BMI Calculator

Enter your height and weight to get your body mass index, the WHO category it sits in, and the weight range that would put you in the healthy band.

The BMI formula

Body mass index is deliberately simple. It divides your weight by the square of your height:

If you work in imperial units the same relationship needs a conversion factor:

The squaring is the part people forget. It means BMI does not scale linearly with height: a person who is 10 % taller needs to be 21 % heavier to keep the same BMI. That is also the root of one of the index's known biases, which we come back to below.

A worked example

Take someone who weighs 82 kg and is 178 cm tall.

1. Convert height to metres: 178 cm ÷ 100 = 1.78 m.
2. Square it: 1.78 × 1.78 = 3.1684.
3. Divide weight by that figure: 82 ÷ 3.1684 = 25.9.

A BMI of 25.9 falls in the 25–29.9 band, which the World Health Organization labels “overweight (pre-obese)”. To reach the top of the healthy band, the same person would need to weigh 24.9 × 3.1684 ≈ 78.9 kg — roughly 3 kg less.

What the categories mean

The WHO cut-offs used by this calculator are the ones most health systems adopt:

These bands are not universal. Several health authorities use lower thresholds for people of South Asian, Chinese and other Asian descent — often 23 for overweight and 27.5 for obesity — because cardiometabolic risk appears at a lower BMI in those populations.

Where BMI breaks down

BMI was devised in the 1830s by Adolphe Quetelet as a way of describing populations, not individuals. It knew nothing about body composition then and it still does not. Specifically:

• It cannot tell muscle from fat. A well-trained rugby player and a sedentary person of the same height and weight get an identical score. One of them is not at risk.
• It ignores where fat sits. Visceral fat around the organs is far more strongly linked to metabolic disease than fat on the hips and thighs. BMI cannot see the difference; waist circumference and waist-to-height ratio can.
• It is biased by height. Because height is squared rather than cubed, BMI tends to overstate adiposity in tall people and understate it in short people.
• It shifts with age and sex. Older adults lose muscle and gain fat at a stable BMI, and at the same BMI women typically carry more body fat than men.
• It does not apply to everyone. Children and teenagers need age- and sex-specific percentile charts; pregnant and breastfeeding women should not use it at all.

Used as one signal among several — alongside waist measurement, blood pressure, blood lipids and fitness — BMI is a reasonable screening tool. Used alone, it is a blunt one.