Body Mass Index (BMI)

Reliability

BMI is generally used as an indicator of body adiposity, although it does not measure body fat directly. However, research has demonstrated the BMI to be a reliable indicator of body fatness when correlated with direct measurements such as underwater weighing, skin fold thickness measurements, computed tomography (CT), and dual-energy X-ray absorptiometry (DEXA). The BMI is a simple, inexpensive method for estimating body adiposity and is a more practical method for health care practitioners and the general public to use.

Calculation

BMI can be calculated using metric units or by an adapted version with imperial units. The same formula is used for both adults and children.

Interpretation

Although the BMI calculation for adults and children is the same, the interpretation of the calculation is different for each age group. Adults, 20 years of age or older, use the standard weight categories for all ages and both genders. Underweight adults have a BMI of less than 18.5; healthy weight adults have a BMI of at least 18.5 but less than 25; overweight but not obese adults have a BMI of at least 25 and less than 30; and obese adults have a BMI of 30 or more. BMI and weight status categories can vary slightly depending on the reporting agency.

BMI interpretation for children and adolescents are age and gender specific and use the BMI percentile. The calculated BMI for children and adolescents is plotted on the BMI-for-age growth chart to obtain the BMI percentile. Percentiles are commonly used in the United States to assess the growth of an individual child as compared with children of the same gender and age. Gender and age are significant when assessing a child's growth pattern because the amount of body fat changes with age and the amount of body fat differs between genders. Weight status based on the BMI percentile for children and adolescents are as follows: underweight children or teens with a BMI-for-age that is less than the 5th percentile; normal weight children or teens with a BMI-forage that is at least the 5th percentile but less than the 85th percentile; at risk for overweight children or teens with a BMI-for-age that is at least the 85th percentile but less than the 95th percentile; and overweight children or teens with a BMI-for-age greater than the 95th percentile.

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