Body mass index (BMI) is a measurement that compares your weight to your height. Population studies use it to identify weight problems in a population group. BMI is easy to measure and calculate, making it convenient to use on large groups.
Definition
BMI is defined as a person' body weight divided by the square of his height. It's therefore measured in units of mass per units of length squared. The most common unit of measure for the BMI is kilograms per square meter (kg/m^2.) You can convert a BMI that's in units of pounds per square inch (lbs/in^2) to a BMI in kg/m^2 by multiplying by the conversion factor of 703.
Origins
The formula for the BMI originated in the 19th century, although the term "body mass index" wasn't used to describe this ratio until the early 1970s. The rapid growth in popularity of the BMI at this time was primarily due to growing concerns about obesity in developed countries.
Usage
BMI is a statistical measure that's appropriate for use only in population studies, where it may be applied to a group of people. The BMI classifications used by the World Health Organization (WHO) assume that the population group is sedentary with an average body composition. BMI shouldn't be used to diagnose individuals, although it's frequently used for this purpose due to its ease of use.
Classification
The WHO BMI categories classify a BMI of less than 18.5 as underweight. A BMI between 18.5 and 25 indicates a normal weight; a BMI that's between 25 and 30 is considered pre-obese. The WHO classifies BMI values between 30 and 35 as obese Class 1. BMI values greater than 35 but less than 40 is obese Class 2; BMI values greater than 40 are obese Class 3.
BMI Prime
The BMI Prime is a modification of the raw BMI score that's defined as the raw BMI divided by the BMI for the upper limit of the normal weight range. This value is currently 25, so a BMI of 23 is equal to a BMI Prime value of 23 / 25 = 0.92. The BMI Prime is useful because it allows a researcher to easily identify the percentage by which a population group deviates from its upper weight limit.
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