Body Mass Index - or BMI - is used medically to assess whether someone is overweight, underweight or normal. It is a ratio that compares a persons weight with their body surface area. As people don't know their surface areas, the person's height is used to calculate the surface area (see the formulas below).

BMI is very useful for normal people. However, if does not take account of very muscular people, as muscle is heavier than fat and so they have an abnormally high BMI even when they are not overweight.

Therefore please feel free to use this calculator as a guide, but if  it shows you to be something you feel is wrong, please discuss it with a health care professional.