The body mass index (BMI) is a measure of association between the mass and size of an individual devised by the Belgian statistician Adolphe Quetelet, so also known as Quetelet index.
It is calculated according to the mathematical expression: body weight / (height *height)
where the mass is expressed in kilograms to the square of the height in squared meters, with the unit of measure of BMI in the MKS system
The value obtained is not constant but varies with age and gender. It also depends on other factors such as the proportions of muscle and adipose tissue. In the case of adults it has been used as one of the means to assess their nutritional status, according to those proposed by the World Health Organization values.
You can find a good BMI calculator here provided by the National Heart, Lung and Blood Institute.