How this is calculated
The Mifflin-St Jeor equation is BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5 for men, or − 161 for women.
Worked example for a 30-year-old man at 80 kg and 180 cm: (10 × 80) + (6.25 × 180) − (5 × 30) + 5 = 800 + 1125 − 150 + 5 = 1,780 calories a day at complete rest.
Total daily energy expenditure multiplies BMR by an activity factor: 1.2 for sedentary, 1.375 for light exercise, 1.55 for moderate, 1.725 for heavy, and 1.9 for very heavy. The same man at moderate activity would need about 2,759 calories to maintain weight.
Why the activity multiplier is where the error lives
The BMR equation is reasonably accurate — typically within 10% for most adults. The activity multiplier is not. It is a coarse five-point scale standing in for enormous individual variation in daily movement, occupation and unconscious activity.
Non-exercise activity thermogenesis, the energy spent fidgeting, standing, walking around and maintaining posture, varies by hundreds of calories a day between people of the same size doing the same formal exercise. That variation is invisible to any multiplier.
The practical consequence: treat the TDEE figure as a starting hypothesis. Eat at it for two or three weeks, track your weight, and adjust from what actually happens rather than from what the equation predicted.
BMR, RMR and TDEE
BMR is measured under strict conditions — fasted, fully rested, thermally neutral. Resting metabolic rate is the same idea measured under looser conditions, and comes out slightly higher, typically by a few percent. The two terms are used interchangeably in practice and the difference rarely matters.
TDEE is the number you actually eat against. It is BMR plus activity plus the thermic effect of food, which is the energy spent digesting what you eat — roughly 10% of intake, and higher for protein than for fat or carbohydrate.
How to use the bmr calculator
- Enter sex, weight, height and age. All four affect the result. Age matters because BMR declines with it, largely through loss of muscle mass.
- Read your BMR. This is resting energy expenditure — the floor, not a calorie target.
- Pick a realistic activity level. Most people overestimate. If you exercise three times a week and otherwise sit at a desk, that is light, not moderate.