Body mass index, or BMI, is a measurement of body fat determined by a simple calculation using height and weight. The Z score, the number resulting from the calculation, indicates where you fall in the range of body fat measurements. For people ages 20 and older, a Z score below 18.5 means you are underweight, 18.5 to 24.9 means you fall in the normal range, 25.0 to 29.9 means you are overweight and a score of 30.0 and higher means you are obese.
Decide whether you will be using pounds and inches or kilograms and meters to determine your BMI.
Convert your height from centimetres to meters for the metric calculation or from feet to inches for standard calculation. For example, a height of 152 centimetres, which is approximately 5 feet, becomes 1.52 meters or 60 inches.
Divide your weight by your height squared. If you are using metric measurements, divide your weight in kilograms by your height in meters squared. For example: 45kg / (1.52m)² = 19.5. If you are using pounds and inches, divide your weight in pounds by your height in inches squared, and then multiply by 703. For example: 45.4kg/ (60 in)² x 703 = 19.5. Both calculations result in a BMI of 19.5, which is in the normal range.
While the BMI calculation is accurate for many people, it does not account for lean muscle mass. If you are very active, your BMI may be high due to your muscle composition rather than body fat.
The Z score resulting from this calculation is intended for measuring women and men who are 20 years old and older. There are separate score charts for children younger than 20 that account for age, sex and the varying degrees of body fat during growth.