Abstract: A method for measuring the air capacity of a user's lungs. The method involves the use of an apparatus which includes a container having a constant volume and an opening adapted to allow flow communication with the lungs. The container further includes a flow sensor and a pressure sensor. The method involves determining the initial air pressure in the container before flow communication with the lungs is established. After flow communication with the lungs is established, the flows and pressures are measured. The lung air capacity is calculated based on the known constant container volume, the initial air pressure, and the changes in volume and pressure of the lungs as determined from the measured flows and pressures in the container.