Formula
BMI = weight (kg) ÷ height (m)² How it works
Body mass index compares your weight with your height. It is a quick screening number used by doctors and insurers, not a diagnosis: it cannot tell muscle from fat, and it reads high for athletic people and can read low for older adults who have lost muscle.
For adults, the WHO categories are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, and 30 or more obesity. Children and teens use age- and sex-specific percentiles instead.
Frequently asked questions
Is BMI accurate for muscular people?
Not very. Muscle is denser than fat, so strength athletes often land in the "overweight" range while having low body fat. Waist measurements or body-fat estimates are more useful for them.
Does the same BMI scale apply to everyone?
Some health bodies use lower cut-offs for people of Asian descent (overweight from 23), because health risks start rising at lower BMIs in those populations.