Understanding Your Body Mass Index

This information was provided by the Department of Health and Human Services – Centers for Disease Control and Prevention, www.cdc.gov.

What is BMI?
Body Mass Index (BMI) is a number calculated from a person’s weight and height. BMI is a reliable indicator of body fatness for people. BMI does not measure body fat directly, but research has shown that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA). BMI can be considered an alternative for direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems. Click here for the CDC's Adult BMI Calculator

How is BMI used?
BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI. However, to determine if excess weight is a health risk, a healthcare provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.

What are some of the other ways to measure obesity?
Other methods to measure body fatness include skinfold thickness measurements (with calipers), underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and computerized tomography. However, these methods are not always readily available, and they are either expensive or need highly trained personnel.

How is BMI Interpreted?
For adults 20 years old and older, BMI is interpreted using standard weight status categories that are the same for all ages and for both men and women. For children and teens, on the other hand, the interpretation of BMI is both age- and sex-specific.

  • If your BMI is less than 18.5, it falls within the “underweight” range. Your health care provider can help you to achieve a healthier total body weight.
  •  If your BMI is 18.5 to 24.9, it falls within the “normal” or Healthy Weight range. Keep up the good work!
  • If your BMI is 25.0 to 29.9, it falls within the “overweight” range. Therefore, you may need to lose weight, especially if you have two or more of the risk factors for diseases associated with overweight.
  • If your BMI is 30.0 or higher, it falls within the “obese” range. Therefore, you should talk to your doctor or health care provider about losing weight.

How reliable is BMI as an indicator of body fatness?
The correlation between the BMI number and body fatness is fairly strong; however the correlation varies by sex, race, and age. These variations include the following examples:

  • At the same BMI, women tend to have more body fat than men.
  • At the same BMI, older people, on average, tend to have more body fat than younger adults.
  • Highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness.

It is also important to remember that BMI is only one factor related to risk for disease.

For assessing someone’s likelihood of developing overweight- or obesity-related diseases, the National Heart, Lung, and Blood Institute guidelines recommend looking at two other predictors:

  • The individual’s waist circumference (because abdominal fat is a predictor of risk for obesity-related diseases).
  • Other risk factors the individual has for diseases and conditions associated with obesity (for example, high blood pressure or physical inactivity).

If an athlete or other person with a lot of muscle has a BMI over 25, is that person still considered to be overweight?
According to the BMI weight status categories, anyone with a BMI over 25 would be classified as overweight and anyone with a BMI over 30 would be classified as obese.

It is important to remember, however, that BMI is not a direct measure of body fatness and that BMI is calculated from an individual’s weight which includes both muscle and fat. As a result, some individuals may have a high BMI but not have a high percentage of body fat. For example, highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness. Although some people with a BMI in the overweight range (from 25.0 to 29.9) may not have excess body fatness, most people with a BMI in the obese range (equal to or greater than 30) will have increased levels of body fatness.

It is also important to remember that weight is only one factor related to risk for disease. If you have questions or concerns about the appropriateness of your weight, you should discuss them with your healthcare provider.

What are the health consequences of overweight and obesity for adults?
The BMI ranges are based on the relationship between body weight and disease and death. Overweight and obese individuals are at increased risk for many diseases and health conditions, including the following:

  • Hypertension
  • Dyslipidemia (for example, high LDL cholesterol, low HDL cholesterol, or high levels of triglycerides)
  • Type 2 diabetes
  • Coronary heart disease
  • Stroke
  • Gallbladder disease
  • Osteoarthritis
  • Sleep apnea and respiratory problems
  • Some cancers (endometrial, breast, and colon)

For more information about these and other health problems associated with overweight and obesity, visit Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults