How to use the BMI calculator
- Enter your height in centimetres (for example 175).
- Enter your weight in kilograms (for example 70).
- Click "Calculate BMI" to see your body mass index and its category.
- Compare the result against the standard categories below.
What BMI measures
Body mass index is a number calculated from your height and weight: weight in kilograms divided by height in metres squared. It is a quick screening tool that places people into broad categories, from underweight to obese. The formula is BMI = weight (kg) / height (m) squared.
BMI categories
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obese | 30.0 and above |
A note on limitations
BMI is a screening measure, not a diagnosis. It does not distinguish muscle from fat, so athletes with a lot of muscle can register as overweight or obese despite being healthy, while some people in the normal range may still carry excess fat. Age, sex and build all influence what a healthy weight looks like. If you have concerns about your weight or health, speak to a doctor or another qualified professional rather than relying on this number alone.
How to measure height and weight accurately
For the most useful result, measure your height without shoes and your weight with light clothing and an empty stomach. Small errors in measurement shift the result, especially for people close to a category boundary, so round to the nearest centimetre and kilogram.
BMI for children and teenagers
The standard adult categories do not apply to children and teenagers, because their bodies are still growing. For young people, BMI is interpreted against age- and sex-specific growth charts rather than the adult ranges. This calculator is intended for adults.
Waist circumference as a companion measure
Many health professionals look at waist measurement alongside BMI, because where fat is stored matters. A large waist can indicate higher risk even when BMI is in the normal range. Waist circumference is a simple addition you can track with a tape measure at home.