BMI Calculator
Enter your weight and height: your body mass index is calculated instantly, along with the WHO classification and the weight range considered healthy for your height.
Enter your weight and height, the result appears instantly.
The WHO body mass index categories
The World Health Organization groups adult BMI into four main categories: underweight below 18.5, normal weight from 18.5 to 24.9, overweight from 25 to 29.9, and obesity at 30 or above. The formula is your weight in kilograms divided by the square of your height in meters: someone weighing 70 kg (about 154 lb) at 1.75 m (5 ft 9 in) has a BMI of 22.9, comfortably in the normal range. Working in pounds and inches? The same result comes from 703 × weight (lb) ÷ height (in)², or simply convert: 1 kg ≈ 2.2 lb, 1 inch = 2.54 cm.
What BMI can't tell you
BMI doesn't distinguish muscle from fat: a very muscular athlete can show up as "overweight" while carrying very little body fat. The standard categories also don't apply to children and teenagers or to pregnant women, and should be interpreted with caution after age 65. This tool provides estimates only and is not medical advice, it's no substitute for a doctor or registered dietitian.
Where an 1830s formula comes from
BMI is weight divided by height squared, devised by the astronomer and statistician Adolphe Quetelet in the nineteenth century to describe populations, not people. It was meant to say what the «average man» of a country looked like, not to assess an individual in a consulting room.
Individual use arrived much later, in the 1970s, because it was cheap and repeatable: a set of scales and a tape measure are enough. It was never designed as a diagnosis, which is why every guideline calls it a population indicator and not a clinical measurement.
Why height squared
The exponent of two is an empirical compromise: the volume of a body grows with the cube of height, but tall people are not enlarged copies of short ones, and the exponent that best fits real data sits between 2 and 2.5.
The practical consequence is that BMI slightly penalises tall people and favours short ones. On a 1.90 m adult the value comes out a point or two higher than body composition justifies, and below 1.60 m the opposite happens.
The measurement that really adds information
The most serious limitation of BMI is that it does not say where the fat is, and position matters more than quantity: abdominal fat is linked to cardiovascular and metabolic risk far more than fat on hips and thighs.
The measurement that fills the gap takes ten seconds: waist circumference divided by height, with the simple threshold of «less than half». Two people with the same BMI can have very different ratios, and that is the number that changes the advice.
Nearby tools
The waist-to-height ratio is worked out in Waist-to-height ratio and body fat percentage in Body fat. For children BMI is read on percentiles: Growth percentiles.