BMI Calculator
Work out your body mass index from height and weight — in metric (cm/kg) or imperial (ft+in/lb), with the WHO category band.
Healthy weight
24.5 BMI
BMI 18.5 – 24.9
A BMI in this range is associated with the lowest population-level health risk for most adults — but it is a single rough indicator, not a diagnosis.
Results update as you type. BMI is a population-level screening tool, not a diagnosis. Athletes, pregnant people, the very tall or very short, and people with very high muscle mass should treat the result with caution.
Formula
BMI is one division:
BMI = weight_kg / (height_m × height_m).
The imperial form folds in the lb→kg and inches→metres conversions:
BMI = (weight_lb × 703) / (height_in × height_in).
Both give the same number for the same person — only the input units change.
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Frequently asked questions
How is BMI calculated?▾
BMI is your weight in kilograms divided by your height in metres squared (kg/m²). For 75 kg at 1.75 m: 75 ÷ (1.75 × 1.75) = 24.5. The same formula in imperial uses a 703 multiplier: BMI = (weight in lb × 703) / (height in inches)². Either way the answer is the same number — only the inputs change.
What are the BMI categories the NHS uses?▾
The NHS uses the WHO bands: under 18.5 = underweight; 18.5–24.9 = healthy weight; 25–29.9 = overweight; 30 and above = obese. Obese is sometimes broken down further: 30–34.9 (Class I), 35–39.9 (Class II), 40+ (Class III). Some NHS guidance lowers the thresholds for South Asian, Chinese, and Black African heritage adults — Class I starts at 27.5 and screening for diabetes from 23 — because cardiometabolic risk rises at lower BMI in those groups.
Is BMI accurate?▾
It is a rough population-level screening tool, not a diagnosis. BMI cannot tell muscle from fat (a 100 kg rugby player and a 100 kg sedentary person at the same height get the same BMI), it doesn't account for fat distribution (visceral fat is the harmful kind, not subcutaneous), and it under- or over-estimates risk in athletes, the elderly, the pregnant, and people who are very tall or very short. Use it as one signal alongside waist measurement, blood pressure, blood-sugar tests, and how you feel.
Why is healthy BMI 18.5–24.9?▾
Population studies (originally Quetelet in 1832, refined through 20th-century actuarial and WHO data) link the lowest all-cause mortality rates with this band. Below 18.5 mortality rises (often from underlying illness or under-nutrition); above 25 it rises gradually; above 30 it rises more sharply. The exact 'healthy' band is a statistical convention, not a sharp clinical cliff.
Should children use this calculator?▾
No — BMI for under-18s is interpreted using age-and-sex centile charts, not adult bands. The NHS Healthy Weight calculator handles paediatric BMI properly. Use this calculator for adults aged 18+.
What about waist measurement?▾
Waist circumference is a useful complement to BMI because it captures fat distribution. NHS guidance: a waist over 94 cm (37 in) for men or 80 cm (32 in) for women indicates increased health risk regardless of BMI; over 102 cm (40 in) men / 88 cm (35 in) women indicates high risk. Lower cut-offs apply for South Asian heritage.