How the BMR equations differ
All three are regression equations: researchers measured the oxygen use of groups of people at rest and fitted a line to weight, height, age and sex. Harris-Benedict dates from 1919 and was refitted in 1984; Mifflin-St Jeor (1990) was fitted on a more modern group of 498 adults and tends to be closer for today’s population; Katch-McArdle ignores age and sex and uses lean mass, which is what actually burns most energy.
Seeing the three side by side is useful: if they’re within 50–100 kcal of each other, you can be fairly confident in the ballpark. A large gap usually means your body composition is unusual for your height and weight, so the lean-mass equation may fit you better.
Worked example
A 40-year-old man, 180 cm, 85 kg: Mifflin-St Jeor = 850 + 1,125 − 200 + 5 = 1,780 kcal. Revised Harris-Benedict = 88.4 + 1,138.7 + 863.8 − 227.1 = 1,864 kcal.
Why your BMR changes
Heavier bodies and more muscle burn more; BMR falls with age, partly through muscle loss. Thyroid conditions, some medicines, pregnancy and illness change it too, which is why a clinician may measure resting energy use directly with indirect calorimetry when accuracy matters.
Questions people ask
What is BMR?
Basal metabolic rate is the energy your body uses at complete rest, lying still, awake, after fasting, in a comfortable temperature. It covers breathing, circulation, cell repair and brain function, and is usually 60–70% of all the calories you burn in a day.
What is a normal BMR?
Many adult women have a BMR of roughly 1,200–1,500 kcal a day and many adult men 1,500–1,900 kcal, but it depends heavily on body size, muscle mass and age.
Is BMR the same as RMR?
Nearly. Resting metabolic rate is measured under slightly less strict conditions and is typically a little higher. The Mifflin-St Jeor equation technically predicts resting energy expenditure, and the terms are used interchangeably in practice.
Can I increase my BMR?
Building muscle through resistance training raises it modestly, since muscle uses more energy at rest than fat. BMR falls slowly with age and when you lose weight.
Should I eat my BMR to lose weight?
Not usually. BMR is what you burn lying still all day; eating at or below it leaves little room for nutrients and activity. Weight-loss targets are normally set below your TDEE, not your BMR.
Sources
- Mifflin MD, St Jeor ST et al. (1990). Am J Clin Nutr 51:241–247.
- Roza AM, Shizgal HM (1984). Am J Clin Nutr 40:168–182; 95% confidence ±213 kcal (men), ±201 kcal (women).
- Frankenfield D et al. (2005). J Am Diet Assoc 105:775–789: Mifflin-St Jeor most reliable of common equations.
- McArdle, Katch & Katch. Exercise Physiology: Katch-McArdle equation.
Every formula on VitalMetrics is listed with its original reference on Sources & methods.