BMI Calculator

Body Mass Index divides your weight in kilograms by the square of your height in meters. The formula was developed in the 19th century by Belgian mathematician Adolphe Quetelet and later adopted by the WHO as a standard adult screening measure. Because height is squared, small changes in height have a larger effect on BMI than equivalent changes in weight.

The WHO defines four primary adult categories: underweight below 18.5, normal weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obesity at 30.0 or above. Some organizations add subcategories within obesity (Class I, II, and III) for clinical risk stratification, though our calculator uses the standard four-tier system.

When using imperial units, weight in pounds is converted to kilograms and height in feet and inches is converted to meters before the calculation runs. The same WHO thresholds apply regardless of which unit system you enter. BMI is intended for adults aged 20 and older; separate growth charts apply to children and adolescents.

• BMI = weight (kg) ÷ height (m)²
• WHO categories: Underweight < 18.5 | Normal 18.5–24.9 | Overweight 25–29.9 | Obesity ≥ 30

A BMI in the normal range (18.5 to 24.9) is associated with the lowest average health risk in population studies. That does not mean everyone in this range is optimally healthy — diet quality, fitness, and body fat distribution all matter independently. It simply means your weight relative to height falls within a range linked to favorable outcomes in large research datasets.

If your BMI falls in the overweight or obesity range, it may indicate excess body fat for many people, though not all. Higher BMI categories correlate with increased risk of type 2 diabetes, cardiovascular disease, and certain cancers at the population level. Individual risk depends on many factors including waist circumference, blood pressure, blood sugar, and family history.

An underweight BMI (below 18.5) can signal insufficient energy intake, malabsorption, or underlying health conditions. It may also reflect a naturally lean build in some individuals. If you are underweight and experiencing fatigue, hair loss, or missed menstrual cycles, consider speaking with a healthcare provider.

Because BMI cannot distinguish muscle from fat, a muscular athlete may register as overweight despite low body fat. In those cases, body fat percentage measured by caliper, bioimpedance, or circumference methods provides a more meaningful assessment of composition.

BMI treats all weight equally — it cannot tell whether extra mass comes from muscle, bone, or adipose tissue. Strength athletes, bodybuilders, and people with above-average lean mass are frequently misclassified as overweight or obese. Conversely, someone with a normal BMI can carry unhealthy levels of visceral fat, a pattern sometimes called "normal-weight obesity."

BMI is not validated for pregnant women, children, the elderly with significant muscle loss, or certain ethnic groups where disease risk thresholds may differ. It should never replace clinical evaluation, blood work, or body composition testing when health decisions are involved.