Calculating Body Mass Index

Calculating Body Mass Index

Body mass index (BMI) is useful for classifying the health risks of body weight if you don’t have access to more sophisticated methods. Though more accurate than height-weight tables, body mass index is also based on the concept that weight should be proportional to height. BMI is easy to calculate and rate. Researchers frequently use BMI in conjunction with waist circumference in studies that examine the health risks associated with body weight.

BMI is calculated by dividing your body weight (expressed in kilograms) by the square of your height (expressed in meters). The following example is for a person who is 5 feet, 3 inches tall (63 inches) and weighs 130 pounds:

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1. Divide body weight in pounds by 2.2 to convert weight to kilograms:

130 4 2.2 = 59.1

2. Multiply height in inches by 0.0254 to convert height to meters:

63 X 0.0254 = 1.6

3. Multiply the result of step 2 by itself to get the square of the height measurement:

1.6 X 1.6 = 2.56

4. Divide the result of step 1 by the result of step 3 to determine BMI:

59.1 4 2.56 = 23

An alternative equation, based on pounds and inches, is

BMI = [weight/(height X height)] X 703

Space for your own calculations can be found in Lab 6.1, and a complete BMI chart appears in Lab 6.2.

A BMI between 18.5 and 24.9 is considered normal and desirable under separate standards from the National Institutes of Health (NIH) and the World Health Organization (WHO). A person with a BMI of 25 or above is classified as overweight, and someone with a BMI of 30 or above is classified as obese (Table 6.1). A person with a BMI below 18.5 is classified as underweight, although low BMI values may be healthy in some cases if they are not the result of smoking, an eating disorder, or an underlying disease. A BMI of 17.5 or

Body Mass Index (BMI) Classifications


Underweight <18.5

Severe thinness <16.0

Moderate thinness 16.0-16.9

Mild thinness 17.0-18.4

Normal 18.5-24.9

Overweight 25.0-29.9

Obese, Class I 30.0-34.9

Obese, Class II 35.0-39.9

Obese, Class III $40.0

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