An initiative of the Robert Wood Johnson Foundation

Methodology and Data Sources

Learn more about the data used on the site, including detailed descriptions of the major federal surveys featured.

The body mass index, a simple ratio of height and weight, is widely used to measure and track obesity rates. But we understand that there are limits to its usefulness, particularly in assessing individual health, and we know that wide overreliance on this measure has caused harm. Read more from Jennie Day-Burget, senior communications officer at the Robert Wood Johnson Foundation.

What is Obesity?

Having a body weight that is higher than what is considered healthy for a given height is described as overweight or obesity. There are many methods of measuring this, some of which are expensive and time consuming. BMI, which is inexpensive and easy to calculate, is typically used as a proxy. Health officials recommend that individual health assessments should consider other factors as well. Research has demonstrated that a high BMI is strongly correlated with negative health consequences, although the association between BMI does vary among ethnic groups.

BMI is a person’s weight in kilograms divided by their height in meters squared. For measurements in pounds and inches, BMI is calculated using the following formula:

BMI Formula

Defining Obesity Among Children and Teens

Because kids are still growing, obesity is measured differently among children than adults. Instead of a simple BMI measurement, a child’s BMI is compared to others of the same age and sex.

According to the Centers for Disease Control and Prevention (CDC), child obesity is defined as a BMI that is at or above the 95th percentile for children and teens of the same age and sex. Overweight is defined as a BMI that is at or above the 85th percentile and below the 95th percentile for children and teens of the same age and sex.

Why is BMI age- and sex-specific for children and teens?

A child’s weight status is determined using an age- and sex-specific percentile for BMI, which is different from BMI categories used for adults. Because children’s body fat levels change over the course of childhood and vary between boys and girls, their BMI levels are expressed relative to other children of the same age and sex.

A table showing body mass index categories for children. Below 5th percentile is underweight. Between 5th percentile and 85 percentile is healthy weight. Between 85th percentile and 95 percentile is overweight. Over 95th percentile is obese.
A table showing body mass index categories for adults. Below 18.5 BMI is underweight. Between 18.5 and 24.9 is healthy weight. Between 25 and 29.9 is overweight. Between 30 and 39.9 is obese. Over 40 is class 3 obesity.

Obesity Rates and Trends Data

This site relies on multiple survey instruments to paint a complete picture of childhood obesity in America:

The NHANES at CDC measures obesity rates among Americans ages two and older and is the primary source for national obesity data. NHANES is particularly valuable in that it covers a wide range of people living in the United States, and combines interviews with physical examinations, so includes measured height and weight data. It is conducted every two years.

See NHANES data and trends over time.

These data come from a report of the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) as the result of a biennial census of families served by the program. A strength of these data is that they are a census of all WIC participants and not just a sample of them. The data include height and weight measurements for children, which are collected by medical staff during certification visits, and then are used to calculate BMI and obesity rates among children ages 2 to 4. The data are gathered in April of even-numbered years, and analyzed by the CDC.

See data for children ages 2-4.

NSCH surveys parents of children ages 10 to 17 about all aspects of their children’s health, including height and weight. An advantage of this survey is that it includes both national and state-by-state data, so obesity rates between states can be compared. A disadvantage is that it relies on parent reports, not direct measures. The survey has been conducted annually since 2016, but because the methodology changed in 2016, it is not possible to compare data collected previously with data collected in 2016 or later. In order to increase sample size and enable more reliable state-level estimates, after a large initial sample size in 2016, data are pooled for analyses across two collection years. The Health Resources and Services Administration’s Maternal and Child Health Bureau (HRSA MCHB) funds and directs the NSCH and develops survey content in collaboration with a national technical expert panel and the U.S. Census Bureau, which then conducts the survey on behalf of HRSA MCHB. See data for youth ages 10-17.

YRBSS tracks high-risk health behaviors among students in grades 9-12, including dietary behaviors and physical inactivity. The survey also measures the prevalence of obesity by asking respondents about their height and weight. As in other surveys that use self-reported data to measure obesity rates, this survey likely underreports the true rates. The survey is conducted every two years by the CDC nationally and by state departments of health and education. It is usually conducted during the spring. Results are available for most states, though Minnesota, Oregon, Washington, and Wyoming do not participate and sometimes other states are not able to collect enough responses to adequately report results.

See data for high school students.

Research shows that children who have obesity at an early age are more likely to have obesity later in life, so this site also includes the latest findings from the BRFSS, which tracks state-by-state adult obesity rates. BRFSS is an annual cross-sectional survey designed to measure behavioral risk factors in the adult population (18 years of age or older) living in households. Data are collected from a random sample of adults (one per household) through a telephone survey. The BRFSS currently includes data from 50 states, the District of Columbia, Puerto Rico, Guam and the Virgin Islands. The self-reported height and weight data are then used to calculate obesity rates for each state, territory, and Washington, D.C. A limit of self-reported data is that people tend to over-report their height and under-report their weight, meaning the obesity rates may be underestimates.

See data for adults.

Additional Resources

For additional data and information regarding our sources, explore the links below.

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