) - measured under looser conditions (no overnight fast, a short rest rather than a full one), so it comes out a few percent higher than BMR. In everyday use the two numbers are close enough to be treated as the same, and this calculator’s result works for either.
• TDEE (Total Daily Energy Expenditure) - your actual daily calorie need: BMR plus the energy for digesting food and for all physical activity. TDEE is what you compare your intake against to lose, gain, or maintain weight.

The equation captures the big drivers, but several factors move resting metabolism up or down:

• Muscle mass. Lean tissue burns more energy at rest than fat, so more muscle raises your BMR. This is the one factor you can meaningfully change, through resistance training and enough protein.
• Body size. A larger body has more tissue to maintain, so taller and heavier people have a higher BMR - the main reason the equation uses your weight and height.
• Age. Resting metabolism falls gradually from your twenties onward, partly from muscle loss and partly from changes in the tissue itself - roughly 1-2% per decade even at a stable weight.
• Gender. Men usually have a higher BMR than women of the same height and weight because they carry more lean mass on average - which is why the equation adds 5 for men and subtracts 161 for women.
• Genetics. Two people with identical measurements can genuinely differ by a few hundred calories a day for inherited reasons the equation cannot see.
• Hormones. Thyroid hormones set the pace of resting metabolism; an under- or over-active thyroid, and some medications, shift BMR up or down noticeably.
• Body temperature and climate. Fighting off sustained cold or heat, a fever, or life at altitude each raise resting energy use modestly.
• Prolonged dieting. A long or aggressive calorie deficit lowers resting burn by more than the weight loss alone predicts - a response called adaptive thermogenesis - and it takes weeks of eating at maintenance to recover.

Any equation is a population average fitted to a research sample. Studies of resting metabolism find that individuals vary by roughly 5-10% around the prediction - and sometimes more - for reasons including genetics, hormones, and past dieting that no formula can account for. A widely cited 2005 review concluded that a substantial share of the variation between people simply cannot be explained by the usual inputs.

The gold-standard measurement is indirect calorimetry - analysing the oxygen you use and carbon dioxide you produce at rest, done in a clinic or sports lab. For everyday planning the equation is close enough: use its number as a starting point, then adjust based on what your weight actually does over a few weeks.

On its own, BMR tells you your resting floor. To turn it into a daily calorie target, multiply it by an activity factor - the table with your result does this for all five common activity levels. From there:

• Pick the activity level that matches your typical week, not an unusually active or lazy one.
• That gives your maintenance calories (TDEE). Eating below it loses weight; above it gains. The Calorie Calculator works out specific loss and gain targets and the weekly change to expect.
• BMR also puts the other body-composition numbers in context - see the Ideal Weight Calculator for a target-weight range.