22 free health and fitness calculators — BMI, BMR, TDEE, calorie deficit, water intake, calories burned, body fat, macros, sleep, life expectancy, pregnancy & due date, age, period & fertility. Clinically validated formulas.
Most of the tools on this page are not independent — they are stages of one chain. It starts with basal metabolic rate, the energy your body spends doing nothing at all, which the Mifflin-St Jeor equation estimates from weight, height, age and sex. For a 35-year-old man at 82 kg and 178 cm that is 10 × 82 + 6.25 × 178 − 5 × 35 + 5 = 1,762 calories a day before he gets out of bed.
Total daily energy expenditure is that figure multiplied by an activity factor: 1.2 if you are sedentary, 1.375 if you train lightly, 1.55 if you train moderately. The same man lands on 2,115, 2,423 or 2,732 calories depending only on which description he picks — a 617-calorie spread from a single dropdown, which is why honest activity selection matters more than decimal precision anywhere else in the calculation.
Weight-change tools then subtract from that number using the convention that a pound of body fat carries roughly 3,500 calories, so a 500-calorie daily deficit predicts about one pound a week. Treat that as an opening estimate rather than a law: metabolic rate falls as body mass drops, so a deficit that works in month one produces less in month six.
Body mass index sits outside this chain and answers a narrower question. It is weight divided by height squared — 82 ÷ 1.78² = 25.9 — and it deliberately knows nothing about body composition. That is why a muscular athlete and a sedentary person of the same height and weight receive an identical BMI and quite different advice. Where that distinction matters, the body-fat and macro tools, which use circumference measurements and goal-based ratios, are the better starting point.