Body Mass Index shows up at the doctor's office, on fitness apps and in health headlines. It's a single number meant to flag whether your weight is in a healthy range for your height. It's genuinely useful as a quick screen — and genuinely limited. Here's how to read it properly.
How BMI is calculated
BMI is simply your weight in kilograms divided by your height in metres squared (kg/m²). That's it — no body-fat scan, no measurements, just weight and height. Its simplicity is exactly why it's so widely used, and also why it can mislead.
The standard categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These ranges are designed for adults and are the same for men and women.
What BMI does well
Across large populations, BMI correlates reasonably well with body fat and with the risk of weight-related conditions. It's cheap, fast and needs no equipment — which makes it a sensible first screening tool for doctors and public health.
Where BMI falls short
- It can't tell muscle from fat. A muscular athlete can register as "overweight" despite very low body fat.
- It ignores where fat sits. Fat around the abdomen carries more health risk than fat elsewhere, and BMI can't see that.
- It varies by age, sex and ethnicity. The same BMI can mean different things for a 25-year-old and a 70-year-old, or across different populations.
- It's not designed for children the same way — kids use age- and sex-specific percentiles.
How to use your BMI sensibly
Treat BMI as one data point, not a verdict. If yours falls outside the healthy range, it's a prompt to look closer — perhaps with waist measurement, body-fat estimates or a chat with a doctor — not a diagnosis on its own. If you're very muscular, BMI will overstate your risk; read it with that in mind.
Check yours
You can work out your BMI in seconds with our free BMI calculator — it accepts both metric and imperial units and shows which category you fall into.