Doctors generally use a person’s body mass index (BMI) to diagnose obesity. However, they can also use other tests that consider alternative factors, including waist circumference measurements.

Obesity is a condition that affects a growing number of people in the United States each year. It can lead to an increased risk of many health conditions, including heart disease, type 2 diabetes, and stroke. Diagnosis and management of obesity are important in reducing a person’s risk of potentially serious conditions.

This article discusses how doctors diagnose obesity in adults and how they diagnose it in children and adolescents. It also explains other tests that can help give an accurate diagnosis.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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According to the Centers for Disease Control and Prevention (CDC), 41.9% of adults in the U.S. were affected by obesity between 2017 and March 2020.

If a person is concerned about their weight and possible effects on their health, they can speak with a healthcare professional who may ask about:

  • medical history
  • medications
  • family history
  • lifestyle
  • concerns about weight

The first step in a healthcare professional giving guidance to a person about their weight typically involves determining whether the individual may have obesity.

Learn more about obesity.

Body mass index (BMI)

The most common way people determine obesity is by calculating a person’s BMI. A BMI is an estimate of an individual’s body that compares their height and weight.

Healthcare professionals use BMI ranges to diagnose obesity and overweight. An individual can also work out their BMI using an online calculator.

BMI rangeDiagnosis
18.5–24.9healthy weight
40 and higherextreme obesity

It is important to remember that BMI does not measure body fat directly, so it may not be reliable for everybody. It also does not take into consideration total body composition, bone density, and muscle mass.

Learn more about the limitations of BMI.

Waist circumference measurement

Another way to diagnose obesity is by measuring a person’s waist circumference. This can also help assess the risk of developing other conditions, such as type 2 diabetes and heart disease.

A person can also check their waist circumference at home. They can correctly measure this as follows:

  • While standing, place a tape measure around the waist just above the hip bones.
  • Make sure the tape is horizontal around the waist.
  • Keep the tape snug, but not compressing the skin around the waist.
  • Take the measurement just after breathing out.

A person’s waist circumference may indicate an increased risk of developing obesity-related conditions if it is:

  • more than 40 inches (in) for males
  • more than 35 in for females

If the measurement is larger than this, a person may be at an even higher risk of developing heart disease and other serious health conditions.

Doctors still use BMI to diagnose obesity in children and adolescents. However, since children’s heights are constantly changing, the BMI charts also consider the child’s sex and age.

Therefore, BMI values for children and adolescents relate to others of the same sex and age and use percentile categories, which can help a doctor diagnose obesity:

Age85th percentile97th percentile99th percentile
0–2 yearsat risk of overweightoverweightobesity
2–5 yearsat risk of overweightoverweightobesity
5–18 yearsoverweightobesitysevere obesity

Those in the 99th percentile with severe obesity are at a higher risk of developing obesity-related conditions in adulthood.

Read more about childhood obesity.

While BMI is the most commonly used calculation for diagnosing obesity, it has limitations. There are other tests doctors can use to confirm obesity and the risk of developing other conditions.

Skinfold measurements

Skinfold calipers are a tool to measure a person’s body fat. It can be more difficult to get an accurate reading in those with obesity. However, if the measurements are correct, they can be a good measure of body fat percentage and how it changes over time.

Readings need to involve three areas of the body when using skinfold calipers to calculate a person’s body fat percentage. These areas are usually:

  • chest
  • abdomen
  • thighs

It is important to always test on the same side and to take two measurements from each area. A person can input the numbers into an online calculator, which gives an individual their body fat percentage.

Waist-to-hip ratio

For this measurement, a person only needs a tape measure. This measurement takes the ratio between the waist and the hips.

This method can help estimate the possible risk of developing obesity-related health conditions — the higher the ratio, the more fat a person is storing in the abdomen. Excess fat in the abdomen can put individuals at higher risk of health conditions such as type 2 diabetes and heart disease.

A person can measure their waist-to-hip ratio as follows:

  • Waist: Measure about halfway between the bottom of the ribs and the top of the hips.
  • Hips: Measure around the widest point of the buttocks.

To find the ratio, divide the waist measurement by the hip measurement. High risk is considered a 0.90 ratio for males and a 0.85 ratio for females.

The following section answers some common questions about obesity.

What are the three types of obesity?

There are three categories of obesity:

  • Class 1: BMI of 30–35
  • Class 2: BMI of 35–40
  • Class 3 or severe: BMI of 40 or higher

What are four conditions linked to obesity?

There are several health conditions linked to obesity. Four of these are:

Diagnosing obesity can help a doctor estimate a person’s risk of developing obesity-related conditions.

The most common way to determine obesity is by calculating someone’s BMI. However, BMI does not take into account overall body composition, muscle mass, and bone density. It uses a person’s height and weight.

Other ways to test a person’s body fat, diagnose obesity, and estimate the risk of other conditions include waist circumference, skinfold measurements, and waist-to-hip ratio.

An individual can speak with a healthcare professional about obesity, weight loss, and individual risk factors.