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BMI

BMI, or body mass index, is an indirect measure of body fat in a person. Because the BMI calculation only requires two pieces of information -- height and weight -- it is an inexpensive and easy method used to screen for weight categories that may lead to health problems such as diabetes, stroke, and heart disease. Calculating BMI can help determine if a person is overweight or obese; BMI calculations are often used in conjunction with other measurements (such as waist circumference) to assess a person's weight.

 

What Is BMI?

Over 100 million adult Americans are overweight or obese. Being overweight or obese increases a person's chances for developing serious health problems, especially heart disease, stroke, diabetes, and cancer. How does someone know if they are overweight or obese? Research on obesity has shown that a measurement called body mass index (or BMI for short) can be used to assess a person's level of body fat.
 
BMI does not measure body fat directly, but it does correspond fairly closely to direct measures of body fat in most people. Therefore, BMI can be considered an alternative for direct measures of body fat.
 
BMI is determined using a person's height and weight. Because the BMI calculation requires only height and weight, it is an inexpensive and easy-to-use method of screening for weight categories that may lead to health problems (see Health Effects of Obesity).
 

How Is BMI Calculated?

There are a few ways to determine your BMI:
 
  • Calculate BMI using the BMI formula
  • Use the BMI chart
  • Use the BMI calculator.
     
Any of these three methods will get the same BMI result. Everyone can use one of these three methods regardless of age, gender, race, or ethnicity. There is no specific BMI chart for women or men.
 

Interpreting BMI Numbers

Once the BMI number is determined, it can be interpreted for adults using standard weight status categories that are the same for all ages and for both men and women. The standard weight status categories associated with BMI ranges for adults are shown below:
 
 
BMI
Weight Status
Below 18.5
Underweight
18.5 – 24.9
Normal
25.0 – 29.9
Overweight
30.0 and Above
Obese
 
One criteria used to diagnose anorexia is a BMI under 17.5.
 
For children and teens, after determining the BMI, the BMI number is plotted on the BMI-for-age growth charts (for either girls or boys) to obtain a percentile ranking. The percentile indicates the relative position of the child's BMI number among children of the same sex and age.
 
BMI-for-age weight status categories and the corresponding percentiles are shown in the following table:
 
Weight Status Category
Percentile Range
Underweight
Less than the 5th percentile
Healthy weight
5th percentile up to the 84th percentile
At risk of overweight
85th to less than the 95th percentile
Overweight
Equal to or greater than the 95th percentile
 

BMI and Weight

If someone has a high BMI does that mean that they are overweight or obese? Furthermore, does that mean that they are at increased risk for health problems? The answers to both these questions depend on several factors.
 
BMI is just one factor to consider when assessing a person's weight. To determine if excess weight is a health risk, a healthcare provider will perform further assessments. According to recent guidelines, assessment of weight involves using three key measures:
 
  • BMI
  • Waist circumference
  • Risk factors for diseases and conditions associated with obesity.
     
Keep in mind that BMI is a screening tool; it is not used to diagnose any medical conditions but instead is used as one measure to assess a person's weight and his or her risk for developing certain medical conditions.
 

High BMI and Medical Conditions

If someone is determined to be overweight or obese, they are at increased risk for certain medical conditions, including:
 
Even a small weight loss (just 5 to 10 percent of your current weight) will help to lower your risk of developing those diseases.
 
(Click BMI and Weight for more information on assessing your risk for obesity-related medical problems.)
 

BMI Versus Other Ways to Measure Body Fatness

The BMI score is valid for both men and women, but it does have some limits. When BMI is used to calculate body fatness:
 
  • It may overestimate body fat in athletes and others who have a muscular build.
  • It may underestimate body fat in older people and others who have lost muscle mass.
     
Besides BMI, there are a number of different ways to measure body fatness. These other methods include:
 
  • Skinfold thickness measurements (with calipers)
  • Underwater weighing
  • Bioelectrical impedance
  • Dual-energy x-ray absorptiometry (DXA)
  • Computerized tomography.
     
These other methods of measuring body fatness are more accurate than BMI. However, these methods are not always readily available, and they are either expensive or require highly trained personnel.
 

Healthy BMI and the United States

Nearly two-thirds of adults in the United States are overweight (defined as having a BMI equal to or greater than 25). This comes to about 130 million American adults. Nearly 61 million adults are obese (defined as a BMI greater than or equal to 30).
 
Even more concerning is that approximately 15 percent of children and adolescents are overweight and another 15 percent are at risk for being overweight (BMI for age between the 85th and 95th percentile). Child obesity is a growing concern in today's world. An alarming number of children are obese and developing diseases normally seen in adulthood.
 
Less than half of US adults have a healthy BMI. This is equivalent to about 68 million adults. About 37 million women between the ages of 20 and 74 have an ideal BMI. About 31 million men between the ages of 20 and 74 have an ideal BMI.
 
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Written by/reviewed by: Arthur Schoenstadt, MD
Last reviewed by: Arthur Schoenstadt, MD