Abstract: A system for measuring body fat percentage is disclosed. The system relies upon the measurement of a subject's weight in two gases of different densities. The difference in weight, along with the difference in atmospheric density, is used to determine the subject's density. Implementing Archimedes' principle of binary mixtures, this figure is used to determine the proportion by weight of fat in the subject. The weight of the subject is measured with a precision balance. The density of the gas is determined with an ultrasonic probe that senses the change in sonic velocity, which is related to the density of the atmosphere.