A new vacuum-gauge calibration method is described. Tile pressure ratio generated by the molecular flow of gas through two known, fixed, series conductances is applied repeatedly, serially in time, to establish a progression of increasing chamber pressures. A single pressure indication serves as the higher value in one ratio and the lower value in the succeeding ratio, thereby correlating the calibration pressures by a fixed known factor. Calibration pressures can be calculated after one of them is sufficiently high to be measured accurately by a reference system. In one modification, gauges are calibrated on a symmetrical, well-baffled chamber connected directly to the pump by a large conductance, thus significantly reducing the possible interaction between the establishment of new calibration pressures, and the pumping or outgassing of gauges being calibrated. A simple, single-pump apparatus suffices for calibrating from10−3Torr to within a decade of the system ultimate pressure. Re-producibility in the order of 2% is achieved in the10−9Torr decade.