Body Mass Index divides your weight in kilograms by the square of your height in metres, giving a single number used to screen for weight categories linked to health risk. It only needs your height and weight to calculate.
The formula is weight (kg) ÷ height (m)². For imperial measurements, height is converted to metres and weight to kilograms before the same formula is applied, so the result is identical regardless of which units you enter. A 75kg person who is 1.8m tall has a BMI of 75 ÷ (1.8 × 1.8) = 23.1.
The standard WHO bands are: under 18.5 (underweight), 18.5–24.9 (healthy weight), 25–29.9 (overweight), 30–39.9 (obese), and 40+ (severely obese). These bands are population-level screening thresholds, not a diagnosis — they were derived from studies linking BMI to health risk across large groups of adults, not from an individual assessment.
NICE guidance recommends lower action thresholds for people of South Asian, Chinese, Black African, African-Caribbean, and Middle Eastern backgrounds, as research shows elevated health risks — particularly type 2 diabetes and cardiovascular disease — arise at lower BMI values in these groups. Switching the "NHS adjustment" toggle above moves the overweight threshold down to 23 and the obese threshold down to 27.5.
BMI doesn't distinguish between muscle and fat, so muscular or very athletic people can be classed as overweight or obese despite low body fat. It's also less reliable for pregnant women, children, and adults over 65, and doesn't account for where fat is carried on the body — a factor that itself affects health risk. It's best used as a quick screening tool alongside other measures, not as a standalone diagnosis.