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How Is BMI Calculated?

Body Mass Index is your weight in kilograms divided by the square of your height in metres: BMI = kg ÷ m². The World Health Organization sorts adult results into bands: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, 30–34.9 is obesity class I, 35–39.9 is obesity class II, and 40 or above is obesity class III.

Worked example: a person who is 1.75 m tall and weighs 75 kg has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 ≈ 24.5 — inside the normal range, close to the overweight line. If the same person weighed 95 kg instead, the result becomes 95 ÷ 3.0625 ≈ 31.0, which falls into obesity class I. Because BMI depends on only two numbers, a few kilograms near a threshold can shift the category label even when nothing meaningful has changed about actual health or body composition.

The WHO thresholds were chosen because, at a population level, BMI above roughly 25 correlates with rising rates of cardiovascular disease, type 2 diabetes and other metabolic conditions. That correlation is useful for screening large groups, but it says little about any single individual.

What You Should Know

BMI cannot separate muscle from fat. A heavily muscled athlete or weightlifter often lands in the "overweight" or even "obese" band despite low body fat, while an older adult who has lost muscle mass can show a "normal" BMI while actually carrying a high fat percentage — sometimes called "skinny fat" or sarcopenic obesity. BMI also does not account for where fat is stored, and abdominal (visceral) fat carries more metabolic risk than fat stored elsewhere at the same total weight.

The standard thresholds were derived mostly from European populations; several health bodies recommend lower overweight/obesity cutoffs for South and East Asian adults, since health risk rises at somewhat lower BMI values in these groups. BMI is not designed for children or teenagers, who should be assessed with age- and sex-specific percentile growth charts, and it is not a useful measure during pregnancy, when weight gain is expected and healthy.

If your result sits outside the normal range, or you are unsure how to interpret it given your muscle mass, age or ethnicity, pair it with a waist circumference or body-fat percentage measurement and discuss the full picture with a doctor or registered dietitian. This tool provides general educational information only, is not a medical diagnosis, and does not replace professional healthcare advice.