Go to the page content

Is BMI a useful tool? Why we still use BMI when talking about obesity

For more than 150 years, the Body Mass Index (BMI) tool has been used to estimate how much body fat someone has to understand whether they are a healthy weight for their height. While it is a quick, easy-to-use, and popular measure, it can also be misleading in certain cases and for some groups of people.

5 min. read
Woman speaking to doctor

The image is a model

In this article, we explore how these shortcomings of BMI can be outweighed by the valuable insights it provides for doctors and health experts to be able to define and diagnose obesity objectively, and to check and keep track of obesity rates in a country. 

What do we mean by BMI?

BMI is a simple mathematical formula using a person’s weight compared to their height. It is calculated as your weight (in kilograms) divided by the square of your height (in metres) or BMI = Kg/M2.

For example, if you weigh 70kg (approximately 11 stone) and are 1.73m (approximately 5 feet 8 inches) tall, you work out your BMI by:

  1. Squaring your height: (1.73 x 1.73) = 2.99
  2. Dividing your weight (70) by this number: 70/2.99 = 23.4

BMI = 23.4

The result of this calculation is a number that can be used to categorise individuals into different weight classifications, ranging from underweight or normal weight to overweight and various levels of obesity. If you want to calculate your BMI, you can use our handy BMI calculator.

What is the latest data saying about BMI?

While the BMI tool has been widely adopted across the world for many years, it’s not perfect. There have been calls to move away from the tool due to the following criticisms:

  • Body composition: Muscle is denser (heavier) than fat. However, BMI cannot differentiate between weight from fat and weight from muscle, meaning some individuals may be wrongly categorised as “overweight” or “obese”. Also, BMI does not consider where a person stores their body fat. This is important, as body fat stored in certain areas, such as the stomach region, may carry greater health risks than other areas.
  • Diversity considerations: BMI doesn't take wider factors of health outcomes such as sex, age, ethnicity, or other health conditions into consideration. So, this ‘one-size-fits-all’ approach can’t account for certain variations, such as how men and women store fat differently or the muscle loss that happens as we age.
  • Self-reported data: Often, BMI data is self-reported, and people can give unreliable numbers when self-reporting their height and weight measurements, leading to incorrect calculations.
  • Types of obesity: BMI does not consider the recent move to categorise different types of obesity to give individuals the most appropriate treatment. This is discussed in more detail in the article: 'Phenotyping obesity: when does excess weight become a disease?' 

So, why are we still using BMI to measure obesity?

Despite its downsides in certain groups of people, BMI is generally an important and beneficial measure of a person’s level of body fat, and it continues to be used globally for the following reasons:

  • Simplicity for individuals: 51% of the global population will be living overweight or obesity within the next 12 years. As a simple and easy-to-use tool, BMI empowers larger numbers of people to self-evaluate their health relative to their weight, regardless of their level of medical knowledge or access to specialist obesity support.
  • Simplicity for doctors: Primary care doctors play an important role in the weight management journey of those living with obesity, although often there is little time to perform a full weight assessment. BMI can be an quick way to identify those who should be referred to an obesity specialist.
  • Non-Invasive: BMI offers a discreet, non-invasive method to help individuals evaluate their weight. While BMI can be calculated by a doctor, some individuals may prefer to do it from the comfort of their own homes if they’re concerned about feeling judged or self-conscious about being touched in places that they find uncomfortable or are sensitive about.
  • Cost-effective: Healthcare costs are soaring. With obesity-related costs predicted to cross $4 trillion per year by 2035, BMI provides a universal, cost-effective way to assess and monitor obesity trends across large numbers of people. This is particularly important in a world where obesity rates and their financial effects vary significantly between countries and how developed they are. For example, in low-income countries, obesity rates among adults are expected to double from 2020 to 2035. Whereas in some high-income countries, the speed at which obesity rates rise appears to be slowing down.

BMI continues to be a valuable and cost-effective tool that is easily accessed and used by doctors and individuals. While it has served as a useful measure to understand obesity at a population level for many years, it's important to remember that obesity is a complex disease. Every individual's journey with weight is unique. The most comprehensive way to understand and manage obesity and your individual circumstances is to have open conversations with healthcare professionals.



  • 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 August 2023.
  • Gutin I. In BMI We Trust: Reframing the Body Mass Index as a Measure of Health. Soc Theory Health. 2018;16(3):256-271. doi:10.1057/s41285-017-0055-0.
  • Khanna D, Peltzer C, Kahar P, Parmar MS. Body Mass Index (BMI): A Screening Tool Analysis. Cureus. 2022;14(2):e22119. Published 2022 Feb 11. doi:10.7759/cureus.22119.
  • Gurunathan, U, and P S Myles. “Limitations of body mass index as an obesity measure of perioperative risk.” British journal of anaesthesia vol. 116,3 (2016): 319-21. doi:10.1093/bja/aev541.
  • Pujia R, Tarsitano MG, Arturi F, et al. Advances in Phenotyping Obesity and in Its Dietary and Pharmacological Treatment: A Narrative Review. Front Nutr. 2022;9:804719. Published 2022 Feb 15. doi:10.3389/fnut.2022.804719.
  • World Obesity Atlas 2023 from WOF. Available at: https://www.worldobesity.org/resources/resource-library/world-obesity-atlas-2023 [Accessed August 2023].
  • Body Mass Index: Considerations for Practitioners. CDC. Available at: https://www.cdc.gov/obesity/downloads/BMIforPactitioners.pdf (Accessed: 19 September 2023).



Was this valuable for you?

You might also like