BMI — Body Mass Index — is the quick number doctors and health sites use to put weight in the context of height. It's easy to calculate and useful as a rough screen, but it's also widely misunderstood. Knowing both the formula and its limits helps you read it sensibly.
Here's how to calculate BMI, what the categories mean, and what the number does and doesn't tell you.
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)². So for 70 kg at 1.75 m: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9.
Imperial: BMI = (weight in lb ÷ height in inches²) × 703. For 154 lb at 69 inches: (154 ÷ 4761) × 703 = 22.7.
Both give essentially the same number — it's just the units that differ.
What the categories mean
- Under 18.5 — underweight
- 18.5 – 24.9 — healthy weight
- 25.0 – 29.9 — overweight
- 30.0 and above — obese
These ranges are for adults. BMI for children and teens is interpreted differently, using age- and sex-specific percentiles.
Calculate yours instantly
Skip the arithmetic with the free BMI Calculator — enter your height and weight in metric or imperial and it gives your BMI and category.
BMI CalculatorCalculate your Body Mass Index.What BMI doesn't tell you
BMI is a useful screening tool, but it's a blunt one. It only knows your height and weight — not your body composition. It can't tell muscle from fat, so a very muscular athlete can register as 'overweight' while carrying little fat. It also doesn't account for where fat is stored, age, sex, or ethnicity, all of which affect health.
Treat BMI as one data point, not a verdict. For a fuller picture, look at measures like body-fat percentage and waist-to-hip ratio too, and talk to a doctor about what's right for you.
Go beyond BMI
- Body Fat Calculator — estimate body-fat percentage.
- Waist-to-Hip Ratio Calculator — where fat is stored matters.
- BMR Calculator — your baseline daily calorie burn.
- Calorie Calculator — daily calories for your goals.