Body Mass Index (BMI)

What is BMI?
Body mass index or BMI is an anthropometric index of weight and height. BMI is commonly accepted as the index for classifying weight in adults and children.

How is BMI used?
BMI is a screening tool used to identify individuals who are underweight or overweight. However, BMI is not a diagnositic tool.

BMI Classifications
The BMI classification is determined differently for adults and children or adolescents. They are based on morbidity and mortality data to determine if an individual is underweight or overweight based on mortality risk.

BMI Classification for Adults
The BMI classification for adults is defined by fixed cut points derived from morbidity and mortality data. Adults classified as underweight or overweight are at a higher relative mortality risk than those who fall in the normal range. BMI is measured as weight divided by height. The classification is the same for males and females.The guidelines below were established in 1998 by the National Heart, Lung, and Blood Institute.

BMI Classification for Children and Adolescents
The BMI classification for children and adolescents are gender specific and age specific. Since BMI changes substantially as childrent get older, BMI-for-age is the measure used for children ages 2 to 20 years.

The normal pattern of growth that occurs in children is typically a decrease in BMI-for-age after about 1 year of age that continues falling during the preschool years until it reaches a minimum around 4 to 6 years of age. Then BMI-for-age begins a gradual increase through adolescence and most of adulthood. This increase in BMI after lowpoint at ages 4 to 6 is referred to as adiposity rebound.

A child who is relatively heavy may have a high BMI for his or her age or high weight-for-height stature. To determine if the child has excess fat, further assessment would be needed. This might include skinfold measurements, assessment of diet, health, physical activity in order to determine an appropriate counseling strategy. The guidelines below were determined by the World Health Organizations Expert Commitee on Physical Status.

BMI Calculator
As an alternative to calculating BMI by hand (weight ÷ height), there are a variety of BMI calculators available. The link to the CDC's National Center for Chronic Disease Prevention and Health Promotion offers a BMI calculator:

BMI Calculator for Adults
http://www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.htm
Enter height and weight to calculate BMI. This link also provides additional information on BMI.

BMI-for-Age for Children & Adolescents
http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.htm
BMI-for-age is plotted on gender specific growth charts. These charts are used for children and teens 2 – 20 years of age. This site has a link to 2000 CDC Growth Charts used to determine BMI for children and adolescents.