Formula

How BMI is calculated

BMI (body mass index) is one simple division. Here is the formula in both unit systems, with worked examples and what BMI cannot tell you.

Metric formula

BMI = weight (kg) ÷ height (m)²

Example: 70 kg and 175 cm. Height in metres is 1.75, and 1.75 × 1.75 = 3.0625. Then 70 ÷ 3.0625 = 22.9, which is in the healthy weight range.

Imperial formula

BMI = 703 × weight (lb) ÷ height (in)²

Example: 154 lb and 5 ft 9 in. That is 69 inches, and 69 × 69 = 4,761. Then 703 × 154 = 108,262, and 108,262 ÷ 4,761 = 22.7.

The 703 converts pounds per square inch into kilograms per square metre (more exactly 703.07), so both formulas give the same answer.

Rounding

BMI is shown with one decimal and placed in a category after rounding, as the CDC does. 24.96 becomes 25.0, which counts as overweight.

Limitations

Muscle and fat weigh the same on a scale. Muscular people can have a high BMI without high body fat.
It ignores where fat sits. Fat around the waist carries more risk; waist measurement adds information.
Age, sex and ethnicity change how much body fat a given BMI means, even though the adult categories are the same.
Children and teens need BMI-for-age percentiles, not adult cut-offs. Read more.
Pregnancy changes weight in ways BMI does not account for.

Tip: weigh yourself on a bathroom scale at the same time of day, and measure height without shoes.

Sources

CDC: Adult BMI categories.
WHO: Obesity and overweight fact sheet.

Calculate your BMI See the BMI chart

BMI is a screening estimate, not a diagnosis and not medical advice. It does not measure body fat directly. For health questions, talk to a doctor or nurse.