How to Calculate BMI (Formula, Categories, Free Calculator)
Learn the BMI formula in metric and imperial units, what each BMI category means, and use a free calculator for an instant result.
Body Mass Index is a rough screening number that compares your weight to your height. It is not a diagnosis, but it is a quick way for doctors and individuals to flag whether further health checks make sense.
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)². If you are 1.75 m and 70 kg: 70 ÷ (1.75 × 1.75) = 22.9.
Imperial: BMI = (weight in pounds × 703) ÷ height in inches². For 5'9" (69 in) and 154 lb: (154 × 703) ÷ (69 × 69) = 22.7.
BMI categories (adults)
- Under 18.5 — Underweight
- 18.5 to 24.9 — Healthy range
- 25 to 29.9 — Overweight
- 30 and above — Obese (class I, II, or III)
Skip the math
Our BMI Calculator accepts metric or imperial input and shows your BMI plus category instantly. Mixing units? Convert with Weight Converter and Length Converter first.
BMI limitations
BMI does not distinguish muscle from fat, so athletes often score 'overweight' despite low body fat. It is also less accurate for older adults, pregnant women, and people under 18. Treat it as a starting conversation, not a verdict.