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
Common questions
How do I set a calorie target to lose weight?
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 your TDEE out with the Mifflin-St Jeor equation and shows the deficit target. This is general information, not personalised dietary advice.
What is the difference between BMR and TDEE?
BMR is the calories you burn at complete rest; TDEE is everything you burn in a full day. Your TDEE is your BMR multiplied by an activity factor, from 1.2 if you are sedentary up to 1.9 if you are extremely active. The activity factor accounts for movement, exercise and daily living on top of resting burn.
What are the TDEE activity multipliers?
Multiply your BMR by 1.2 for sedentary (little or no exercise), 1.375 for lightly active (1 to 3 days a week), 1.55 for moderately active (3 to 5 days), 1.725 for very active (6 to 7 days), and 1.9 for extremely active (a physical job or training twice a day). Pick the factor that best matches a normal week.
Is BMI a reliable measure of health?
BMI is a useful first screen but not a diagnosis, because it does not separate muscle from fat, so very muscular people can read as overweight while still being lean. The NHS now recommends checking your waist-to-height ratio alongside it, keeping your waist under half your height. This is general information, not a medical assessment.
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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