How to Calculate Fat Mass

Your body weight is composed of lean body mass, such as muscles, bones and tendons, and body fat. Weight itself is not bad, but an excessive amount of body fat can lead to severe health issues and decrease physical attractiveness. Body fat is typically expressed as a percentage of your body weight and is measurable with special scales, calipers or density tests. However, if you do not have access to such equipment, you can estimate your lean body mass from body measurements, and from that, you can derive your total fat mass and body fat percentage. The formula for calculating body fat depends on the gender of the patient.

Women

Multiply your body weight in pounds by 0.732 and add 8.987. As an example, If you were a 120-pound woman, multiply 120 times 0.732 and add 8.987 to get a total of 96.8.

Measure the circumference of your wrist in inches at its widest point and divide the result by 3.14. Add this result to the previous calculation. Continuing with the example, if your wrist measured 6 inches, divide 6 by 3.14 and add the resulting 1.91 to 96.8 to get 98.7.

Measure your waist at the naval and multiply the result by 0.157. Subtract this figure from the previous total. If your waist measured 24 inches, multiply 24 times 0.157 to get 3.77. Subtract this from the previous total to get 95.0.

Measure your hips at their fullest point, multiply this measurement by 0.249 and subtract the result from the previous total. If your hips measured 34 inches, multiply 34 times 0.249 to get 8.47. Add this to the previous result to get 86.5.

Measure your forearm at its widest point, multiply the result by 0.434 and add this to your running total. If your forearm measured 9 inches, multiply by 0.434 to get 3.91. Add this to your previous total to calculate your lean body mass of 90.4 pounds.

Subtract your lean body mass from your total weight to derive your body fat mass. Continuing with the example, subtracting 90.4 from 120 calculates 29.6 pounds of body fat.

Divide your body fat mass by your total weight and multiply by 100 to calculate your body fat percentage. In the example, 29.6 divided by 120 results in 0.247. Multiply by 100 to convert the decimal form to 24.7 percent.

Men

Multiply your total body weight times 1.082 and add 94.42. As an example, if you weighed 180 pounds, multiply 180 times 1.082 and add 94.42 to get a total of 289.2.

Measure your waist, multiply the measurement by 4.15 and subtract this from the previous total to calculate your lean body mass. In the example, if your waist measured 34 inches, multiply 34 times 4.15 and subtract the resulting 141.1 from 289.2. This calculates your lean body mass of 148.1 pounds.

Subtract your lean body mass from your total body weight to calculate your body fat mass. Continuing with the example, 180 minus 141.1 calculates your body fat mass of 31.9 pounds.

Divide your body fat mass by your total body weight and multiply by 100 to calculate your percent body fat. In the example, 31.9 divided by 180 and multiplied by 100 calculates your percent body fat of 17.7 percent.

Tips

Men and women differ in the amount of acceptable body fat. Ten to 12 percent body fat is essential for women, but men only require 2 to 4 percent. Female athletes typically have 14 to 20 percent, but male athletes have 6 to 13 percent. Fitness-conscious women tend to have 21 to 24 percent, but a male fitness enthusiast may only have 14 to 17 percent. An acceptable upper limit is 25 to 31 percent for women or 18 to 25 percent for men. Women are considered obese when their body fat percentage is greater than 32 percent, and men are obese with body fat greater than 25 percent.