Developing and maintaining a healthy weight is one of the most important things you can do for overall health. Some people have a hard time understanding what constitutes healthy weight. Your body mass index, or BMI, and body-fat percentage are two different ways of evaluating weight. Your BMI can be converted into body-fat percentage values, but this technique may not give an accurate body-fat percentage estimate.
What is BMI and Body Fat Percentage?
Your BMI is calculated by dividing your weight in kilograms by the square of your height in meters. For example, if you weight 68 kilograms and your height in 1.65 meters, your BMI is 68/(1.65 x 1.65) or 24.96. A healthy BMI is between 18.5 and 24.9 for adults. Your body-fat percentage is essentially a calculation of how much of your body weight comes from fat. According to the American Council on Exercise, the average body fat percentage is between 25 and 31 percent for women and 18 to 24 percent for men.
Converting BMI to Body Fat Percentage
It is possible to take BMI values and convert them to an estimate of body-fat percentage. According to Dr. Steven Halls, the formula for this conversion is (1.2 x BMI) + (0.23 x age) - 5.4. Men also get to subtract an additional 10.8 from this formula to calculate body fat since they naturally have less fat. For example, a 30-year-old man with a BMI of 25 would do the following calculation: (1.2 x 25) + (0.23 x 30) - 5.4 - 10.8 equals 20.7 percent body fat. These calculations were generated from a study that measured the BMI and body-fat percentages in more than 5,000 people. An equation was then developed to accurately estimate body-fat percentage from BMI values.
Problems With The Formula
Although Halls' formula is one way to get an estimate of body-fat percentage, it may not be accurate for all people. Athletes, for example, tend to have more muscle mass, which can lead to a high BMI even in the absence of a high body-fat percentage. People with the same BMI can have very different body-fat percentages. In addition, the above formula does not take into account race, which, according to a 2002 article in the International Journal of Obesity, can affect the relationship between BMI and body-fat percentage. For example, the above formula underestimates the percentage of body fat in black women by an average of 6 percent, meaning that if a black woman's body fat was estimated at 25 percent using the above equation, her actual body fat could be 106 percent of that calculation, or 26.5 percent.
Why Use BMI to Estimate Body Fat Percentage?
Although formulas that convert BMI to body-fat percentage are not always accurate, they do represent a convenient and rapid way of estimating body-fat percentage. The most accurate ways of measuring body-fat percentage, such as underwater weighing or X-ray densitometry, require specialized equipment and trained professionals. Even simpler methods of measuring body-fat percentage, such as measurements of different parts of your body can require several different measurements and suffer from inaccuracies. As a result, while it is not the most accurate way to estimate body-fat percentage, using the BMI conversion formula is a fast and simple.
References
- "International Journal of Obesity"; The effect of sex, age and race on estimating percentage body fat from body mass index: The Heritage Family Study; Jackson et al.; June 2002
- Centers for Disease Control and Prevention; Body Mass Index; 2011
- American Council on Exercise; Healthy Body Fat Percentage; Dr. Natalie Muth; 2011
- Dr. Steven Halls; BMI to Body Fat Percentage; 2008
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