Body Mass Index (BMI) is a tool for indicating nutritional status in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9.

BMI = 70 (kg) / 1.752 (m2) = 22.9

For adults over 20 years old, BMI falls into one of the following categories:

See BMI chart below.

The BMI ranges are based on the effect excessive body fat has on disease and death.  As BMI increases, the risk for some diseases increases.  Some common conditions related to overweight and obesity include: premature death, cardiovascular disease, high blood pressure, osteoarthritis, some cancers and diabetes.

BMI is the current frequently used weight/height ratio in schoolage children and adolescents. With the release of the new WHO standards for BMI-for-age in 2006 for pre-school children (0-5 years), this index is also recommended for use in this population group.

During childhood and adolescence the ratio between weight and height varies with sex and age. The cut-off points of the 2006 BMI-for-age reference for children from 0 to 5 years of age for the diagnosis of overweight and obesity have been set as the 97th and the 99th percentile respectively.