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.
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.
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
Calculation: [weight (kg) / height (m) / height (m)]
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: Height = 165 cm (1.65 m), Weight = 68 kg
Calculation:
68 ÷ (1.65)2 = 24.98 |
| Pounds and inches |
Formula: weight (lb) / [height
(in)]2 x 703
Calculation: [weight (lb) / height (in) / height (in)] 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 |
Interpretation of BMI for adults
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.
| BMI |
Weight Status |
| Below 18.5 |
Underweight |
| 18.5 – 24.9 |
Normal |
| 25.0 – 29.9 |
Overweight |
| 30.0 and Above |
Obese |
For example, here are the weight ranges, the corresponding BMI ranges, and the weight status categories for a sample height.
| 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 |
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.
excerpted from http://www.cdc.gov/nccdphp/dnpa/bmi/adult_BMI/about_adult_BMI.htm
