What the formula calculates
Body Mass Index (BMI) divides body weight in kilograms by height in metres squared. The result is a dimensionless index that correlates loosely with body fat percentage across large population groups. The World Health Organization classifies adults as underweight below 18.5, normal weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obese at 30.0 and above.
Imperial conversion
In US customary units, weight is measured in pounds and height in inches. A correction factor of 703 converts the result into the same dimensionless index. A person weighing 180 lb at 5 ft 9 in (69 in) has a BMI of 703 × 180 / 69² = 703 × 180 / 4761 ≈ 26.6.
Worked example
A person weighing 75 kg at 1.75 m height: BMI = 75 / (1.75 × 1.75) = 75 / 3.0625 ≈ 24.5. This falls in the normal weight range according to WHO thresholds.
Limitations
BMI does not distinguish between muscle and fat mass. Elite athletes frequently fall in the overweight category despite low body fat. It also does not account for fat distribution, which is a key predictor of metabolic risk. Waist-to-height ratio and body fat percentage measurements complement BMI for a fuller picture.