The BMI formula
In metric units, BMI = weight (kg) / height (m)². For example, 70 kg at 1.75 m gives 70 / (1.75 x 1.75) = 22.9. In imperial units, BMI = 703 x weight (lb) / height (in)².
What the categories mean
Standard adult ranges are: Underweight below 18.5, Normal 18.5-24.9, Overweight 25-29.9, and Obese 30 and above. These are general guides, not a diagnosis.
The limits of BMI
BMI does not measure body fat directly and cannot tell muscle from fat. Athletes can read as "overweight" despite low body fat, and BMI is less reliable for older adults, children, and pregnancy. Treat it as a starting point, not the full picture.