BMI calculator: body mass index from height and weight

What body mass index actually measures

Body mass index is your weight in kilograms divided by the square of your height in metres. That is the whole formula. It was devised in the 1830s by Adolphe Quetelet, a Belgian statistician who was explicit that it described populations and was not a measure of individual fatness.

It became the standard because it is cheap and reproducible: two measurements anyone can take, no equipment beyond a scale and a tape. Across a large group it tracks body fat well enough to be useful in public health.

Where the BMI calculator misleads

• It cannot tell muscle from fat. A muscular athlete and a sedentary person of the same height and weight get the same BMI and are in completely different health.
• It ignores where fat sits. Visceral fat around the organs carries most of the metabolic risk, and BMI is blind to it. Waist circumference and waist-to-height ratio do better on that specific question.
• The cutoffs are not universal. Several health bodies use lower thresholds for people of South Asian descent, where cardiometabolic risk rises at a lower BMI.
• It behaves badly at the extremes of height, because the square in the denominator is an approximation that fits average heights best.

What a healthy BMI range means for you

Treat it as one input among several, which is how clinicians use it. A BMI in the overweight range alongside a normal waist measurement, good blood pressure and normal blood glucose is a different situation from the same BMI with the opposite. If you want a second measurement that adds real information, take your waist: over half your height is the threshold most often cited.

For an estimate that separates fat from lean mass, the body fat calculator uses the tape method instead. For intake, the calorie calculator works from the same height and weight.