What is the Body Mass Index Formula?

The body mass index (BMI) formula is a calculation meant to measure body fat. The formula is based on a person’s height and weight and is presented in ranges to classify people as underweight, normal weight, overweight, and obese. There are several possible computations for the body mass index formula depending on the units of measurement. A Belgian mathematician developed the body mass index formula in the mid-1800s; while it is still used today, the concept has come under some criticism for its failure to account for different body types.

Using pounds and inches, the formula for body mass is to multiply an individual’s weight in pounds by 703. That number is then divided by height in inches squared. The result is the BMI. For instance, the BMI for a 150 pound person who is 5 feet, 5 inches tall is (150*703)/(65*65). This person’s BMI equals 24.95, which likely would be rounded up to 25, making this individual overweight.

There also is a body mass index formula in metric units. The metric formula is weight in kilograms divided by meters tall squared. The resulting BMI falls into the same classification as the calculation using pounds and inches.

A body mass index chart plots BMI results to determine a person's weight category. A BMI of below 18.5 is considered underweight. Normal weight falls into a range of 18.5 to 24.9. Overweight is considered 25 to 29.9; any result above 30 is obese. Some charts further break down obese categories into class I (a BMI of 30 to 34.9), class II (35 to 39.9, and class III or morbid obesity (40 and above). BMI may be computed by hand, but there also are a number of online sites that provide a body mass index calculator.

A Belgian mathematician and statistician named Adolphe Quetelet developed the body mass index formula in 1832; it was first known as the Quetelet Index. Quetelet studied the physical characteristics that made up an average person. Measuring hundreds of subjects, he determined that weight was proportionate not to height, but to height squared. The formula remain unchanged and came to be known as the body mass index in 1972.

While still used as a standard measurement of body fat, the BMI formula is subject to some criticizism. The calculation assumes an average body type that does not take bone density or muscle mass into account. Both of these factors affect BMI and could deem a fit, muscular person overweight as often is the case with professional athletes. Some medical staff prefer additional tests, such as waist circumference, to determine whether a person is at risk because of his or her weight.