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. Every calculator shows the formula it uses and a worked example.
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.
The macro calculator converts a calorie target into grams, and the conversion is not uniform: protein and carbohydrate carry about 4 calories per gram, fat about 9. A 2,000-calorie day split 30/40/30 comes to 150 grams of protein, 200 grams of carbohydrate and 66.7 grams of fat. The fat figure looks wrong to people expecting three similar numbers, but it is right: the same 600 calories buys 150 grams of carbohydrate or only 67 grams of fat.
Protein targets are set per kilogram of bodyweight rather than as a share of calories, which is why the protein tool asks for weight instead. The dietary reference intake is 0.8 grams per kilogram — 66 grams a day for an 82-kilogram adult — while the ranges usually cited for people training with resistance run from 1.6 to 2.2 grams per kilogram, or 131 to 180 grams. The gap between those two figures is larger than most people assume, and which one applies depends on what you are asking the diet to do.
Body-fat estimation exists on this page precisely because body mass index cannot do it. The circumference method uses neck, waist and height — and for women, hip — to estimate composition rather than mass, so two people at 82 kilograms and 1.78 metres share a BMI of 25.9 and can return materially different body-fat results. Neither number is a diagnosis, and the circumference approach carries its own error margin, but it at least answers a question about the body rather than about the scales.
Water intake is the tool on this page with the weakest evidence base behind the advice it is usually asked to confirm. The familiar eight-glasses-a-day figure has no clear origin in the research literature, and intake requirements vary with body size, activity, climate and the water already present in food. The site's research section covers where that rule came from and what the evidence actually supports, and the calculator is built to estimate from your own inputs rather than to repeat the eight-glass convention.