BMI (body mass index) is your body weight in kilograms divided by your height in meters squared. The World Health Organization (WHO) uses it to sort adults into weight categories.
Formula and categories
BMI = weight (kg) ÷ height (m)²
According to the WHO, a BMI of 18.5 to 24.9 counts as normal weight, 25 to 29.9 as overweight and 30 or more as obesity. These categories do not account for age, sex or build.
Example: A lifter at 1.80 m and 90 kg has a BMI of 90 ÷ 1.80² = 27.8. By WHO standards that is overweight, even if his body fat percentage is low. This is a worked example.
Why it is of little use for bodybuilders
BMI only knows weight and height. It does not distinguish between muscle and fat. If you carry a lot of muscle, you quickly land in the overweight range even though you are lean. For lifters, body fat percentage, FFMI and waist circumference say more. FFMI uses the same math but only counts fat-free mass.
BMI is not much help for tracking a lean bulk or cut either: it moves exactly like your weight and does not reveal whether a change comes from fat or muscle. Watching your trend weight and waist works better. As a rough classification for the general population, BMI is still useful.