Understanding and Interpreting Body Mass Index (BMI)

Understanding and Interpreting Body Mass Index (BMI)

Table of Contents

I. What Is My BMI?

II. How to Calculate BMI

III. How to Interpret BMI

IV. Is BMI Reliable?

V. Conclusion

Calculating your body mass index (BMI) is an easy way to get a quick snapshot of your weight status. BMI uses a simple formula that considers your height and weight to classify you into categories like underweight, normal weight, overweight, or obese. The good news is that you can easily determine your own BMI using a free online BMI calculator. You just enter your height and weight, and the calculator will give you a BMI score that you can compare to standard BMI charts.

But what do these BMI numbers really tell you, and why is BMI used in the first place? In this article, we will look at what BMI is, how to figure out your BMI, how to interpret your result, and if BMI is useful for everyone. 

What Is My BMI?

Body Mass Index (BMI) is the easiest way to get a sense of whether your weight is in a healthy range. The BMI formula only requires two simple measurements: your height and weight. Your height is factored in because taller individuals naturally have more body tissue compared to shorter individuals, which impacts overall weight. 1

It is important to note that BMI is not a perfect measure. It does not directly measure body fat, so it cannot distinguish between fat and muscle mass. This means that some people, like athletes with a lot of muscle, may have a high BMI but not be carrying excessive fat. But for most of us who are not athletes, BMI gives a rough idea of whether we are carrying too many extra pounds. 1

Studies have shown that BMI correlates with gold-standard methods for measuring body fat, making it a useful tool for doctors to identify individuals who may be at a higher risk for health problems due to their weight. 1 So, if you're curious about your weight status, calculating your BMI is a good place to start. Based on your results, you can determine whether you need to follow a healthier diet for weight loss or increase your exercise.

How to Calculate BMI

a doctor calculating a patient's BMI

BMI is calculated the same way for both adults and children. The calculation uses a simple formula that only requires two measurements: your weight in pounds and your height in inches. 1

The formula to determine your BMI is: 2

BMI = weight (lb) / [height (in)]2 x 703

For example, let's say you're an adult who weighs 150 pounds and is 65 inches (5'5") tall. Plugging those numbers into the formula, we get:

150 / (65)2 x 703 = 24.96

In this case, your BMI would be 24.96.

If you want to skip the math, you can use the CDC's BMI calculator for adults. Just enter your height and weight, and it will instantly calculate your BMI. It will also tell you which weight status category you fall under, whether it be underweight, normal weight, overweight, or obese.

How to Interpret BMI

For adults aged 20 and older, BMI is interpreted using four standard weight categories:

  • Below 18.5: Underweight
  • 5 to 24.9: Healthy weight
  • 25 to 29.9: Overweight
  • 30 and above: Obese 2

This BMI chart is the same for both men and women regardless of body type or age. 2

For children and teens, interpreting BMI is more complex since healthy body fat levels vary based on:

  • Gender
  • Age 2

Because of these variables, the Centers for Disease Control and Prevention (CDC) has developed standardized BMI charts for children aged 2 to 20. Based on these BMI charts:

  • BMI between the 85th and 95th percentile: Overweight
  • BMI at or above the 95th percentile: Obese 2

The most accurate way to determine if your child has a healthy weight is to consult your pediatrician. They can properly assess your child's growth and development based on their age, sex, and overall health. 

Is BMI Reliable?

a doctor calculating an obese woman's BMI

While BMI can be a useful tool, it is not perfect. There are several important limitations to keep in mind.

  • BMI does not account for high muscle mass or pregnancy: Because of this, athletes and pregnant women may have a high BMI without high body fat.
  • BMI cutoffs were based on a predominately white population: Body composition varies across ethnicities, so BMI may overestimate risk in Black individuals and underestimate it in Asian individuals. This could lead to inappropriate counseling and care, worsening healthcare disparities. That's why the WHO and NIH recommend lower BMI cutoffs for those of Asian descent. Changes to cutoffs for other ethnic groups are being considered as well. 3

While imperfect, BMI does provide some useful information. In general, the higher your BMI, the greater your chances of health issues like:

Conclusion

Your BMI can provide valuable insights into your weight category and potential health risks. While it may not be a perfect measure, it can still be a useful tool for the average person. However, some groups, like pregnant women or athletes, may find that BMI does not accurately reflect their situation.

The best approach is to use your BMI as one factor among many when assessing your health. Focus on living an active lifestyle and following a healthy diet. If you're already doing that but not seeing BMI improvements, ask your doctor about weight loss medications or other strategies. To learn about weight loss tips to lower your BMI, visit our weight loss blog.

References

  1. Harvard School of Public Health Why use BMI?
  2. CDC About adult BMI
  3. Shmerling, R. How useful is the body mass index (BMI)?