BMI calculator
Calculate your Body Mass Index (BMI) in either metric or imperial units. BMI is a rough population-level screening tool, not a diagnosis — see the guide below.
What BMI is
Body Mass Index (BMI) is a number that compares your weight to your height. The formula is weight in kilograms divided by height in metres squared — or, in imperial units, weight in pounds times 703, divided by height in inches squared. It was invented in the 1830s by the Belgian statistician Adolphe Quetelet as a population-level statistic.
The WHO classification
| BMI range | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese class I |
| 35.0 – 39.9 | Obese class II |
| 40 or above | Obese class III |
These ranges come from the World Health Organization. Different countries and professional bodies use slight variations — for example, several Asian health ministries use a lower overweight threshold (23.0) because health risks appear at lower BMI in populations of East and South Asian ancestry.
What BMI does well — and what it doesn't
BMI is a useful, fast, low-cost population-level indicator. Plotting average BMI against incidence of cardiovascular disease, type 2 diabetes, and certain cancers reveals strong correlations. For governments planning public-health interventions, BMI is a defensible metric.
At the individual level, BMI is much less useful. It cannot distinguish muscle from fat. A bodybuilder with 8% body fat will register as "obese". An older adult with low muscle mass and high body-fat percentage may register as "normal". BMI ignores fat distribution; visceral fat (around organs) carries far higher health risk than subcutaneous fat (under the skin) at the same total.
For these reasons, modern clinical practice combines BMI with waist circumference, body-composition scans, and patient history. Don't make decisions based on this calculator alone.
Children and adolescents
For people under 20 years old, BMI is interpreted against age-and-sex-specific growth percentiles, not against the fixed adult thresholds above. This calculator uses the adult formula; if you're under 20, your number is correct but the classification is not meaningful for you.
The formula in both unit systems
Metric: BMI = weight (kg) / (height in metres)²
Imperial: BMI = 703 × weight (lb) / (height in inches)²
The 703 multiplier is the conversion constant that makes the imperial formula produce the same numeric result as the metric formula.
Privacy
Your weight and height never leave your device.