Boyle's Law is used to calculate the unknown volume within the lungs. First, the change in volume of the chest is computed. The initial pressure of the box times its volume is considered equal to the known pressure after expansion times the unknown new volume. Once the new volume is found, the original volume minus the new volume is the change in volume in the box and also the change in volume in the chest. With this information, Boyle's Law is used again to determine the original volume of gas in the chest: the initial volume (unknown) times the initial pressure is equal to the final volume times the final pressure. Starting from this principle, it can be shown that the functional residual capacity is a function of the changes in volume and pressures as follows: