What is BMI?
The body mass index, or BMI, is a way of looking at how much weight you are carrying on your frame; the mass of your body. The BMI is a good way to estimate how much fat you are carrying on your body but is not a direct measure of fat. There are separate tables for children, teens and adults. Tables for children and teens are age- and sex-specific and look at percentiles rather than absolute numbers.
Preparatory Calculations
To calculate BMI for an adult (age 20 and older), you need the person's height in inches and weight in pounds. A woman 5 feet 5 inches tall would, for example, be 65 inches tall. Multiply the weight in pounds by 703. For a woman weighing 125 pounds, for example, the result would be 87875.
Calculating the BMI
The weight in pounds times 703 is divided by the height in inches. For a 5 foot 5 inch women weighing 125 pounds, we would divide 87875 by 65, with a result of 1351.92. The resulting number is divided again by the height in inches. In our example, we would divide 1351.92 by 65, for a result of 20.8. In our example, the woman's BMI is 20.8.
Evaluating the BMI
The BMI figure is then compared to the BMI index for adults. These are the ranges for BMI: 18.5 or less, underweight; 18.5 to 24.9, healthy; 25.0 to 29.9, overweight; 30.0 to 39.9, obese; 40.0 or greater, morbidly obese.
Special Considerations
If you are a bodybuilder, you probably have additional muscle mass, and may have a higher BMI without having excess fat. If you are an elderly woman, a BMI between 25 and 27 may help you prevent osteoporosis. Otherwise, if your BMI falls outside the healthy range, talk with your doctor about making changes.
Children and Teens
For children and teens, the BMI is calculated in the same way. It is often called "BMI for age." There are two evaluation charts, based on sex and age. These are the ranges for BMI for children and adults: 5th percentile or less, underweight; 5th to 85th percentiles, healthy; 85th to 95th percentiles, overweight; 95th percentile or greater, obese.
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