BMI Calculator

Understanding BMI

Body Mass Index, or BMI, is a simple index of weight-for-height that is commonly used to classify 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 above. This applies to men and women over the age of 18.

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People who are overweight or obese (i.e. a higher BMI), have a greater chance of developing many diseases, such as Heart Disease, diabetes, high blood pressure, and several types of cancer.


BMI does not measure body fat, and cannot differentiate between body fat and muscle mass. People who have the same BMI can have a different percent body fat. For example, a bodybuilder with a large muscle mass and a low percent body fat may have the same BMI as a person who has more body fat, because BMI is calculated using height and weight only.


In adults, BMI changes very little with age. However, in children, BMI changes substantially with age. Therefore, child BMI needs to be assessed using BMI-for-age charts, where the child's BMI is plotted against their age. This will indicate whether the child is underweight, in the normal range, overweight or obese.

Waist Circumference

The size of your waist (regardless of your height) provides a good indication of your risk of developing chronic diseases such as diabetes, heart disease and some cancers. The World Health Organisation defines the following cut points for the waist circumference in Caucasians. These figures may very slightly within different ethnic groups.

Increased Risk Men: more than 94 centimetres Women: more than 80 centimetres

Greatly Increased Risk Men: more than 102 centimetres Women: more than 88 centimetres