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.
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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. Why does CDC use BMI to measure overweight and obesity? Calculating BMI is one of the best methods for population assessment of overweight and obesity. Because calculation requires only height and weight, it is inexpensive and easy to use for clinicians and for the general public. The use of BMI allows people to compare their own weight status to that of the general population. |
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 calculated and interpreted?
Calculation of BMI:
BMI is calculated the same way for both adults and children. The calculation is based on the following
formulas:
| Measurement units | Formula and calculation |
| Kilograms and meters (or centimeters) | Formula: weight (kg) / [height (m)]2
With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Since height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters. Example: Weight = 68 kg, Height = 165 cm (1.65 m) Calculation: 68 ÷ (1.65)2 = 24.98 |
| Pounds and inches | Formula: weight (lb) / [height (in)]2 x 703
Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703. Example: Weight = 150 lbs, Height = 5’5” (65") Calculation: [150 ÷ (65)2] x 703 = 24.96 |
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.
The standard weight status categories associated with BMI ranges for adults are shown in the following
table.
For example, here are the weight ranges, the corresponding BMI ranges, and the weight status categories
for a sample height.
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:
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:
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:
Is BMI interpreted the same way for children and teens as it is for adults?
Although the BMI number is calculated the same way for children and adults, the criteria used to
interpret the meaning of the BMI number for children and teens are different from those used for
adults. For children and teens, BMI age- and sex-specific percentiles are used for two reasons:
Because of these factors, the interpretation of BMI is both age- and sex-specific for children and teens.
The CDC BMI-for-age growth charts take into account these differences and allow translation of a BMI
number into a percentile for a child’s sex and age.
For information on calculating BMI for children and teens
see About BMI for Children and Teens.
For adults, on the other hand, BMI is interpreted through categories that are not dependent on sex
or age.
To automatically calculate your BMI, fill in the blanks below and click
calculate:
BMI Weight Status Below 18.5 Underweight 18.5 – 24.9 Normal 25.0 – 29.9 Overweight 30.0 and Above Obese Height Weight Range BMI Weight
Status 5’ 9” 124 lbs or less Below 18.5 Underweight 125 lbs to 168 lbs 18.5 to 24.9 Normal 169 lbs to 202 lbs 25.0 to 29.9 Overweight 203 lbs or more 30 or higher Obese