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The body mass index (BMI) formula is used to determine a person's body mass index, an indirect measure of body fatness. The same formula is used for both adults and children, regardless of gender or ethnicity. Before you start using the formula to calculate your body mass index you'll need to decide which units of measurement you're using -- the formula changes somewhat, depending on whether you're using the metric system or the English system of measurement.

BMI Formula: An Overview

Body mass index, or BMI for short, is an indirect measure of a person's body fatness. The BMI formula is the same for men, women, and children of all ethnic backgrounds (see BMI for Children for information on interpreting a child's BMI results). The formula does differ slightly, depending on whether you are using the metric system or the English system.
 
(Click BMI and Weight to learn more about other factors healthcare providers consider when determining risk for conditions related to weight.)
 

Using the Metric System

If you're using the metric system, the formula for BMI is:
Weight in kilograms divided by height in meters squared (weight (kg) / [height (m)2]).
 
Since height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters before you use the formula.
 
To calculate BMI, take the weight (kg) and divide it by height (m). Then divide that number once again by height (m). Round to the second decimal place.
 
An example of calculating BMI using the body mass index formula: Height = 165 cm (1.65 m), Weight = 68 kg
 
BMI Calculation: 68 ÷ (1.65)2 = 24.98
 
Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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