RunToolRunBMI Calculator
Calculate your body mass index.
How to Calculate Your BMI
Enter height and weight
Type your height in centimeters and weight in kilograms.
BMI is computed instantly
The standard formula weight ÷ height² runs in your browser.
Read your category
See your value on the WHO scale from underweight to obesity.
What BMI is and where it came from
Body mass index is weight in kilograms divided by height in metres squared. It was devised in the 1830s by a Belgian statistician studying the characteristics of populations — not by a physician, and not as a measure of individual health.
That origin explains both its usefulness and its limits. Across a large population it correlates usefully with body fat and with several health outcomes, which is why public health bodies still use it. Applied to one person, it is a rough screening number rather than a diagnosis.
What the number cannot see
BMI uses only height and weight, so it cannot distinguish muscle from fat. A well-trained athlete is frequently classified as overweight by BMI while carrying very little body fat. In the other direction, an older person who has lost muscle can fall in the healthy range while carrying a great deal of fat.
It also says nothing about where fat is distributed, which is one of the things that matters most. Abdominal fat carries higher metabolic risk than fat elsewhere, and two people with identical BMI can have quite different risk profiles for that reason alone.
The thresholds are not universal
The familiar categories — under 18.5, 18.5 to 25, 25 to 30, above 30 — were derived largely from European-descended populations. Several health authorities use lower thresholds for people of South Asian, Chinese and other Asian descent, because increased risk appears at lower BMI values in those groups.
The cut-offs are also boundaries on a continuum rather than steps. There is no meaningful health difference between a BMI of 24.9 and 25.1, and treating the category change as an event rather than a position on a scale misreads what the number is.
Who it does not apply to
Adult BMI categories do not apply to children and teenagers, who are assessed against age- and sex-specific growth charts because body composition changes throughout development. They also do not apply during pregnancy, and they are unreliable for people with very high muscle mass, limb loss, or certain medical conditions.
For a fuller picture, waist circumference, waist-to-height ratio and body composition measurement each add information that BMI structurally cannot provide.
Using it sensibly
BMI is most useful as one input among several, and as a way of tracking change in yourself over time, where the confounders stay roughly constant. It is least useful as a verdict on a single person at a single moment.
This is a calculator, not medical advice, and it cannot know anything about you beyond two numbers. Anything with health consequences belongs with a clinician who can see the rest of the picture. The calculation runs entirely in your browser, so the height and weight you enter are not transmitted or stored anywhere.