Your total body volume is one component in certain equations to figure your percentage of lean body mass. Finding volume for a regular-shaped object, such as a cube, is relatively straightforward, but determining this calculation for an irregular object, such as the human body, is nearly impossible using a simple equation. Total body volume is most often found using water displacement or air displacement methods, which call for specialized equipment and trained test administrators.
Water Displacement to Find Volume
Hydrostatic weighing helps you find the volume of the human body. You’re first weighed on dry land and then submerged in a large pool and weighed using specialized equipment. The human body weighs less underwater. The difference between your dry land weight and underwater weight is the weight of the water you disperse when submerged, as per the Archimedes principle. Use the mass and density of water displaced to calculate the volume of the water displaced. The volume of the displaced water is equal to the volume of your body.
Challenges With Calculating Body Volume
Using water displacement to calculate body volume is easy enough if you have the right equipment, a trained professional and a large pool, but it's not something you can readily do at home. Also note that your body contains a certain amount of gas, which has a low density and alters the measure of your volume.
Air or gas in the intestinal tract can be eliminated from the equation by fasting prior to the underwater weigh-in, but air in your lungs is usually present. When you undergo an underwater weighing test, you're asked to exhale as much air as possible to minimize the chance that it will skew your results. Air that cannot be exhaled is estimated and figured into the equation.
Air Displacement Method of Finding Body Volume
A newer method of determining density measures volume using air displacement. Specialized chambers perform the measurement and figure the calculation. The process still requires expensive equipment and people trained to perform the tests, but it eliminates the hassle of using water. Some health care centers and fitness establishments have machines, known as the Bod Pod, that perform the measurement.
Magnetic resonance imaging, or MRI, can also be helpful in determining the volume of tissue in a particular region of the body. Again, it's a procedure that requires specialized equipment that's only available in health-care settings staffed with trained technicians.
Mathematical Calculations to Estimate Body Volume
An equation is available to estimate body volume, but it's based on males and doesn't take into account age. The advantage to the equation is that it's easy to use and adequate for many clinical and practical uses, such as medication dosing.
The equation reads: volume in liters = surface area in square meters (51.44 x weight in kilograms / height in centimeters + 15.3)
Surface area is determined using a specialized chart available to medical professionals that estimates the number based on height and weight. Charts also exist that estimate volume based on height and weight.