What Is BMR? Basal Metabolic Rate Explained

Your body spends energy even when you do nothing at all. Cells repair themselves, the heart beats, you breathe, and your temperature is held steady. BMR puts a number on that background cost, in kilocalories per day, the same unit used for food. It is the floor of your daily energy budget: everything you add by walking, working or training sits on top of it.

Strictly speaking, a true basal rate is measured in a lab, after resting, with no recent food and a comfortable room. The figure a calculator gives you is a prediction from an equation, which the original paper calls resting energy expenditure. In everyday use the two names are treated as the same thing, and this glossary does the same.

How BMR is calculated

CalorTracker uses the Mifflin-St Jeor equation, published by Mifflin and colleagues as a predictive equation for healthy adults. It needs three body measurements and one adjustment:

• 10 × weight in kilograms
• plus 6.25 × height in centimetres
• minus 5 × age in years
• then plus 5 for men or minus 161 for women, rounded to a whole number

The calculator converts pounds and inches first, so the same equation serves every unit system. The full method is written out on how we calculate, and the paper is listed with our other sources.

Worked example

Take a man aged 30, weighing 80 kg and standing 180 cm tall. The same function that powers the BMR calculator gives:

That is about 74 kcal an hour, spent without moving. A woman with exactly the same weight, height and age comes out at 1,614 kcal, because the equation's last adjustment differs. The same man ten years later, at 40, comes out at 1,730 kcal, because age enters the equation with a minus sign.

What BMR is used for

On its own, BMR is rarely a calorie target, since it leaves out every minute you are awake and moving. Its job is to be the starting point. Multiply it by an activity factor and you get TDEE, the estimate of what you use in a whole day. Subtract or add a fixed amount to TDEE and you get a goal, as described under calorie deficit and calorie surplus.

What BMR does not tell you

An equation works from averages, so two people with identical inputs can differ in practice. Body composition is one reason: the equation sees weight, not how much of it is lean tissue, which is why some calculators ask for body fat as a separate input. Treat the result as a reasonable starting estimate, watch what your weight actually does over a few weeks, and adjust from there.

BMR is also not a medical measurement. If you have a medical condition, or you are pregnant, a clinician can give you figures that fit your situation better than any online calculator.