What is body mass index (BMI)?

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What is body mass index (BMI)?

Body mass index (BMI) is a screening tool that measures the ratio of your height to your weight. Healthcare providers calculate BMI by using weight in kilograms (kg) divided by the square of height in meters (m2). In most people, BMI relates to body fat, but it’s not accurate in some cases. BMI alone does not diagnose body fatness or health. Healthcare providers use BMI and other tools and tests to assess someone’s health status and risks.

The following BMI ranges (in kg/m2) classify different weight types:

  • Underweight: Less than 18.5.
  • Optimum range: 18.5 to 24.9.
  • Overweight: 25 to 29.9.
  • Class I obesity: 30 to 34.9.
  • Class II obesity: 35 to 39.9.
  • Class III obesity: More than 40.

BMI is not always an accurate representation of an individual’s health. For example, if you have more or less muscle than what is considered optimal, your BMI may not be an accurate measurement of how much body fat you have. In addition, you could have health conditions that are highly associated with having class III obesity, such as Type 2 diabetes or high blood pressure, without having obesity.

The standard BMI scale is often not accurate for the following people:

  • Bodybuilders and professional athletes: Since muscle is more dense than fat, people who are very muscular may have a high BMI even though they have optimal levels of fat.
  • People older than 65: For people over the age of 65, it's often better to have a BMI between 25 and 27, rather than under 25. This is because a slightly higher BMI may help protect them from developing weakened bones (osteoporosis).
  • Children: While many children have obesity, you shouldn't use the standard BMI chart to evaluate a child’s weight. Talk to your child's healthcare provider about the optimum weight range for your child's age and height.

The standard BMI scale may also be inaccurate for people of different races when assessing their risk for certain health conditions. Studies have shown that, in general, people of Asian descent are more likely to have health risks at a much lower BMI, and Black people are more at risk for health issues at a higher BMI than what is considered at-risk in the standard scale.

As an example of BMI inaccuracies according to race, the standard cutoff BMI number associated with a higher risk of developing Type 2 diabetes (T2D) varies for people of different races. Differences include:

  • A BMI of 23.9 and or above is linked to a higher risk of T2D for people of South Asian descent.
  • A BMI of 26 or above is linked to a higher risk of T2D for people of Arabic descent.
  • A BMI of 28 or above is linked to a higher risk of T2D for Black people.
  • A BMI of 30 or above is linked to a higher risk of T2D for white people.

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