BMI Calculator

How to use the BMI calculator

Enter a height and a weight, and the tool divides weight in kilograms by height in metres squared to return your body mass index, the WHO band it falls in, and the weight range that would land in the healthy band at that height. Both fields carry a unit toggle, and pounds and inches are converted internally because the index is defined in metric.

The fields arrive pre-filled with a representative profile, 70 kg at 175 cm, which scores 22.9 and sits inside the healthy band. Height matters more than it looks: because it is squared, a measurement taken in shoes or with a slouch shifts the ratio noticeably more than an equivalent slip on the scale.

The cut points come from the World Health Organization adult classification and apply to adults from age 20, both sexes, with no separate male and female thresholds. The full ladder runs like this.

Read the healthy weight range rather than fixating on a single figure inside it. At 175 cm the healthy band covers 56.7 kg to 76.6 kg, a spread of nearly 20 kg, and no point in that range is more correct than any other.

The table below sets out the same range for heights from 150 cm to 195 cm along with the weight at which the index reaches 30, so you can see how the bands translate into actual kilograms and pounds at your own height.

What this page cannot do is tell you what you should weigh. That depends on build, muscle, age, medical history, medication and a dozen things four digits cannot see, and it belongs in a conversation with a doctor or a registered dietitian.

What BMI cannot see

The single most important thing to understand about this number is what it does not measure. Body mass index is a ratio of total mass to height, and it has no way of knowing what that mass is made of.

A rugby player carrying substantial muscle and a sedentary person of the same height and the same weight produce exactly the same index, because the arithmetic sees one weight and one height and nothing else. If body composition is the question, the body-fat-calculator estimates it from circumference measurements, and the waist-to-height-ratio tool looks at where mass is distributed.

The formula, worked line by line

Body mass index is one division. Take weight in kilograms, take height in metres, square the height, divide. The ratio was devised by the Belgian statistician Adolphe Quetelet in the 1830s as a way of describing the average build of a population, which is why it was known for a century as the Quetelet index.

The name body mass index arrives with Ancel Keys and colleagues in the Journal of Chronic Diseases in 1972, in a paper comparing several relative-weight indices and concluding that weight over height squared was the least bad of them for population work.

That origin explains both its strength and its limit. It was built to describe groups, and it does that job cheaply: two measurements anyone can take, no equipment beyond a scale and a wall. It was never built to characterise an individual body, and squaring height is itself a compromise, chosen because it happened to correlate least with height across the samples Keys examined, not because human mass genuinely scales with the square of stature.

BMI = kg / (m x m)
imperial: BMI = 703 x lb / (in x in)
healthy weight range for a height:
  low  = 18.5 x m x m
  high = 25 x m x m

The athlete and the screening trade-off

Now the illustration that matters most. A person of 92 kg at 183 cm scores 27.5 and is labelled overweight. That is the same label whether the extra mass is fat or trained muscle, because the calculation has seen exactly two numbers and neither of them describes tissue.

This is not a flaw in the arithmetic; it is the arithmetic working as specified. Body mass index is a screening ratio, and screening means sorting a large population cheaply into groups worth a closer look, accepting that individual cases will be misplaced.

Athletes are the best-known misplacement, but they are not the only one: people who have lost muscle with age or illness can score inside the healthy band while carrying a body composition that the number flatters.

Two further limits are worth stating plainly. First, the cut points are adult cut points, valid from age 20 and identical for both sexes. Children and adolescents cannot be assessed this way at all, because the normal relationship between mass and height changes through growth, so paediatric assessment uses age-and-sex percentile charts instead.

Second, the index cannot see where mass sits, and distribution carries information that total mass does not. The National Institute for Health and Care Excellence recommends in guideline NG246 that waist-to-height ratio be used alongside BMI for adults with an index under 35, on the basis that central fat tracks cardiometabolic risk more closely than total mass does. That is what the waist-to-height-ratio tool on this site calculates.

Everything here is general information rather than medical advice, and a number that is going to inform a decision about your own health should be taken to a doctor first.

Questions people ask