ACE Health Coach Certification Practice Test

How is body-fat percentage calculated in hydrostatic weighing?

1. Skinfold thickness

2. Body density

3. Weight and height

4. Body mass index

The correct answer is: Body density

Body-fat percentage is calculated through hydrostatic weighing by first determining body density. This method is based on the principle of buoyancy, where an individual's body is submerged in water. The weight of the person is measured both in air and submerged in water to calculate body density. The difference between these two weights allows for the calculation of volume, and by combining this volume with the person's weight, body density can then be derived. Once body density is known, it can be compared to known densities of fat and lean tissue to estimate body-fat percentage. This method is regarded as one of the most accurate for measuring body composition. While the other options—skinfold thickness, weight and height, and body mass index—are methods for assessing body fat to some degree, they do not operate on the principle of hydrostatic weighing, nor do they directly measure body density in the way that hydrostatic weighing does. Instead, those methods can provide estimations rather than precise calculations, reinforcing why body density is the correct approach in the context of hydrostatic weighing.