The BMI is calculated from the weight and the length of a person, to see if the patient is the correct weight for his length.
The BMI quantifies the amount of tissue mass (muscle, fat, and bone) in an individual, and is used to categorize that person as underweight, normal weight, overweight, or obese based on that value. (According to Wikipedia).
To work out your BMI:
divide your mass in kilograms (kg) by your height in metres (m)
then divide the answer by your height again to get your BMI.