BMI Calculator

Body mass index divides your weight by the square of your height. With kilograms and metres that is the whole formula; with pounds and inches it needs a conversion factor, 703.

BMI = weight (kg) ÷ height (m)² BMI = 703 × weight (lb) ÷ height (in)²

The 703 is the unit conversion, not a correction. A pound is exactly 0.45359237 kg and an inch exactly 0.0254 m, and 0.45359237 ÷ 0.0254² = 703.07. Hand calculations round it to 703; this calculator uses the exact values, so the same body gets the same BMI in either unit system.

The healthy weight range shown with your result is the normal-weight band worked backwards for your height: the weights at which your BMI would be exactly 18.5 and exactly 24.9. That is why the range moves with your height while the category boundaries stay fixed.

The ratio is almost two centuries old. Adolphe Quetelet described it in the 1830s while studying the proportions of the “average man”, and the physiologist Ancel Keys named it the body mass index in 1972. It was designed to compare populations; using it as a screening number for one person came later.

These are the World Health Organization’s cut-points for adults. They are the same for every adult, whatever their age or sex, because the formula has no input for either. The last column turns each band into weights for a height of 175 cm.

Lower action points for Asian populations. A 2004 WHO expert consultation found that in many Asian populations the health risks linked to body fat start at lower BMI values, and it proposed 23 and 27.5 as additional points for public-health action. Some national guidelines use lower cut-offs as a result. This calculator applies the international cut-points in the table.

• Height in centimetres. The formula needs metres: 70 ÷ 1.75² = 22.9, but 70 ÷ 175² = 0.002.
• Forgetting to square the height. 70 ÷ 1.75 = 40, a reading in the highest obesity class, for a body whose BMI is 22.9.
• Rounding the height first. Rounding 1.75 m to 1.8 m moves the answer from 22.9 to 21.6. Square the full height and round only the result.
• Mixing unit systems. Kilograms go with metres and pounds with inches. The factor 703 belongs to the pounds-and-inches version only.
• Using the adult table for children and teenagers. Young people are assessed on age-and-sex percentile charts from the WHO or the CDC, because a healthy BMI changes as they grow.
• Treating a boundary as a verdict. At 175 cm, a BMI of 24.9 and one of 25.1 are about 0.6 kg apart. The cut-points are population thresholds, not a line your health crosses.
• Using BMI during pregnancy. Advice on weight gain in pregnancy is based on the BMI from before the pregnancy, not on a reading taken during it.

BMI is a screening number: quick, free and roughly right for most adults, but not a diagnosis. It cannot see:

• Body composition. Muscle is denser than fat, so a muscular person can read overweight while lean, and an adult with little muscle, which includes many older people, can read normal while carrying more fat than the number suggests.
• Where fat is stored. Fat around the abdomen carries more risk than fat elsewhere, and BMI cannot tell the two apart. That is why a waist measurement is its usual companion.
• Everything else about your health. Age, sex, ethnicity, blood pressure, blood sugar, cholesterol, fitness and family history all change the picture, and none of them is in the formula.

If your result is outside the normal band, or inside it and something still worries you, a doctor can put the number next to the measurements that matter for you.