What BMR is and why the equation matters
Basal metabolic rate is the energy your body uses at complete rest to maintain vital functions. It is the largest component of most people's daily energy expenditure, so it forms the base that activity multipliers are applied to when estimating total daily calories. Because BMR cannot be measured without lab equipment, everyday tools estimate it with a regression equation — and different equations were fitted to different populations, so they disagree.
The three equations, one person
Using a single reference person — a 30-year-old man, 80 kg, 180 cm, at an assumed 15% body fat (68 kg lean mass) — here is what each equation produces. The formulas differ in whether they use total body weight or lean body mass, which is the main reason their results diverge.
| Equation | BMR estimate | Uses |
|---|---|---|
| Mifflin-St Jeor (1990) | 1,780 kcal | Weight, height, age, sex |
| Harris-Benedict (revised 1984) | 1,854 kcal | Weight, height, age, sex |
| Katch-McArdle | 1,839 kcal | Lean body mass (needs body-fat %) |
Why they disagree
Mifflin-St Jeor was published in 1990 and is widely regarded as the most accurate for the general modern population; it is the equation many dietitians default to. The revised Harris-Benedict equation dates to 1984 (updating the original 1919 study) and tends to estimate slightly higher for many adults, as it does here by about 74 kcal.
Katch-McArdle takes a different approach: instead of total body weight, it uses lean body mass, so it needs a body-fat percentage. For two people of the same weight, the leaner one has a higher Katch-McArdle BMR. This makes it well suited to lean, muscular individuals and less useful when body-fat percentage is unknown or only estimated, since an error in body fat feeds directly into the result.
Which one should you use?
For most people, Mifflin-St Jeor is the reasonable default. If you have a reliable body-fat measurement and are lean or athletic, Katch-McArdle may fit better. Whichever you choose, treat the number as a starting estimate, not a measurement: real BMR varies with genetics, body composition and other factors an equation cannot capture. The most reliable approach is to pick one equation, apply a consistent activity multiplier, and adjust based on real-world results over a few weeks.
Questions fréquentes
Which BMR equation is most accurate?
For the general population, the Mifflin-St Jeor equation is generally considered the most accurate. Katch-McArdle can be better for lean, athletic people because it uses lean body mass.
Why do BMR calculators give different numbers?
They use different equations fitted to different study populations. For one 80 kg, 180 cm, 30-year-old man the estimates range from 1,780 to 1,854 kcal — about a 4% spread.
Do I need my body-fat percentage for BMR?
Only for the Katch-McArdle equation, which is based on lean body mass. Mifflin-St Jeor and Harris-Benedict use weight, height, age and sex.
Is BMR the same as the calories I should eat?
No. BMR is your at-rest energy use. Daily calorie needs are BMR multiplied by an activity factor (your TDEE), then adjusted for any weight goal.
Références
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr. 1984;40(1):168-182. https://doi.org/10.1093/ajcn/40.1.168
- Katch FI, McArdle WD. Nutrition, Weight Control, and Exercise — lean body mass basis for resting metabolic rate. Academic reference.