Body mass index — weight in kilograms divided by height in metres squared — is one of the most used numbers in health. It is also one of the most misused. Is BMI accurate? It is accurate at describing populations and misleading at judging individuals. Both halves of that sentence matter.
What BMI actually measures
BMI measures weight relative to height. It does not measure fat, muscle, bone density or where fat sits on the body. Two people can share a BMI of 27 — one carrying extra muscle from years of training, the other carrying extra abdominal fat — and have very different health profiles. Use the BMI Calculator to find your number, then read it with the caveats below.
Where BMI is useful
Across large groups, higher BMI tracks with higher risk of type 2 diabetes, heart disease and several other conditions, and the WHO bands (underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, obese 30 and above) give researchers and clinicians a cheap, consistent screening tool. As a first flag — “this is worth a closer look” — BMI works.
Where BMI misleads
- Muscle: athletes and regular lifters are routinely classed overweight or obese despite low body fat.
- Fat distribution: abdominal (visceral) fat carries more risk than fat on hips and thighs, but BMI cannot tell the difference.
- Age and sex: older adults lose muscle, so a “healthy” BMI can hide excess fat; women naturally carry more essential fat than men at the same BMI.
- Ethnicity: risk rises at lower BMIs in some South Asian populations and at higher BMIs in some others, which is why some guidelines use lower cut-offs.
- Short and tall people: the squared-height formula slightly penalises shorter people and flatters taller ones.
What to check instead — or as well
Waist circumference, waist-to-height ratio (aim for waist under half your height), how your weight trends over months, blood pressure, blood lipids and glucose, fitness and strength, and how you sleep and feel. No single number tells your story; a handful of them together do.
The bottom line
Use BMI as a signal, not a verdict. If it puts you in the overweight or obese band, treat that as a prompt to look at waist, habits and blood markers — not as proof you are unhealthy. If it puts you in the healthy band, do not let it excuse a sedentary life. Health is a pattern, and BMI is only one line of it.
See the methodology for the WHO bands and the formula.