BMI Calculator

Body Mass Index (BMI) is the standard NHS and WHO screening measure for healthy weight in adults. The formula is weight in kilograms divided by height in metres squared (kg/m²). The standard categories used by the NHS are: under 18.5 underweight, 18.5-24.9 healthy weight, 25-29.9 overweight, 30 and over obese, with obesity sub-divided into Class 1 (30-34.9), Class 2 (35-39.9) and Class 3 or "severe" obesity (40+). These thresholds were derived from large-population studies showing rising all-cause mortality risk above 25 and below 18.5. NICE updated guidance in 2023 recommends lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African Caribbean ethnicity, because metabolic risk rises at lower BMI in these groups: overweight from 23 and obese from 27.5. This calculator applies the appropriate threshold automatically if you select your ethnicity. The NHS uses BMI alongside waist circumference for adults aged 18-65 — for the most accurate health risk picture, combine both: a waist over 94cm (men) or 80cm (women) indicates increased risk regardless of BMI. BMI has limitations and should be a screening tool, not a diagnosis. It tends to overestimate body fat in very muscular adults (rugby players, weightlifters often score "obese" on BMI alone), and underestimate body fat in older adults with sarcopenia (low muscle mass) or in some Asian ethnicities. It is not validated for use under 18 — children use age-and-sex-specific BMI percentiles via the UK90 growth charts. It is not used in pregnancy. People over 70 may have a higher "healthy" range as a slightly higher BMI is associated with better outcomes in elderly populations. Use the result as a starting point for conversations with your GP or for tracking change over time. If your BMI puts you in the overweight or obese band and your waist measurement is also elevated, the NHS offers free weight management programmes — including the NHS Digital Weight Management Programme accessible via GP referral. The calculator displays your BMI, the corresponding NHS category, an ethnicity-adjusted note where applicable, and your distance (in kg) from the nearest healthy threshold. It does not calculate body fat percentage, lean mass, or visceral fat — those require body composition scales or DEXA scans. For calorie planning use the Calorie Calculator; for ideal weight targets use the Ideal Weight Calculator.