Since there is at present no standard means of calibrating vacuum gauges at pressures below 1 Torr, one often finds that such gauges frequently differ in their indications by 30% or more. In many areas of research it is necessary to measure pressures accurately to within 2%. Thus, to achieve this goal, it is necessary to devise a means to calibrate the measuring gauges. This calibrator permits one to calibrate a vacuum gauge over the pressure range of10−4Torr to atmospheric or above increments as small as10−4Tort or as large as 3.5 Torr. Virtually any value in between is possible. Three volumes are interconnected by a pair of solenoid valves. Volume 1, with the gauge to be calibrated connected to it, is evacuated to some very low pressure, say10−6Torr, and sealed off. Volume 3 is sealed off at some readily measurable pressure. Then volume 2 is filled from volume 3 and emptied into volume 1. This process is termed a step. If the three volumes are measured, then the pressure inV1can be calculated at the end of every step and compared with the gauge indications. The general equations describing the pressure rise inV1as a function of step number are obtained. The effect of an initial pressure, not zero, and a finite leak and/or desorption rate are also considered. A description of the actual apparatus is also included.