What BMR measures
Even lying perfectly still, your body spends energy: the brain, liver, heart and kidneys together account for most of it. Basal metabolic rate is that minimum daily cost. It depends mainly on how much lean tissue you have, which is why it rises with weight and height, is higher on average in men, and declines slowly with age. Knowing your BMR is the starting point for any calorie plan, because everything else you burn is added on top.
The three equations
Mifflin-St Jeor (1990) Men: 10W + 6.25H − 5A + 5 Women: 10W + 6.25H − 5A − 161 Revised Harris-Benedict (Roza & Shizgal, 1984) Men: 13.397W + 4.799H − 5.677A + 88.362 Women: 9.247W + 3.098H − 4.330A + 447.593 Katch-McArdle 370 + 21.6 × LBM, LBM = W × (1 − body fat %)
W = weight in kg, H = height in cm, A = age in years. TDEE = BMR × activity factor: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 very active, 1.9 extra active.
Worked example
A 40-year-old man, 5 ft 11 in (180.3 cm), 185 lb (83.9 kg), about 20% body fat:
- Mifflin-St Jeor: 839.1 + 1,127.2 − 200 + 5 ≈ 1,771 kcal
- Harris-Benedict: 1,124.2 + 865.4 − 227.1 + 88.4 ≈ 1,851 kcal
- Katch-McArdle: lean mass 67.1 kg → 370 + 21.6 × 67.1 ≈ 1,820 kcal
Moderately active, his TDEE would be about 1,771 × 1.55 ≈ 2,746 kcal/day.
Limitations
- These are population averages; measured BMR for a given person can differ by 10–15%.
- Katch-McArdle is only as good as the body-fat number you give it.
- Illness, thyroid conditions, some medications, pregnancy and recent major weight loss all change metabolic rate.
Sources: Mifflin, M. D. et al. (1990), Am J Clin Nutr 51:241–247; Roza, A. M. & Shizgal, H. M. (1984), Am J Clin Nutr 40:168–182; McArdle, Katch & Katch, Exercise Physiology.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR (basal metabolic rate) is the energy your body uses at complete rest to breathe, circulate blood and keep organs working. TDEE (total daily energy expenditure) adds everything you do on top of that — moving, exercising and digesting food. TDEE is the number to compare with how much you eat.
Which BMR equation is most accurate?
For most adults, Mifflin-St Jeor is the best general choice and is the one most dietitians use. If you know your body-fat percentage reasonably well (from DEXA or a careful tape measurement), Katch-McArdle can be better for lean or muscular people because it works from lean mass rather than total weight.
Why do the three equations disagree?
They were fitted to different groups of people at different times. Harris-Benedict dates from 1919 (revised 1984) and tends to run a little high today; Mifflin-St Jeor was built in 1990 from a more modern sample; Katch-McArdle ignores age and sex and depends entirely on lean mass. A spread of 50–100 calories is normal.
Can I raise my BMR?
Somewhat. Lean mass is the biggest driver you can change, so strength training that builds or preserves muscle helps, especially while dieting. Age, sex and height can't be changed, and very low-calorie diets tend to lower metabolic rate.
Is BMR the same as RMR?
Not quite. BMR is measured under strict lab conditions (fasted, fully rested, thermoneutral room); resting metabolic rate (RMR) is measured under looser conditions and is usually slightly higher. The equations here are commonly used for both and the difference is small.
This calculator is for general information and screening only and is not medical advice. Talk to a doctor or registered dietitian before making changes based on the result, especially if you are pregnant, under 18, or managing a health condition.