Body mass index, or BMI, is an easy-to-calculate measure of obesity based on the ratio of your weight and your height. It's convenient to use because it doesn't require expensive equipment or a clinical setting to measure -- just knowledge of your height and weight -- and it's useful for most people to get a general idea of your disease risk.
Because BMI is simply a measure of your weight versus your height, it doesn't take into account where that weight comes from -- lean tissue or fat. For this reason, you might have a normal "healthy" weight, according to your BMI, but still face health risks due to excess body fat.