Fitness calculators
FitCalcs' calculators are free, instant and transparent. Each one shows the exact equation it uses and cites the UK data or sports-science source behind it, so the number is one you can check. There is no sign-up and we store nothing you enter. These are informational tools, not medical or dietary advice.
Calories and TDEE
Maintenance calories, a safe deficit, and your BMR (Mifflin-St Jeor).
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.
BMI and waist-to-height
NHS BMI categories plus waist-to-height ratio, the measure the NHS now prefers.
How this is worked out
BMI = weight (kg) divided by height (m) squared. NHS categories: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and over obese.
BMI = weight ÷ (height_m × height_m)
Waist-to-height ratio = waist ÷ height (same units). The NHS now treats 0.5 or above as a sign of increased health risk regardless of BMI, because it reflects central fat. It is a better single number than BMI for many people.
Informational only, not a medical diagnosis. BMI does not distinguish muscle from fat. Editorially reviewed by FitCalcs, with each figure citing its source.
Race time predictor
Predict your 5K, 10K, half and marathon from one recent result (Riegel).
How this is worked out
Predictions use Riegel's endurance formula, the model running coaches and race calculators have used since 1981:
T2 = T1 × (D2 ÷ D1)^1.06
T1 is your known time over distance D1; the 1.06 exponent reflects how pace slows as distance grows. It assumes equivalent training and even pacing, so longer races (especially the marathon) are best-case estimates, not guarantees.
For information only. Build distance gradually and train for the event you are targeting.
Every calculator on FitCalcs
Calories, diet and body
Strength and activity
Running and cardio
Common questions
Which FitCalcs calculator should I start with?
Start with the calorie and TDEE calculator if your goal is weight or diet, since your daily calorie burn feeds the macro, protein and weight-loss tools. If you run, start with the race time predictor. Every calculator works on its own, so you can also jump straight to the one you need.
What equations do these calculators use?
Each tool names its method and cites its source. Calories and BMR use the Mifflin-St Jeor equation, BMI uses the NHS categories and waist-to-height ratio, race times use the Riegel endurance formula, heart-rate zones use the Karvonen method, and strength uses the Epley and Brzycki formulas. The working is shown on every result.
Are the results medical or dietary advice?
No. FitCalcs tools are for general information only and are not a substitute for personalised medical or dietary advice. Health calculators such as BMI, calories and body fat are estimates that cite primary sources like the NHS. If you have a health condition or specific concern, speak to your GP or a registered professional.
Do you store the numbers I enter?
No. The calculators run in your browser and there is no sign-up, so we do not store the figures you type in. That means your weight, height and other inputs stay on your device and are cleared when you leave the page.
Email me when this updates
No account, no spam. Just a short note when we refresh our fitness calculators or add a related tool. Unsubscribe any time by replying.
Editor, FitCalcs
Oliver Mackman is the named editor of FitCalcs and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every result to cite the underlying equation and the named UK source behind it, with health tools carrying the appropriate not-medical-advice caveats.
Last reviewed: 12 June 2026