Mifflin-St Jeor BMR + activity multiplier. Metric or imperial.
Mifflin-St Jeor BMR × activity multiplier. The cleanest field formula for the general population (±10 % vs indirect calorimetry).
Counting calories without knowing how many calories you actually burn is a recipe for frustration. "Eat 2 000 a day" is a sticker on a cereal box; your real number depends on your sex, your age, your weight, your height, and how much you move during a typical week. Get the maintenance figure right and the rest of the diet stack falls into place: subtract about 500 kcal a day for steady fat loss, add about 300 for a lean gain, hold the line for body recomposition. Get it wrong by 300 kcal and you spend three months wondering why the scale will not move. The two-step Mifflin-St Jeor calculation behind this calculator is the cleanest field formula for the general adult population — its standard error against indirect calorimetry is around ten percent, better than any of the older Harris-Benedict variants and within touching distance of the gold-standard chamber test. Step one estimates the energy your body burns at complete rest (the basal metabolic rate, BMR); step two scales that figure by an activity multiplier to give the total daily energy expenditure (TDEE). The calculator surfaces both numbers and three derived targets — cut, maintain, lean gain — so the visitor leaves with a number they can plug into a meal-plan app on the next click.
BMR (Mifflin-St Jeor 1990) in kilocalories per day, with weight in kg, height in cm, age in years: men: BMR = 10 × weight + 6.25 × height − 5 × age + 5. Women: BMR = 10 × weight + 6.25 × height − 5 × age − 161. The five-versus-minus-161 offset captures the average sex difference in lean body mass at the same weight and height; for a 75 kg, 178 cm, 35-year-old, BMR works out to 1 658 kcal/day for a man and 1 492 for a woman, an 11 % gap. TDEE = BMR × activity factor, where the factor is 1.2 (sedentary, desk job, no exercise), 1.375 (light, 1–3 sessions a week), 1.55 (moderate, 3–5 sessions a week), 1.725 (active, 6–7 sessions a week), or 1.9 (very active, physical job plus training). The same 75 kg man at moderate activity has a TDEE of 1 658 × 1.55 ≈ 2 570 kcal/day. From the TDEE the calculator derives a cut target (TDEE − 500), a maintain target (= TDEE), and a lean gain target (TDEE + 300). The cut deficit corresponds to about 0.5 kg of fat per week (500 × 7 ≈ 3 500 kcal, roughly half of the 7 700 kcal/kg energy density of body fat). The lean gain surplus is small enough to limit fat accumulation in trained lifters; untrained beginners can use a larger surplus, advanced lifters need an even smaller one to stay lean.
Six inputs: sex, age, height (cm or inches), weight (kg or lb), and activity level. The unit selectors handle conversion internally so the visitor enters whatever units their bathroom scale and growth-chart used. The defaults — male, 35 years, 178 cm, 75 kg, moderate — represent an average European adult man training three to four times a week. The result panel shows TDEE as the headline KPI, BMR alongside, the activity factor used, and three goal-keyed daily targets. Plug the cut number into a meal-tracking app for fat loss, the maintain number for body recomposition, the lean-gain number for hypertrophy phases.
A 30-year-old woman, 165 cm, 62 kg, light activity (gym 2 days/week): BMR = 10 × 62 + 6.25 × 165 − 5 × 30 − 161 = 620 + 1 031 − 150 − 161 = 1 340 kcal/day. TDEE = 1 340 × 1.375 = 1 843 kcal/day. Cut = 1 343, maintain = 1 843, lean gain = 2 143. Compare to a 40-year-old sedentary man at 90 kg, 180 cm: BMR = 900 + 1 125 − 200 + 5 = 1 830. TDEE = 1 830 × 1.2 = 2 196. Cut = 1 696. The same man five years younger and active rather than sedentary jumps to BMR 1 855 × 1.725 ≈ 3 200 kcal/day — the activity multiplier dominates the age difference for non-elderly adults. A bigger lift than that is hard: doubling activity from sedentary (1.2) to very active (1.9) increases TDEE by 58 %, while losing twenty kilos at constant activity drops TDEE by only ten percent. Move more matters more than weigh less, all else equal.
First, overstating activity level. Most people self-report their activity as one bracket too high; the survey-validated correction is to round down. "Moderate" means three to five hard training sessions of 45–60 min a week, not three lazy gym walk-throughs. Second, ignoring NEAT. Non-exercise activity thermogenesis — fidgeting, walking around the office, doing housework — accounts for several hundred kilocalories of variation between similarly-built people who train identically. The activity factor approximates this but a sedentary office worker who tracks 12 000 steps a day will burn closer to a moderately-active person's TDEE than to the sedentary bracket the formula assigns them. Third, expecting the formula to track changes in body composition that the inputs do not see. Two 75 kg people with very different body fat percentages have measurably different BMRs (lean mass is metabolically more active), and Mifflin-St Jeor cannot tell them apart. The Katch-McArdle formula uses lean mass directly and is preferred for athletic populations who know their body fat percentage. Fourth, treating the TDEE as fixed. As you lose weight, BMR drops in proportion (about 10 kcal per kg lost) and adaptive thermogenesis cuts another 100–200 kcal. Re-run the calculator monthly during a cut. Fifth, using TDEE without checking the scale. Ten days of weight tracking will tell you whether your real maintenance is the calculator's number or 200 kcal away from it; trust your scale over the formula.
Mifflin-St Jeor is the consensus pick for healthy adults aged 18 to 79 across both sexes. Katch-McArdle uses lean body mass: BMR = 370 + 21.6 × LBM (kg). It is more accurate for athletes and significantly leaner-than-average individuals, but requires a body fat measurement — see the body-fat-percentage calculator. Harris-Benedict revised (Roza-Shizgal 1984) is the older standard and reads about 5 % higher than Mifflin-St Jeor on average; most modern sources have moved away from it. Cunningham 1980 is essentially Katch-McArdle with a slightly different constant. For children and adolescents, the Schofield equations (FAO 1985) split the formula by age band and are still the WHO reference. For pregnant or lactating women, add 350 kcal/day in the second trimester, 450 in the third, 500 during exclusive breastfeeding. None of those replace the basic two-step approach used here — BMR plus activity multiplier — they refine the BMR step or extend the activity step for special populations.