How to Calculate and Interpret BMI (Body Mass Index)
The BMI formula, WHO categories, its limitations and when to talk to a doctor.
Body mass index (BMI) relates weight to height. It is not a precise health measure, but it gives a quick general picture.
How it is calculated
BMI = weight (kg) ÷ height (m)². For someone 70 kg and 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9.
WHO categories
- Below 18.5: underweight
- 18.5 – 24.9: normal
- 25 – 29.9: overweight
- 30 and above: obesity
Limitations
BMI cannot tell muscle from fat, so a muscular athlete may appear overweight. Age, sex and body build also matter. Adult categories do not apply to children, who are assessed with age- and sex-specific growth charts, and BMI is not suitable during pregnancy.
Waist circumference matters too
Fat around the abdomen can predict health risk better than weight alone. It is more informative to look at BMI together with waist size and overall health.
When to see a professional
If your BMI is outside the normal range, if your weight changes quickly, or if you have a chronic condition, talk to a doctor or dietitian. This tool is for information only and is not medical advice.