In the mid 1800's Belgian scientist Adolphe Quetelet developed the Quetelet index, which is known today as the body mass index (BMI). This statistical measurement compares a person's height and weight and is a useful tool to identify weight problems within a population or for an individual.

Though a BMI Calculator cannot measure the actual percentage of body fat, its ease of calculation makes it a popular diagnostic tool for health experts.

The body mass index is defined as an individual's body weight (in kilograms) divided by the height (in meters) squared. For imperial units, the formula is modified as follows:

Height in Inches: [weight (lb) * 703] / [height (in) * height (in)]

Height in Feet: [weight (lb) * 4.88] / [height (ft) * height (ft)]

Body mass index was always intended to be a general guide for health professionals. However, doctors must consider other factors on a case-by-case basis. For example, muscle density, bone density, and ethnic origin can skew the results of a BMI Calculator. For a general guideline, the body mass index is broken into separate categories outlined below with the corresponding BMI:

Severely Underweight - Less than 16.5

Underweight - 16.5 to 18.5

Normal - 18.5 to 25

Overweight - 25 to 30

Obese Class I - 30 to 35

Obese Class II - 35 to 40

Obese Class III - Over 40

Using the BMI Calculator, a health and nutrition survey in 1994 found that 59% of American men and 49% of women have a body mass index over 25. Individuals with a BMI higher than 40 are considered morbidly obese, which accounts for 2% of the men and 4% of the women in America. A more recent survey in 2007 shows an increase in America's weight, with 63% of Americans considered overweight and 26% in the obese category.

For individuals who are underweight, a BMI close to 15 is an indicator of starvation, and 17.5 is an informal indicator of eating disorders such as anorexia nervosa.

It is very common to consider the BMI Calculator in the medical underwriting of private health insurance plans. Providers will use a high BMI as a cut-off point to raise insurance rates or deny coverage. Furthermore, the BMI is often used by surgeons to determine if a patient qualifies the Gastric Band procedure.

Typically, doctors are looking for a BMI of 35 or higher when considering potential candidates. Each case should be considered on an individual basis, but the BMI calculator does serve as a useful tool.