BMI formula
BMI = weight (kg) ÷ height (m)². In US units: BMI = 703 × weight (lb) ÷ height (in)². Adult categories used by many health agencies: underweight under 18.5, healthy 18.5–24.9, overweight 25–29.9, obesity 30 and above.
BMI does not measure body fat, muscle, or health. Athletes can land in “overweight” with low fat. It is a screening number, not a diagnosis. Talk to a clinician about your own case.
Worked example
A person 170 cm and 70 kg has BMI 24.2, in the standard adult “healthy” range (18.5–24.9). At 6 ft 0 in and 200 lb, BMI is about 27.1, in the “overweight” band on that same chart.
BMI does not see muscle, bone, or where fat sits. Athletes, older adults, and pregnancy make the chart a poor fit. It is a screening number, not a diagnosis. Not medical advice.
Common questions
Is BMI different for children?
Yes. Children use age-and-sex percentiles, not these adult cutoffs. This page is for adults.
Why do some charts start obesity at 30?
That is the common WHO adult split. Some Asian-population guides use lower cutoffs.