What is BMI?
Body Mass Index (BMI) is a simple numerical measure of a person's body size calculated from their height and weight. The formula is weight (kg) divided by height squared (m²). BMI was developed in the 19th century by Belgian mathematician Adolphe Quetelet and has been widely adopted as a screening tool by health organizations worldwide. It is not a direct measure of body fat, but provides a useful population-level indicator of weight status.
BMI categories explained
The World Health Organization defines four BMI categories: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). These thresholds were developed from population health data and are associated with different risks for conditions like heart disease, diabetes, and hypertension. Being outside the normal range is a signal to consult a healthcare provider, not a diagnosis.
Limitations of BMI
BMI has well-known limitations: it does not distinguish between muscle and fat, so very muscular individuals may be classified as overweight. It also does not account for age, sex, or ethnicity — older adults typically have more body fat at the same BMI than younger adults, and BMI cut-points differ across ethnic groups. For children, age and sex-specific percentile charts are used instead. Always interpret BMI in context with other health measures.
Metric and imperial inputs
This calculator supports both metric units (kilograms and centimetres) and imperial units (pounds and feet/inches). When imperial units are selected, the calculator converts to metric internally before computing BMI, since the BMI formula uses SI units. The conversion is precise: 1 pound = 0.453592 kg, 1 inch = 0.0254 m. Results are identical regardless of which unit system you use.