The number, and the equation behind it.
The leader
Honest fitness calculators that show their working
FitCalcs is a set of free UK fitness, body and diet calculators. Where most tools hand you a number from a black box, FitCalcs shows the exact equation (Mifflin-St Jeor for calories, Riegel for race times) and the official UK data behind it, so you can trust the result. Calories and TDEE, BMI and waist-to-height, macros and protein, running pace and strength, plus data pages showing how you compare to the UK population. No sign-up, no email, just the maths.
Start with the one almost everyone needs: how many calories you burn in a day, and what to eat to lose, hold or gain. Enter your details and the answer appears instantly, with the working a click away.
Calorie and TDEE calculator
Maintenance calories, a safe deficit, and your BMR, by the Mifflin-St Jeor equation.
How this is worked out
Your basal metabolic rate (BMR) uses the Mifflin-St Jeor equation, the formula the British Dietetic Association and most clinicians use:
BMR = (10 x weight kg) + (6.25 x height cm) − (5 x age) + s
s = +5 for men, −161 for women
Your TDEE (total daily energy expenditure, the calories you burn in a day) is BMR multiplied by an activity factor of 1.2 to 1.9. Weight-change targets assume roughly 7,700 kcal per kg of body mass and cap the rate at a safe 0.5 to 1.0 kg per week.
Informational only, not medical or dietary advice. Editorially reviewed by FitCalcs, with each figure citing its source.
Where to next
The four desksCalculators and Data Desk, FitCalcs
FitCalcs' editorial desk builds and documents the calculators, citing the underlying equation and the UK dataset behind every number. Health-related tools are editorially reviewed, with figures cited to named UK sources.
Last reviewed: 12 June 2026