Devised in way back in 1830, the BMI is used to as a way of measuring a person's body shape. Put simply, it is calculated by dividing the body's weight by the square of their height. Eg: Kilograms: weight, divided by Metres (Meters): height.
Bmi is a variant of the Quetelet index. If you have a Prime of less than 0.74, you are underweight. If your prime is more than 0.74 and less than 1.00, your weight is ideal. If your BMI prime is more than 1 you are probably overweight.
BMI is particularly inaccurate for people who are very fit or athletic, as their high muscle mass can classify them in the overweight category by BMI, even though their body fat percentages frequently fall in the 10-15% category, which is below that of a more sedentary person of average build who has a normal BMI number. See wikipedia.org/wiki/Body_mass_index#Limitations for further clarification.