The BMI formula
Metric: BMI = weight (kg) ÷ [height (m)]².
Imperial: BMI = [weight (lb) ÷ height (in)²] × 703.
A worked example
Someone 70 kg and 1.75 m tall: 1.75 × 1.75 = 3.06, then 70 ÷ 3.06 ≈ 22.9 — within the healthy range. In imperial, 154 lb and 69 in: 69² = 4761, then (154 ÷ 4761) × 703 ≈ 22.7.
What your BMI means
- Under 18.5 — underweight
- 18.5–24.9 — healthy weight
- 25–29.9 — overweight
- 30 and over — obesity
The limits of BMI
BMI uses only height and weight, so it can overestimate fat in muscular people and doesn’t show where fat is stored. Pair it with your waist-to-height ratio for a fuller picture. Children and teens use age- and sex-specific BMI percentiles instead.