How to Calculate BMI (Formula, Chart & Limits)

BMI (body mass index) is calculated as weight (kg) ÷ height (m)², or in imperial units 703 × weight (lb) ÷ height (in)². A BMI of 22, for example, sits in the "normal" range of 18.5–24.9. Here's the formula, the categories, its limits, and a free BMI calculator.

The BMI formula

Metric: BMI = kg ÷ m². A 70 kg person who is 1.75 m tall has a BMI of 70 ÷ (1.75²) = 22.9. Imperial: BMI = 703 × lb ÷ in². The BMI Calculator handles both units automatically.

BMI categories

• Under 18.5 — underweight
• 18.5–24.9 — normal weight
• 25–29.9 — overweight
• 30+ — obese

What BMI doesn't tell you

BMI is a quick screening tool, not a diagnosis. It ignores muscle mass (very fit people can read "overweight"), fat distribution, age, and sex. Treat it as one signal alongside others like waist measurement and, for fitness goals, body fat percentage.

Beyond BMI

Planning a change in weight? Estimate your daily energy needs with the Calorie Calculator and see how many calories you should eat.

Bottom line

BMI = kg ÷ m² (or 703 × lb ÷ in²) — a fast weight-status check, best read alongside other measures. Calculate your BMI free.