Body mass index, or BMI, is a body fat measurement that is based on height and weight. BMI is a simple, easy-to-perform alternative to direct body fat assessment and is considered reliable by the Centers for Disease Control, or CDC, in most populations. BMI is used as as a quick screening tool to place individuals in weight categories and assess their risk of weight-related diseases.
Calculation
BMI can be found with a simple calculation that utilizes only height and weight. The formula to calculate BMI according to the metric scale is weight in kilograms divided by height in meters squared. However, measurements are generally taken in pounds and inches as opposed to kilograms and meters, thus a formula utilizing these measurements is also available. To utilize this formula weight in pounds is divided by height in inches squared. The resulting value is multiplied by 703 to obtain BMI. Although these calculations can be easily performed by hand, there are BMI calculators available online for added ease and convenience. The CDC provides BMI calculators for adults as well as a version for children and teens.
Interpretation for Adults
There are four general BMI standards for adults that are used to interpret BMI values and classify individuals according to their weight status. Any individual with a calculated BMI between 18.5 and 24.9 is considered to be of normal weight. Any value lower than 18.5 labels the individual as underweight, while a value falling between 25.0 and 29.9 places the person in the overweight category. BMI values greater than 30.0 indicate obesity.
Interpretation for Children and Teens
BMI is specific to the child's sex and age with children and teens and is often referred to as BMI-for-age. BMI values are calculated using height and weight measurements of the child, and the resulting value is plotted on BMI percentile charts provided by the CDC. There are four general categories that the child can be placed in using their percentile ranking. Healthy weight children are considered to be any child that falls between the 5th and 85th percentile for their age and sex. Any child who exhibits a BMI that places them lower than the 5th percentile is considered underweight, while a child with a BMI that falls between the 85th and 95th percentiles is considered overweight. Children with calculated BMI values greater than or equal to the 95th percentile are considered to be obese. Percentiles are used with children and teens instead of the general categories used with adults, because body fat differs between the sexes, and the amount of body fat changes with growth and development as well as age.
Reliability
For the general population, BMI values are a fairly reliable indicator of body fat percentages, although there are some variations that should be noted. Women tend to exhibit more body fat than men even if they present the same calculated BMI values. This can be attributed to the differences in body makeup between the sexes and the presence of mammary tissue in women. Age can also play a role in BMI variation. Younger individuals tend to have a higher percentage of muscle than older individuals. This will increase their BMI values, however, the increase will be due to an increase in muscle weight, not in body fat. Highly trained athletes have increased BMI values for the same reason thus BMI is not the best choice for body fat assessment in these individuals.
Other Considerations
Although BMI is a reliable calculation that can help screen for health risks, it is important to remember that this is not the only indicator of health status and disease risk. Blood pressure, cholesterol and waist circumference measurements are also important values to consider to comprehensively evaluate health status. It is also important to remember that BMI is simply an estimate of body fat percentage. In order to obtain an exact measurement, a direct assessment method such as underwater weighing or dual-energy X-ray measurement is used.
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