What Is Body Mass Index (BMI)?

What to know

Body mass index (BMI) is a calculated measure of body weight relative to height. Annual BMI screening is recommended as part of routine pediatric care. Although BMI does not directly measure adiposity (body fatness), BMI is correlated with direct measures of adiposity.

The letters "B, M, I" spelled out on blocks below a tape measure.

What is BMI?

A measure of weight relative to height

Body mass index (BMI) is a calculated measure of body weight relative to height.[1] Metric measures are in kilograms (kg) and meters (m) or centimeters (cm). The US customary unit measures are in pounds and inches (in). The equations for calculating BMI by hand are:

  • Metric units: BMI = weight (kg) / [height (m)]2
  • Metric units: BMI = [weight (kg) / height (cm) / height (cm)] x 10,000
  • US customary units: BMI = weight (pounds) / [height (in)]2 x 703

BMI Calculator‎

CDC's Child and Teen BMI calculator calculates and plots BMI.

A screening measure

The American Academy of Pediatrics (AAP)/Bright Futures Preventive Care Guidelines recommend annual BMI screening for children and adolescents 2 years and older as part of routine pediatric care.[2] Screening can identify whether a BMI falls into the underweight, healthy weight, overweight, obesity, or severe obesity category. Obesity is associated with health problems such as asthma, sleep apnea, bone and joint problems, type 2 diabetes, and risk factors for heart disease such as high blood pressure.

Children are growing. As a result, their BMI values need to be expressed relative to other children of the same sex and age. These are called BMI-for-age percentiles. Read more about Child and Teen BMI Categories.

An indirect measure of adiposity

BMI is correlated with direct measures of adiposity (body fatness), such as dual energy x-ray absorptiometry (or DEXA) scans. However, BMI is not a direct measure of adiposity. BMI does not distinguish between fat and lean body mass, nor does it capture the body's fat distribution. Compared to direct measures of adiposity, BMI is simpler, less expensive, less invasive, and is collected in routine pediatric health care.

Recommendations

To monitor the growth of children and adolescents 2 years and older, CDC recommends that health care providers use the:

Test your knowledge

  1. Body mass index (BMI) is a calculated measure of body weight relative to height.
    1. True
    2. False
  2. BMI percentiles are age- and sex-specific for children and adolescents. Calculating BMI as a percentile based on age and sex allows health professionals to screen children and adolescents for underweight, healthy weight, overweight, obesity, or severe obesity:
    1. True
    2. False
  3. Which statement accurately describes BMI?
    1. BMI is correlated with direct measures of body fat.
    2. BMI directly measures body fat.

See answers.A

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Content Source:
National Center for Chronic Disease Prevention and Health Promotion (NCCDPHP)
  1. Question 1: True. BMI is a calculated measure of body weight in kilograms divided by height in meters squared. Question 2: True. Because children and adolescents are growing, their BMIs need to be expressed relative to other children or adolescents of the same sex and age. Question 3: A. BMI does not directly measure body fat nor distinguish between fat and lean body mass. However, BMI correlates with direct measures of body fat.
  • Keys A, Fidanza F, Karvonen MJ, et al. Indices of relative weight and obesity. J Chronic Dis. 1972;25(6):329–343. doi: 10.1016/0021-9681(72)90027-6
  • Hampl SE, Hassink SG, Skinner AC, et al. Executive Summary: Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics. 2023;151(2):e2022060641. https://doi.org/10.1542/peds.2022-060640