What is BMI?
Body mass index (BMI) is your weight divided by the square of your height. It shows how heavy you are for your height. Because it is simple and correlates well with body fat, it is the most widely used screen for underweight, overweight and obesity in adults.
BMI = weight (kg) ÷ height (m)²
For example, 65 kg at 170 cm gives 65 ÷ (1.7 × 1.7) ≈ 22.5. With pounds and inches, use 703 × weight (lb) ÷ height (in)²: 5 ft 9 in (69 in) and 160 lb give 703 × 160 ÷ 69² ≈ 23.6. This calculator rounds BMI to one decimal place and then compares it with the chosen table.
Why the cut-offs differ by country
The World Health Organization (WHO) classes a BMI under 18.5 as underweight, 18.5 to 24.9 as normal, 25 or more as overweight and 30 or more as obese, with obesity split into three classes at 30, 35 and 40. Evidence has built up, however, that Asian populations show a different link between BMI, body fat and health risk than European populations, and that many Asian people already have a high risk of type 2 diabetes and heart disease below a BMI of 25. In 2004 a WHO expert consultation kept the international cut-offs but listed extra public health action points at BMI 23, 27.5, 32.5 and 37.5 for countries to consider.
- Korea: the Korean Society for the Study of Obesity (KSSO) 2022 guideline calls 23 to 24.9 pre-obese (overweight or at-risk weight) and 25 or more obese, with classes I to III starting at 25, 30 and 35. Class III is also called severe obesity.
- Japan: the Japan Society for the Study of Obesity (JASSO) calls 25 or more obese, in grades 1 to 4. Being obese does not by itself mean "obesity disease" that needs weight loss, and a BMI of 22, where illness is least common, is used for standard body weight.
- China: health standard WS/T 428-2013 (weight criteria for adults) classes 18.5 to 23.9 as normal, 24 to 27.9 as overweight and 28 or more as obese.
So the same BMI of 24 is normal under WHO and Japanese cut-offs, pre-obese in Korea and overweight in China. Switch the standard in the calculator above to compare.
Limits of BMI
- It cannot tell muscle from fat. Very muscular people can have a high BMI without much body fat.
- It does not show where fat sits. Fat around the waist is more closely tied to health risk, so waist circumference is checked too. KSSO, for example, defines abdominal obesity in Korean adults as a waist of 90 cm or more in men and 85 cm or more in women.
- Older adults and people who are pregnant or breastfeeding: the link between BMI and body fat is weaker.
- Children and teens are still growing, so adult cut-offs do not apply. They are assessed with age- and sex-specific growth references; WHO uses BMI-for-age for ages 5 to 19.
BMI is a starting point, not a diagnosis. Blood pressure, blood sugar, cholesterol and habits matter too, so discuss your results with a health professional.
How to measure accurately
Body weight changes during the day, so weigh yourself at the same time and under the same conditions each time, for example in the morning before eating, after using the toilet and in light clothing. Measure height without shoes, with heels, buttocks and shoulders against a wall. Your last values, units and standard are saved only in this browser, so they are still there next time.