How to Calculate BMI (Body Mass Index)

Formula

BMI = weight (kg) / height (m)²

• weight: your body weight in kilograms
• height: your height in metres. Divide centimetres by 100: 170 cm is 1.7 m.
• BMI: the result is in kg/m² and is rounded to one decimal. The category uses the rounded value.

The World Health Organization (WHO) adult categories are: below 18.5 underweight, 18.5–24.9 normal, 25.0–29.9 overweight, 30.0 or more obesity.

Step-by-step example

An adult who weighs 68 kg and is 170 cm tall:

1. Convert the height to metres: 170 cm = 1.7 m.
2. BMI: 68 / 1.7² = 23.5 kg/m².
3. Category: 23.5 is in the 18.5–24.9 range, so the result is normal.
4. Healthy weight range: from 18.5 × 1.7² = 53.5 kg to 24.9 × 1.7² = 72 kg.

Examples of the BMI categories

Rounding matters at the limits. At 170 cm, 72.2 kg gives a BMI of 25.0, which is overweight; the top of the healthy range for that height is 72 kg.

What BMI does not show

BMI does not tell muscle, fat and bone apart. A muscular person can have a high BMI with little body fat. The categories are for adults of 18 or older; children and teens use percentiles by age and sex, and BMI is not read in pregnancy. Waist size and other risk factors are not taken into account either. Ask your doctor about decisions on your weight.