Body Mass Index Guidelines, Accuracy, and Advice

Understand body mass index to make sense of the results

BMI accuracy and guidelines
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If you don't understand BMI guidelines, you can easily get confused or even offended when you get your body mass index results. Whether this happens in a doctor's office, a health club or even in your own home, the number may cause you to become concerned about your health. But once you learn more about body mass index principles, how to calculate your BMI and BMI accuracy you should be able to make sense out of your number.

What Is BMI?

The body mass index (BMI) is used to help health practitioners estimate a patient's body fatness without the use of expensive or complicated testing procedures. Your doctor or provider uses your height and weight to determine your number (see how to calculate BMI below). Your result may be used as a starting point for a broad conversation about your weight and your health.

The Body Mass Index was developed by the World Health Organization in 1997 to help estimate and classify the global population according to body weight. The organization maintains a large database of information that is used to monitor, report and create awareness for nutritional, weight and health issues that need to be addressed.  

Your BMI result provides information about how you fit into a large category of individuals. It doesn't necessarily provide specific data or recommendations about your own health and fitness level.

For example, if your BMI is 23 you fall into the normal weight category. The number does not necessarily indicate that your body weight is normal. Instead, it indicates that most people with a BMI of 23 are normal weight. If your body mass index is 29 you fall into the overweight category. This does not necessarily mean that you need to lose weight.

It means that most people with a BMI of 29 are overweight. 

In order to get a personalized assessment of your weight, you and your health care provider should use BMI along with other measurements to evaluate your body composition and body fat distribution.

How to Calculate Your BMI

Now that you understand how BMI works, you may be curious about your own number. To calculate your body mass index you can use an online calculator. Or if you prefer to do the math on your own, you can use this formula to calculate your result.

BMI = (weight in kilograms) ÷  (height in meters)2

Now that you know your number, compare it to the body mass index classifications in the chart below.

BMI Chart

This body mass index chart from the Centers for Disease Control and Prevention allows you to compare your BMI and see if your weight falls into a healthy category.

Body Mass Index Scores
BMI ScoreDescription
  >18.5Underweight
  18.5 to 24.9Normal Weight
  25.0 to 29.9Overweight
  30.0 or higherObese

The World Health Organization also provides sub-classifications for different levels of obesity.

  • Obese Class I = 30.00 - 34.99
  • Obese Class II  = 35.00 - 39.99
  • Obese Class III = 40.00 or above

Further subclassification levels are also provided within each obesity subclass.

How Accurate Is BMI?

You shouldn't worry about the accuracy of BMI when you get your number. Why? Because the body mass index is not a diagnostic measure. BMI is only a classification system. Health care providers use it as a screening tool to begin an evaluation of your weight and to estimate your possible risk for certain types of disease. It is not designed to be an accurate measurement of how fat you are.

Critics of body mass index point out that BMI does not take into account a patient's body fat distribution or lean muscle mass. So, a very athletic individual may fall into the overweight category simply because they have a larger body due to healthy muscle mass. The WHO also acknowledges that the body mass index may not be as helpful when used with certain ethnic populations. 

Of course, these facts might not help you to feel better if your doctor tells you that you have a BMI that falls into the obese or overweight category. Those words may be hard to hear. But if that happens, take a deep breath and put the number into perspective. Your BMI is simply a starting point for a larger discussion about weight.

If you have a high body mass index, you may want to use other methods to assess your weight. You start with simple measurements like waist circumference or hip to waist ratio. You can also measure your body composition at the doctor's office, at the health club or with a body fat scale at home. 

Sources:

Global Database on Body Mass Index. Background. World Health Organization. Accessed: February 19, 2016. http://apps.who.int/bmi/index.jsp?introPage=intro_1.html

Calculate Your BMI. National Heart Lung and Blood Institute. United States Department of Health and Human Services Accessed: February 19, 2016. http://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm

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