BMR

Foundation · 12 min

Energy is spent even at rest

Even on a day you spend without moving at all, your body is at work: the heart pumps blood, the lungs breathe, the brain produces signals, cells renew themselves, body temperature is maintained. The energy cost of all this vital work is called the basal metabolic rate, BMR for short. In most people, BMR is the largest item of total daily expenditure. True BMR can only be measured under laboratory conditions; in daily life it is estimated with formulas.

In everyday speech, metabolism is described like a personality trait, fast or slow; yet BMR is the sum of measurable factors, and most of the difference between people is explained by body size and composition.

What affects BMR

BMR varies from person to person, and the main factors are these: body weight and height (a larger body means more tissue to supply with energy), age (BMR tends to decline gradually as age advances), sex (on average men carry a higher share of muscle mass, so BMR is higher), and body composition (muscle tissue spends more energy at rest than fat tissue). Genetics, hormones and certain health conditions also play into the picture; formulas cannot see these details.

Most of these factors change slowly; BMR therefore changes not day to day but gradually over years. When your weight or muscle mass changes noticeably, the calculation should be refreshed.

The Mifflin-St Jeor formula

The most widely recommended estimation formula today is Mifflin-St Jeor:

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

Example: for a 30-year-old woman weighing 65 kg and 165 cm tall: 10 × 65 = 650; 6.25 × 165 = 1031; 5 × 30 = 150. The sum: 650 + 1031 − 150 − 161 = 1370. The estimated BMR comes out at about 1370 kcal per day. Note the structure of the formula: weight and height raise the value, age lowers it, and the difference between the sexes is carried by the constant term.

This is an estimate

Mifflin-St Jeor is an equation developed from averages; it does not know your muscle share, your genetics or your hormonal situation. Research reports that it can deviate from an individual's true value by around ten percent; a calculation that lands on 1370 kcal really points to a range of roughly 1230-1510. So read the result not as an exact number but as a starting point. How this value meets daily life is the subject of the next lesson: TDEE and the activity factor.

Most calculators on the internet also use this formula, or its close relatives, behind the scenes; the value of calculating by hand once is that you can now read the inside of every number you see on a screen.