A method and apparatus for conducting the method is disclosed for non-invasive, quantitative respirator fit testing. The method includes the step of having the wearer properly position the respirator over his nose and mouth, inhale to create a negative pressure inside the respirator cavity volume, hold his breath and record the pressure differential versus time decay rate between the pressure inside the respirator cavity volume and that of the surrounding environment. The method may also include establishing a leakhole of known dimension, repeating the above steps and determining the volume of the respirator cavity based upon the results of the recorded differential pressure versus time by comparing the result to calibration curves.The apparatus of the present invention includes modifying a conventional face mask respirator by providing the respirator with a pressure sensor and a leakhole of known dimension. Preferably, the apparatus can also include a calculator to continuously calculate a quantitative factor to indicate the degree of protection, which is based upon the volume of the respirator cavity divided by the volumetric flow rate through the leakhole or holes of unknown dimension and location for a standard unit of time, given an initial negative pressure in the respirator cavity.