PurposeThe BMI gives healthcare professionals a consis-
tent way of assessing their patients’ weight and an
objective way of discussing it with them. It is also
useful in suggesting the degree to which the patient
may be at risk for obesity-related diseases.DescriptionBMI is a statistical calculation intended as an
assessment tool. It can be applied to groups of people
to determine trends or it can be applied to individuals.
When applied to individuals, it is only one of several
assessments used to determine health risks related to
being underweight, overweight, or obese.The history of BMI
The formula used to calculate BMI was developed
more than one hundred years ago by Belgian mathe-
matician and scientist Lambert Adolphe Quetelet
(1796-1874). Quetelet, who called his calculation the
Quetelet Index ofObesity, was one of the first statis-
ticians to apply the concept of a regular bell-shaped
statistical distribution to physical and behavioral fea-
tures of humans. He believed that by careful measure-
ment and statistical analysis, the general characteristics
of populations could be mathematically determined.
Mathematically describing the traits of a population
led him to the concept of the hypothetical ‘‘average
man’’ against which other individuals could be meas-
ured. In his quest to describe the weight to height
relationship in the average man, he developed the for-
mula for calculating the body mass index.
Calculating BMI requires two measurements:
weight and height. To calculate BMI using metric
units, weight in kilograms (kg) is divided by the height
squared measured in meters (m). To calculate BMI in
imperial units, weight in pounds (lb) is divided by
height squared in inches (in) and then multiplied by- This calculation produces a number that is the
individual’s BMI. This number, when compared to the
statistical distribution of BMIs for adults ages 20–29,
indicates whether the individual is underweight, aver-
age weight, overweight, or obese. The 20–29 age group
was chosen as the standard because it represents fully
developed adults at the point in their lives when they
statistically have the least amount of body fat. The
formula for calculating the BMI of children is the
same as for adults, but the resulting number is inter-
preted differently.
Although the formula for calculating BMI was
developed in the mid-1800s, it was not commonly
used in the United States before the mid-1980s. Until
then, fatness or thinness was determined by tables that
set an ideal weight or weight range for each height.
Heights were measured in one-inch intervals, and the
ideal weight range was calculated separately for men
and women. The information used to develop these
ideal weight-for-height tables came from several
decades of data compiled by life insurance companies.
These tables determined the probability of death as it
related to height and weight and were used by the
companies to set life insurance rates. The data excluded
anyone with a chronic disease or anyone who, for
whatever health reason, could not obtain life insurance.
Interest in using the BMI in the United States
increased in the early 1980s when researchers became
concerned that Americans were rapidly becoming
Body/mass index can be calculated by locating your height
and weight on the chart and drawing a diagonal line between
the two. Where the line crosses over the third bar is the
approximate BMI.(Illustration by Argosi, Inc./Thomson Gale.)Body mass index