In obese subjects the body fat percentage calculated with Siri's formula is an overestimation.

By means of theoretical calculations it is shown that the chemical composition of the fat-free mass is affected by the amount of fat in the body. With increasing fatness the relative amounts of minerals and protein in the fat-free mass will decrease and the relative amount of water will increase. These changes in the chemical composition of the fat-free mass are confirmed by data from the literature. Because minerals and protein are the most dense components in the fat-free mass, and water is the less dense component, the density of the fat-free mass will decrease with increasing body fatness. As a consequence, the normally used computation formula for the calculation of the body fat percentage from whole-body density (Siri's equation) will systematically overestimate the real body fat percentage by as much as 2-4 per cent in severely obese subjects. This overestimation is large enough to correct for in studies on energy metabolism and body composition.