BMI calculator: find your body mass index in seconds

For most adults, the World Health Organization defines a BMI between 18.5 and 24.9 as the healthy weight range. Below 18.5 is classed as underweight, 25–29.9 as overweight, and 30 or above as obesity. These bands were built from population health data, not from any single person's body composition.

No. BMI uses only height and weight, so it can't tell muscle from fat. Athletes and very muscular people are often classified as overweight despite having low body fat, and the standard cut-offs were derived mainly from European populations — several Asian health bodies use lower thresholds because health risk rises at a lower BMI in those groups. Treat BMI as a quick screening number, not a diagnosis.

Metric: weight in kilograms divided by height in metres, squared (kg/m²). Imperial: weight in pounds divided by height in inches squared, multiplied by 703. Both formulas describe the same ratio of weight to height — our calculator does the conversion for you automatically.

The standard adult BMI formula and its categories don't adjust for age or sex. Children and teenagers are assessed differently, using age- and sex-specific growth charts (BMI-for-age percentiles) rather than the fixed adult cut-offs used on this page.

Use it as a starting point for a conversation, not a verdict. A doctor or registered dietitian can look at your waist circumference, body composition, family history and lifestyle alongside BMI to get a fuller picture, and can advise on next steps if any are needed.

The standard WHO healthy BMI range of 18.5–24.9 applies to both men and women. However, women naturally carry more body fat than men at the same BMI due to differences in hormonal and reproductive physiology. Some research suggests women have lower cardiovascular risk at a slightly higher BMI than men, but current clinical guidelines use the same cut-offs for both sexes.

A BMI of 30 or above is classified as obesity by the WHO. This is further divided into three classes: Class I (30–34.9), Class II (35–39.9), and Class III (40 and above, sometimes called severe obesity). These classifications inform clinical treatment decisions but are based on population risk data — individual health status depends on much more than a single number.

At a population level, BMI correlates reasonably well with health outcomes. At an individual level, it's imprecise — it can misclassify muscular people as overweight and thin people with high body fat as healthy. Combining BMI with waist circumference, waist-to-hip ratio, or a body fat percentage measurement gives a more complete picture.

They measure different things. BMI uses only height and weight; it's a proxy for body composition but doesn't measure fat directly. Body fat percentage measures the actual proportion of fat in your body, which makes it more meaningful for tracking body composition changes over time. BMI is faster and only requires two measurements; body fat percentage requires additional measurements (circumference method) or equipment. Use BMI as a quick screen and body fat percentage for deeper assessment — our body fat calculator uses the US Navy circumference method.