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How to Measure Body Fat From Water

Your body fat rate is simply the percentage of your body weight that is made up of fat. If you weigh 200 lbs., for instance, and have 10 percent body fat, then your body consists of 20 lbs. of fat and 180 lbs. of lean muscle mass such as bone, muscle, organ tissue and fluids. There are several methods to measure body fat percentage. Of those, the technique known as hydrostatic weighing, or underwater weighing, is considered the most accurate. Because it requires some specialized equipment, it is also a complex method.

Things You'll Need

  • Scale
  • Nose clip
  • Spirometer
  • Stainless-steel weighing tank
  • Underwater mounted chair
  • Underwater scale
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Instructions

    • 1

      Weigh the person on a regular scale to determine his dry weight. Record this number because you will be using it in a calculation later in the process.

    • 2

      Estimate the person's residual volume, which is the amount of air remaining in the lungs after he exhales the most air he possibly can. To measure the residual volume, you must first determine the person's vital capacity, which is the amount of air he can maximally exhale. The unit of measurement used is the liter. Estimates for vital capacity are 4.8 liters for men and 3.1 liters for women. Estimates for residual volume are 1.2 liters for men and 1 liter for women.

    • 3

      Place a nose clip on the person, place the mouthpiece of the spirometer in his mouth and have him maximally exhale to measure his vital capacity. Perform two or three trials and use the highest volume the spirometer records. Multiply the person's vital capacity by .24 for males or .28 for females to determine the residual volume.

    • 4

      Have the person sit on the weighing device's specialized seat and exhale the air from his lungs. Lower him into the weighing tank until his body is submerged. The person must remain motionless while the underwater scale's computer gets a reading on his underwater weight.

    • 5

      Calculate the person's body density using the measurements you have recorded so far. Use the following formula: Body density = dry weight / [((dry weight - wet weight) / water density) - RV - 0.1].

    • 6

      Calculate the person's body fat percentage using the number you calculated for his body density. Use the following formula, which is called the Siri Equation: Body fat percentage = (495 / body density) - 450

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