What is BMI and what value is normal?
BMI (body mass index) is your weight in kilograms divided by the square of your height in metres; for adults, the World Health Organization considers a value between 18.5 and 24.9 normal.
Above 25 means overweight, and from 30 up, obesity. BMI is a screening tool, not a diagnosis.
How is BMI calculated?
Divide your weight (kg) by your height in metres squared. The result is then placed in the WHO categories for adults over 18.
The calculator also shows the weight range that would give a BMI between 18.5 and 24.9 at your height.
Formula
- BMI = weight (kg) ÷ height (m)²
- Weight with normal BMI = 18.5 × height² to 24.9 × height²
Worked example
A person weighing 70 kg at 175 cm: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which is normal weight. At 175 cm, a normal BMI corresponds to a weight between 56.7 and 76.3 kg.
What are the WHO BMI categories?
| Category | BMI (kg/m²) | Weight at 170 cm |
|---|---|---|
| Underweight | below 18.5 | below 53.5 kg |
| Normal weight | 18.5–24.9 | 53.5–72.0 kg |
| Overweight | 25.0–29.9 | 72.3–86.4 kg |
| Obesity class I | 30.0–34.9 | 86.7–100.9 kg |
| Obesity class II | 35.0–39.9 | 101.2–115.3 kg |
| Obesity class III | 40 or above | above 115.6 kg |
What does BMI not tell you?
- It does not distinguish fat from muscle: a muscular athlete can have an overweight BMI without excess fat.
- It does not show where fat is stored; waist circumference says more about cardiometabolic risk.
- The categories are for adults; for children and teenagers the WHO growth charts by age and sex are used.
- It does not apply during pregnancy, and for older adults and some Asian populations the risk cut-offs may differ.
When should you talk to a specialist?
See your GP if your BMI is below 18.5 or above 30, if you have lost or gained a lot of weight without trying, or if you have other signs such as high blood pressure or raised blood sugar.
A nutritionist or dietitian can measure your body composition (fat, muscle) and build a plan that suits you.
Working with a nutritionist? See how they follow you in Rod