What Your BMR Actually Measures

Quick Answer

What basal metabolic rate measures is the energy your body spends staying alive while completely at rest: circulation, breathing, holding temperature, and repairing tissue. This basal metabolic rate calculator estimates it with the Mifflin St Jeor equation (usually written Mifflin-St Jeor) from your sex, age, height, and weight, then shows what each activity multiplier adds on top. The key thing to know is that BMR is not what you should eat. It is the resting floor, and total daily energy expenditure, the number that answers how much to eat, is BMR multiplied by an activity factor between 1.2 and 1.9. Finding the resting figure an app used before it applied its own multiplier is a good use of this metabolism calculator, which is also called a resting metabolic rate calculator.

How BMR Works

Even lying still in a dark room, your body is expensive to run. The brain, liver, heart, and kidneys account for most of it despite weighing very little, which is why resting energy expenditure scales with overall body size rather than with how much you exercise. For most adults, resting metabolic rate is 60 to 70 percent of the whole day's burn. That proportion is the honest reason "eat less, move more" is mostly a conversation about the smaller slice: your activity is real, but it sits on top of a much larger fixed cost you cannot negotiate with.

The Mifflin-St Jeor Equation

The formula is 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age in years, then plus 5 for men or minus 161 for women. Weight is the dominant term, height comes second, and age subtracts slowly over a lifetime. The sex term is a flat offset of 166 calories, which is why two people of identical size get different figures. Published in 1990, the Mifflin-St Jeor equation replaced the 1919 Harris-Benedict equation because Harris-Benedict tends to overestimate resting energy for modern body compositions by around 5 percent.

That is how the equation is built, and it explains what a normal BMR looks like at a given size and age: there is no single normal figure, because the dominant term is your body mass. A 30 year old man of 175 cm and 70 kg lands near 1,650 calories a day; a woman of the same age at 165 cm and 60 kg lands near 1,320. A figure that looks low next to someone else's usually means you are smaller or older, not that your metabolism is broken.

BMR Versus TDEE

The two get confused constantly, and the confusion is expensive. BMR is rest; total daily energy expenditure is rest plus everything else, including the energy spent digesting food. A sedentary day multiplies BMR by 1.2, a moderately active one by 1.55, and hard daily training by 1.9. Eating at your BMR while living a moderately active life is a deficit of several hundred calories a day, which is a far more aggressive cut than most people mean to make. The calculator shows all five multipliers at once so the size of that choice is visible before you commit to one.

Examples

Man, 30, 70 kg, 175 cm. For example, BMR is about 1,649 calories a day, roughly 69 calories every hour spent doing nothing at all.

Woman, 30, 60 kg, 165 cm. For example, BMR is about 1,320 calories. The gap to the case above is mostly size, with 166 calories of it coming from the sex term.

Same man, moderately active. For example, multiplying 1,649 by 1.55 gives roughly 2,556 calories, so activity is adding over 900 calories to the resting figure.

Common Mistakes