Health5 min readElena VaughnUpdated 2026-08-19
How accurate is BMI, really
BMI is everywhere, but it has clear limits. Here is what it is good for and when you need other numbers.
The body mass index shows up at every check-up and in every health app. It is quick to work out, which is exactly what it was built for. As a rough sort for large groups it is useful. As a verdict on one person it has clear weaknesses.
What BMI measures
BMI relates only weight to height. The WHO groups adults like this: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight and 30 and above is obesity.
18.5-24.9
Where BMI falls short
BMI cannot tell muscle from fat. A muscular athlete often lands in the overweight band even though their body fat is low. The other way round, someone can have a normal BMI and still carry a lot of belly fat.
- It ignores where the fat sits. Belly fat carries more health risk.
- It fits poorly for very muscular or very tall people.
- Different thresholds apply for some population groups.
94 / 80 cm
Frequently asked questions
Only to a point. Muscle is heavy, so muscular people often land in the overweight band even though their body fat is low. Add a body fat measurement alongside it.
The WHO names 18.5 to 24.9 as the healthy range for most adults. Values below that count as underweight, 25 and up as overweight.
For training goals, yes. Body fat percentage says more about your composition. For a quick screening, BMI stays practical.