What Is the Formula to Calculate Body Fat?

The formulas for determining your body fat content are based on your body measurements. The most accurate measurements can be taken when you get out of bed, following seven to eight hours of sound sleep. On his website, ShapeUp.org, Dr. Everett Koop, former Surgeon General of the United States, says that 8 to 19 percent body fat is a good target for men under 39; women the same age should have between 21 and 32 percent body fat. Over 39, men should range from 11 to 24 percent, and women from 23 to 35 percent.

Women, Begin with Your Weight

• Grab a paper and pencil or, if you don't want to do the math, grab a calculator. First, multiply your total body weight by 0.732. To the result, you need to add 8.987. Here's an example: if you weigh 150 pounds, multiply that by 0.732. The answer is 109.8--yes, you'll need to use the decimal places. Add 8.987 to that. The total now is 118.787. Let's call this "Factor 1." Now, we move on to the next step: taking body measurements.

Measurements

• Use a tape measure to measure around the fullest part of your wrist. Take that measurement and divide it by 3.140. Again, here's an example: if your wrist measures 9 inches around, divide 9 by 3.140; the answer, which we'll call "Factor 2," is 2.866. Record this, and keep writing down the results as you go. There will be a test at the end.

  Next, measure around your waist. No, not where you wear your pants--up higher, at the navel. Now multiply that by 0.157. This means your 32-inch waist is now called "Factor 3," and we discover that Factor 3 is 5.024.

  Factor 4 is your hips. Measure around them at the fullest point and multiply by 0.249. "Fullest point" means include your backside in the measurement. If that measurement equals 36 inches, multiply the result by 0.249, and you get 8.964. Move on to Factor 5, the forearm measurement.

  Measure around your forearm at its fullest point, then multiply by 0.434. If your forearm is 12 inches around, multiply 12 by 0.434 and you get Factor 5, which equals 5.208.

  Add Factors 1, 2, 3, 4 and 5 together, and you get your lean body mass: 140.849

How's Your Body Fat?

• This part is easy. Your body fat weight is your total body weight minus your lean body mass. If you weigh in at 150, and your lean body mass is 140.849, subtract 140.849 from 150. Your body fat weight is 9.151. To determine the percentage of body fat, divide your body fat weight (9.151) by your total body weight (150). Then, multiply the result, 0.061, by 100, which equals 6.100, or 6.1 percent.

Men, Yours Is Easier

• Men deal with two factors: body weight and waist measurement.

  Factor 1 is your weight. If you weigh in at the same 175 pounds that you weighed in high school, then multiply that weight by 1.082. Add the result, 189.35, to 94.42 and record your answer: 283.77.

  Factor 2 is just as simple to obtain: measure your waist, at the navel, not where you wear your jeans. If your waist is 36 inches, you need to multiply that by 4.15. This means that Factor 2 is 149.4.

  Your lean body mass is found by subtracting Factor 1 from Factor 2: 283.77 minus 149.4. This means your lean body mass is 134.37.

Your Body Fat Percentage

• Just as with women, your body fat weight is your total weight, 175, minus your lean body mass. If you subtract 134.37 from 175, this means your body fat weighs in at 40.63 pounds. To determine your percentage of body fat, first you divide the weight of your body fat by your total weight. In the example, this means you divide 40.63 by 175. Then, multiply the result, 0.2321, by 100, which is 23.21, or 23.21 percent.