28 Body Volume by Displacement

The (submersion, or dunking method) can be used to accurately measure the of the human body and other oddly shaped objects by measuring the volume of fluid displaced when the object is submerged, as illustrated in the figure below.

A toy dinosaur is shown next to a cylinder of water with marking indicating volume. To the right, dinosaur is now submerged in the water and the water level is higher in up the cylinder.
When the dinosaur is submerged some of the water is displaced and the water level rises. The displaced volume is measured by reading the gradings, in this case 49 to 53, for a total of 4 volume units (which could be cm3 for a toy dinosaur or m3 for a real one). Image credit: Greg Golz, Exploring Science

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Measuring body with the requires specialized equipment, such as a large tub of water with volume grading (markings) or a special scale that can measure the of a submerged person. Recently technologies have been developed that allow for air rather than water to be used as the submersion fluid, opening up the method to a broader set of the population [2]

A baby can be seen through an observation window on a large machine in which the baby is enclosed. A healthcare professional operates the machine to measure the baby's body fat percentage.
Infant body composition through air displacement plethysmography” by Cosmed via Wikimedia Commons

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