What is a BMI?

Body Mass Index (BMI) is a simple tool used to estimate whether a person’s weight falls within a healthy range for their height. It helps give an idea of a person’s body fat levels and is often used by healthcare professionals to assess the risk for certain health conditions, such as heart disease, diabetes, and high blood pressure.

You can calculate your BMI using this formula:

BMI = weight (kg) ÷ height (m)²

Or, you can use any number of free online BMI calculators for quick results.

For example, a person that weighs 70 kilograms and is 1.7 metres tall would have a BMI value of 24.2. This would classify them as being of normal weight, according to the BMI categories outlined below. These categories were developed based on large studies that looked at how different BMI levels relate to the risk of certain health problems, upon which ranges were defined within which people tend to have the lowest health risks and where the risks start to increase.

BMI Category  BMI Value 
Severely underweight ≤16.5
Underweight 16.5–18.5
Normal weight 18.5–24.9
Overweight 25–29.9
Class I obesity 30–34.9
Class II obesity 35–39.9
Class III obesity ≥40

What does it mean if my BMI is too high or too low?

As a helpful tool that gives a general idea of whether your weight is in a healthy range for your height, the BMI can help identify whether someone may be at a higher risk for certain health problems. A BMI in the overweight or obese range may suggest a higher level of body fat, which is linked to an increased risk of the following conditions:

  • Heart disease;
  • Type 2 diabetes;
  • Certain cancers;
  • Osteoarthritis;
  • Liver and kidney disease;
  • Sleep apnea;
  • Depression.

On the other hand, a BMI that is too low can also pose health risks:

  • Hormonal or reproductive issues;
  • Weakened immune system;
  • Heart disease;
  • Certain cancers.

Both underweight and overweight classifications can negatively impact your health, so it’s important to aim for a BMI in the normal range. However, BMI is just an estimate and should be interpreted with caution. That’s why it’s always best to speak with your doctor to understand what your BMI means for your personal health and well-being.

The controversy with BMI

Overall, BMI is a useful tool for estimating body fat levels and can provide helpful information about health risks related to excess fat. It compares fairly well with more precise methods like DXA scans and can predict certain fat-related health markers with good accuracy. However, BMI has some important limitations, so it’s not always a perfect measure of health:

  • Muscle mass: BMI cannot distinguish between muscle and fat. Since muscle weighs more than fat, people with high muscle mass, like athletes or those who exercise regularly, may have a higher BMI despite having low body fat and good health. In these cases, BMI may overestimate health risks.
  • Visceral fat: This is fat stored deep around internal organs, which can increase the risk of serious conditions like heart disease and diabetes. Visceral fat is not visible from the outside, so someone with a normal or even low BMI might still have unhealthy levels of this type of fat, which BMI does not detect.
  • Fat location: The health risks associated with fat depend on where it is stored in the body. For example, fat around the belly is linked to a higher risk of heart disease compared to fat stored in the hips or thighs. BMI does not account for where fat is located, so two people with the same BMI could have very different health risks.
  • Sex differences: BMI uses the same calculation for both men and women, but men typically have more muscle mass, while women naturally have a higher percentage of body fat. This difference means BMI may not always accurately reflect health risks for either sex.

 

Because of these factors, BMI should be seen as a general guide rather than a definitive measure of health. It’s important to consider other factors and talk to your healthcare provider for a complete assessment.

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