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BMI is weight divided by height squared
Nothing more. It was devised in the 1830s as a way to describe populations, not individuals, and it survives because it needs only two numbers that anyone can measure. What it gives you is a position on a distribution, not a verdict about a body.
The cut-offs are not the same everywhere
The WHO scale calls 25 overweight and 30 obese. The Asia-Pacific scale, used across much of East and South Asia, moves those to 23 and 25, because metabolic risk appears at a lower BMI in those populations. The same number reads differently depending on which scale is applied, so this page shows both.
It cannot tell muscle from fat
A trained athlete and a sedentary person of the same height and weight get the same BMI, and the reading is wrong for at least one of them. It also misses normal-weight obesity, where BMI sits inside the healthy band but body fat is high. Waist circumference catches what BMI misses and takes ten seconds to measure.
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Frequently asked questions
QWhich scale should I use?
Use the one your local health service uses. In Europe, the Americas and Africa that is the WHO scale, with overweight at 25 and obesity at 30. Across much of East and South Asia the Asia-Pacific scale applies, moving those to 23 and 25. Both readings are shown here so you can see how far apart they are.
QIs BMI valid for athletes?
Not really. Muscle is denser than fat, so a heavily trained body often lands in the overweight or obese band on a healthy composition. If you train seriously, body-fat percentage or waist-to-height ratio describes you far better.
QDoes BMI work for children?
Not directly. Children are assessed on growth percentiles for their age and sex, because the healthy range shifts as they grow. Using an adult cut-off on a child gives a meaningless answer.
QWhat about older adults?
The healthy band tends to sit slightly higher after about 65. Some loss of muscle is normal with age, and a BMI at the low end is associated with worse outcomes rather than better ones. Read the number as one input, not a target.