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About obesity

Obesity classes 1, 2 and 3: What is the difference?

3 min. read

Obesity is a complex disease, and can be difficult to understand. Obesity is frequently divided into three types: class 1, class 2 and class 3; but what is the difference between these types, and why do they exist? This article aims to support your understanding of obesity, and the common language used by doctors to explain it.  

First, let’s start with understanding BMI, or Body Mass Index, as this is used to differentiate between the different obesity classes. To put it simply, BMI compares height with weight to get a proxy measure of body fat. A BMI between 18.5 kg/m2 – 24.9 kg/m2 indicates a "healthy" weight, and anything outside of this range is either considered underweight or overweight.

  • If your BMI is less than 18.5, it falls within the underweight range.
  • If your BMI is 18.5 to < 25.0, it falls within the “healthy” weight range.
  • If your BMI is 25.0 to < 30.0, it falls within the overweight range.
  • If your BMI is 30.0 or higher, it falls within the obesity range.

Find your BMI and health risks

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Metrics / Imperial


Clinically, a higher BMI is often used as an indicator of health risks that are associated with obesity, but it is important to note the limitations of using the body mass index as a diagnosis of obesity. For example, BMI can overestimate or underestimate body fat by not accounting for muscle mass or loss of muscle mass in some individuals. It also cannot indicate body fat distribution, and therefore doesn’t distinguish between men and women.

Recommended classification of obesity based on BMI

  • Obesity Class 1: BMI of 30 to < 35
  • Obesity Class 2: BMI of 35 to < 40
  • Obesity Class 3: BMI of 40 or higher.

The above obesity classes are the general guidelines that healthcare providers use when assessing obesity, but these can change based on gender or ethnicity differences. For example, research into health complications has shown that Asian populations are more likely to have a higher amount of body fat at a lower BMI, and therefore, lower BMI cut-off values is commonly used for these populations.

The different obesity classes indicate the severity of obesity. Class 3 is the least common but most severe. Depending on the severity of obesity, different treatment options may be available. For those with either a BMI of 30 or greater, or with a BMI of 27 or above and weight-related health problems such as high blood pressure or type 2 diabetes, weight loss medications that help regulate appetite and help prevent weight regain may be available via a healthcare provider.


How does losing weight improve your health?

See how losing 13% of your body weight can lower your risk of developing certain obesity-related complications.

If you are concerned that your BMI may be high, specially greater than 30, it is highly recommended to speak to a healthcare provider. Several options are available to support you. Find in this list here.

Even as little as 5% weight loss (5 kgs if your weight is 100 kgs) can offer significant improvements to your overall health. In fact, a study found that people with a BMI over 30 can reduce their risk of developing type 2 diabetes by up to 58% if they lose around 7% of their body weight and maintain this weight loss.

Weight loss like this is achievable but is often much harder to achieve alone. For further resources to support your weight loss journey, or if you would like to read a bit more about obesity and its causes, you can visit the article section on Truth About Weight here.



1.       Rueda-Clausen CF, Poddar M, Lear SA, Poirier P, Sharma AM. Canadian Adult Obesity Clinical Practice Guidelines: Assessment of People Living with Obesity. Available from: https://obesitycanada.ca/guidelines/assessment. Accessed 18 October 2022.

2.       De Lorenzo A, Soldati L and Sarlo F et al. (2016) New obesity classification criteria as a tool for bariatric surgery indication. World Journal of Gastroenterology. Available: 10.3748/wjg.v22.i2.681

3.       Deurenberg-Yap, M et al, “The paradox of low body mass index and high body fat percentage among Chinese, Malays and Indians in Singapore.” Int J Obes Relat Metab Disord. 2000 Aug;24(8):1011-7. doi: 10.1038/sj.ijo.0801353. PMID: 10951540.

4.       World Obesity. World Obesity Atlas 2022. Available at: https://www.worldobesity.org/resources/resource-library/world-obesity-atlas-2022. Last accessed: October 2022.

5.       NIH (2022), Prescription Medications to Treat Overweight and Obesity. Available at: https://www.niddk.nih.gov/health-information/weight-management/prescription-medications-treat-overweight-obesity. Accessed 8 November 2022

6.       National Diabetes Prevention Program:  10-year follow-up of diabetes incidence and weight loss in the Diabetes Prevention Program Outcomes Study  Available:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3135022/ Accessed November 2022


Find your local weight management provider

Talk to your weight management provider about treatment options that could prevent the weight you lose from coming back.

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