Calories and diet
To lose weight you need to eat fewer calories than you burn. The honest way to set a target is to estimate the calories you burn in a day (your TDEE), then subtract a modest amount: about 500 a day for roughly half a kilogram of loss a week. FitCalcs works this out with the Mifflin-St Jeor equation, the formula UK dietitians use, and shows every step. This is general information, not personalised dietary advice.
Calorie and TDEE calculator
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
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.
More calorie, diet and body calculators
Calculators 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