Obesity is often defined as having a body mass index (BMI) of 30 or higher. However, there are actually three different classes of obesity, and they’re organized according to severity.

Obesity is a chronic medical condition that’s characterized by having excess body fat. Having obesity can increase your chance of several health conditions, including heart disease, stroke, and diabetes.

The prevalence of obesity is increasing in the United States. Data collected between 2017 and 2020 estimates that 41.9% of people in the country had obesity at that time.

While experts still categorize obesity based on severity, the medical community is moving away from terms like “morbid obesity.” Instead, obesity is divided into three different classes according to its severity as well as your BMI measurements.

Keep reading to learn more about these three classes.

You technically have obesity if your BMI is 30 or higher. Obesity is then further defined into different classes based on severity.

The table below shows the three different classes of obesity based on BMI.

Obesity classBMI measurement
Class 130 to less than 35
Class 235 to less than 40
Class 340 or higher

Class 3 obesity is the most severe class of obesity. As such, you may also hear experts refer to class 3 obesity as “severe” or “extreme” obesity.

The prevalence of each class of obesity generally decreases as severity increases. For example, a 2022 study including data from over 44,000 adults in the United States found that:

  • A total of 20.17% of participants had class 1 obesity.
  • A total of 8.98% of participants had class 2 obesity.
  • A total of 6.32% of participants had class 3 obesity.

BMI is used as a measure of the different obesity classes. It estimates your body fat based on your height and weight.

Experts calculate BMI by dividing your weight in kilograms by the square of your height in meters. There are many BMI calculators available online that can do this for you, including one by the National Institutes of Health (NIH).

A note on BMI measurements

BMI is used as a screening tool and isn’t a definitive measure of your actual level of body fat or your overall health. Other factors, such as the distribution of fat, may also be important.

For example, measurements like waist circumference and waist-to-hip ratio take abdominal fat into account. Abdominal, or visceral, fat is associated with a variety of health risks, including in people with typical BMI measurements.

Studies have investigated the effects of the different classes of obesity on various aspects of health and healthcare, including:

  • the chance of death (mortality)
  • the chance of developing additional health conditions (comorbidities)
  • the cost of healthcare

Increased chance of mortality

Obesity is generally associated with a higher chance of death (mortality). In fact, one 2022 study found that excess weight was associated with more than 1,300 excess deaths per day in the United States.

Mortality risk is particularly high for class 3 obesity. A 2014 study found that class 3 obesity, compared with no obesity, was linked to significantly higher mortality, with most excess deaths occurring due to heart disease, diabetes, or cancer.

Increased chance of comorbidities

Obesity increases the chance of a variety of different health conditions. These include but aren’t limited to:

When two or more conditions exist at the same time, they’re called comorbidities. One 2022 cohort study found that the chance of developing one or more obesity-related comorbidities increases with obesity class.

Increased healthcare costs

Obesity and its complications can lead to an increase in doctor’s visits and associated medical costs. Overall, researchers have estimated that obesity is associated with $1,861 in yearly excess medical costs per person.

A 2021 study broke the increase in costs down by obesity class. Compared with having a typical BMI, it found that:

  • Having class 1 obesity increased expenditures by 68%.
  • Having class 2 obesity increased expenditures by 120%.
  • Having class 3 obesity increased expenditures by 233.6%.

Obesity is divided into three different classes. Each class is made up of a specific BMI range.

The different classes of obesity are based on severity. Class 1 is the least severe class, while class 3 is the most severe.

Obesity is associated with an increased chance of death and comorbidities as well as higher medical expenses. These effects tend to be larger as obesity class increases.

If you’ve had a diagnosis of obesity, talk with your doctor about ways to manage your weight. They can recommend weight management strategies that are safe and appropriate for your situation.