BMI Calculator — Body Mass Index in Metric or Imperial

1. 1Choose metric or imperialMetric takes centimetres and kilograms, imperial takes feet, inches and pounds. Both give the same BMI.
2. 2Enter your height and weightType your height and weight. In imperial mode, feet and inches go in separate fields — 5 ft 7 in is 5 and 7, not 5.7.
3. 3Read your BMI and categoryYour BMI appears with its WHO category, a marker on the scale, and the healthy weight range for your height.

Body mass index is a single number relating weight to height. Devised in the 1830s by Adolphe Quetelet to describe populations, it became a screening tool because height and weight are quick to measure and because across large groups BMI correlates reasonably well with body fat percentage.

What it does not do is measure body fat, separate fat from muscle, or say where fat is carried. Two people with the same BMI can have very different health risks. It indicates whether a closer look is warranted, not health itself.

BMI = weight in kilograms ÷ (height in metres)². At 1.70 m and 68 kg that is 68 ÷ 2.89 = 23.5. Results are conventionally reported to one decimal place, in units of kg/m² that are usually left unwritten.

Imperial input is converted rather than given its own formula: inches × 2.54, pounds × 0.45359237, then the same division. The direct imperial equivalent is 703 × pounds ÷ inches². The healthy range is the formula rearranged — 18.5 and 24.9 times your height in metres squared.

The World Health Organization classification for adults is: under 18.5 underweight, 18.5–24.9 healthy weight, 25.0–29.9 overweight, 30.0–34.9 obesity class I, 35.0–39.9 class II, and 40.0 or above class III. Those are the boundaries used here, and they apply to adults aged 20 and over of any sex.

The cut-offs are not universal. WHO publishes lower action points for Asian populations, where cardiovascular and diabetes risk appears at a lower BMI, and several countries define overweight from 23.0 for people of South and East Asian descent. The boundaries are also abrupt in a way risk is not: 24.9 and 25.1 describe near-identical bodies.

BMI treats all weight as equivalent, so muscle counts the same as fat and trained athletes are routinely classified as overweight while carrying low body fat. It ignores distribution too, though abdominal fat carries higher metabolic risk than the same mass on the hips — which is why waist circumference is often measured alongside it. It does not apply to children and teenagers, who are assessed on percentile charts, nor to pregnant women.

Height and weight are health data and are treated as such: the calculation runs in your browser and the figures are never transmitted, logged or stored. Nothing is kept after you close the tab.