While a person sits inside the enclosure, a rapidly vibrating diaphragm in the wall of the test chamber produces small changes in air pressure between it and an adjoining reference chamber. The changes in air pressure can be used to determine the amount of air the person displaces in the test chamber. In turn, that measurement leads to an estimation of the person’s body volume, corrected for the volume of air in the lungs at the time of the measurement.
Then, taking the person’s body mass (obtained by a scale) and dividing it by the person’s body volume, the body density is determined. With that, a person’s percent body fat can be calculated.