The Body Mass Index, or BMI, is an index that is commonly used to identify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m squared).

BMI = weight (kg) / height (m) x height (m)

The classification of overweight and obesity, according to BMI, is shown in the table below. This applies to men and women over the age of18. There is a different index for adolescents and children (see links below).

YOUR BMI AND YOUR HEALTH

People who are overweight or obese, have a greater chance of developing many diseases, such as heart disease, diabetes,suffer from high blood pressure, and even some types of cancer - breast, colon, endometrium and prostate. 

BMI - BODY FAT Vs MUSCLE

BMI does not measure body fat, and it does not differentiate between body fat and muscle mass. So be careful how you use the index. People who have the same BMI can have a different percent body fat. For example, an athlete with a large muscle mass and a low percent body fat may have the same BMI as a person who has lots more body fat, because BMI is calculated using height and weight only.

BMI IN CHILDREN VARIES WITH AGE

In adults, BMI changes very little with age. However, in children, BMI changes substantially with age. Therefore, child BMI needs to be assessed using age related charts. This will indicate whether the child is underweight, in the normal range, overweight or obese.

BMI CALCULATORS