Body mass index (BMI) uses height and weight to produce a screening number. You can calculate it manually with either the metric or imperial formula below, or use the adult BMI calculator.
Metric BMI formula
BMI = weight in kilograms ÷ height in metres²
Example: A person who weighs 70 kg and is 1.80 m tall has a BMI of 70 ÷ (1.80 × 1.80) = 21.6.
- Convert height to metres if it is recorded in centimetres: 180 cm = 1.80 m.
- Square the height: 1.80 × 1.80 = 3.24.
- Divide weight by the squared height: 70 ÷ 3.24 = 21.6.
If height stays in centimetres, use BMI = weight in kg ÷ height in cm² × 10,000.
Imperial BMI formula
BMI = 703 × weight in pounds ÷ height in inches²
Example: A person who weighs 145 lb and is 6 ft 1 in tall is 73 inches tall. BMI = 703 × 145 ÷ (73 × 73) = 19.1.
- Convert total height to inches: (6 × 12) + 1 = 73 inches.
- Square the height: 73 × 73 = 5,329.
- Multiply weight by 703, then divide by squared height.
Interpreting the result
For adults aged 20 and older, a BMI below 18.5 is underweight, 18.5 to below 25 is healthy weight, 25 to below 30 is overweight, and 30 or higher is obesity. You can also look up values in the metric BMI chart or imperial BMI chart.
BMI is a screening measure, not a diagnosis. It does not directly measure body fat or distinguish fat from muscle and bone. Children and teenagers require age- and sex-specific interpretation rather than adult categories.
Formula and reference information reviewed 29 August 2026 using CDC calculation guidance.