BMI Calculator

This calculator gives your body mass index from your weight and height, the WHO category it falls into, and the weight range that would put you between a BMI of 18.5 and 24.9 at your height. Switch between metric (kg, cm) and imperial (lb, in) — the result is the same either way.

BMI is a screening number, not a diagnosis. It is quick and needs no equipment, which is why doctors and public-health bodies use it, but it cannot tell muscle from fat or show where fat is stored. Use it alongside a waist measurement or a body fat estimate, and talk to a doctor or nurse before acting on it.

The adult categories here apply to people aged 18 and over. Children and teenagers are assessed with BMI-for-age percentiles instead, because healthy body proportions change as they grow.

The formula is the Quetelet index, introduced by Adolphe Quetelet in the 1830s and adopted as "body mass index" in 1972. The healthy weight range shown is simply the formula run backwards: 18.5 × height² to 24.9 × height².

Two related figures are folded into this page: BMI Prime is your BMI divided by 25 (the top of the healthy range), so 1.00 is the upper limit and 1.20 means 20% above it; the ponderal index divides weight by height cubed (kg/m³) and is sometimes preferred for very tall or very short people because it scales better with height.

These are the World Health Organization's adult categories, used by the NHS and the US CDC. Some health services use lower cut-offs for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, because the same BMI is linked with a higher risk of type 2 diabetes in these groups: in UK NICE guidance, 23 marks the start of overweight and 27.5 the start of obesity.

• Muscular builds: athletes and people who lift weights can read as overweight or obese with low body fat, because muscle is denser than fat.
• Older adults: muscle mass falls with age, so a "healthy" BMI can hide a high body-fat percentage. Some guidance treats a BMI slightly above 25 as acceptable over 65.
• Pregnancy: BMI is not meaningful during pregnancy; use your pre-pregnancy weight when discussing weight gain with a midwife.
• Where fat sits: fat around the waist carries more health risk than fat on the hips. A waist measurement more than half your height is a common warning sign at any BMI.
• Children: use BMI-for-age charts, not these adult bands.

Weigh yourself first thing in the morning, after using the toilet and before eating, in light clothing. Measure height barefoot against a wall with your heels, back and head touching it, and mark the top of your head with a flat object such as a book. Small errors matter less than you might think: being 1 cm out on height changes BMI by roughly 0.3.