Density is a physical property defining the amount of mass contained within a given volume. Calculating body density provides a direct measure of body composition, which is important for health and fitness assessments. Since the body is composed of various tissues with different densities, this measurement is always represented as an average.
The Average Density of the Human Body
The average density of a living human body typically falls between \(0.98 \text{ g/cm}^3\) and \(1.05 \text{ g/cm}^3\). This range is close to the density of pure water, which is approximately \(1.0 \text{ g/cm}^3\). An object less dense than water will float, while a denser object will sink. This near-equal density explains why a person can float or sink depending on their position and breath management. Slight differences in individual body composition determine whether they are closer to the floating or sinking end of this spectrum.
Determining Density Through Water Displacement
The most accurate method for determining human body density is hydrostatic weighing, or underwater weighing. This technique relies on Archimedes’ principle, which states that the buoyant force on a submerged object equals the weight of the water the object displaces. The procedure measures the person’s weight outside the water, and then again while fully submerged in a specialized tank.
During the underwater measurement, the person must exhale as much air as possible to minimize the buoyant effect of air in the lungs. The difference between the land weight (mass) and the underwater weight allows researchers to calculate the body’s volume. This volume is combined with the mass to yield the average body density, which is used to estimate the ratio of lean tissue to fat tissue.
Biological Factors That Change Individual Density
Individual body density varies significantly due to differences in tissue composition and air volume. The two primary components influencing overall density are adipose tissue and lean body mass. Adipose tissue (body fat) is less dense than water, measuring approximately \(0.90 \text{ g/cm}^3\) to \(0.92 \text{ g/cm}^3\). Lean body mass, which includes muscle, bone, and organs, is significantly denser, typically around \(1.06 \text{ g/cm}^3\) to \(1.10 \text{ g/cm}^3\). A person with a higher percentage of body fat will have a lower overall density, making them more buoyant.
The second major variable is the volume of air contained within the lungs. Air is extremely light, and holding a full breath dramatically increases body volume without significantly increasing mass, which decreases the average density and aids in floating. When a person fully exhales, the air that remains is known as the residual volume. Maximal exhalation increases the body’s average density by removing a large volume of low-density air, which can cause the body to sink. The act of breathing can cause a change in a person’s density by around \(5\%\).