The Primary Standards Laboratory at Lockheed Martin Missiles and Space has developed an orifice flow vacuum calibration station for in-house calibration of spinning rotor and ion gages. A flowmeter supplies gas to a vacuum chamber partitioned by an orifice of calculable conductance. The pressure generated within the chamber is defined by the flowrate and the orifice conductance for molecular flow conditions. Orifice conductance is calculated from dimensional measurements, the flowrate is determined by measuring pressure drop at constant volume within the flowmeter. The design, development, and operation of the flowmeter is fully explained and illustrated. Chamber pressures as predicted from flowrate measurements are compared to those measured using a spinning rotor gage calibrated at the National Institute of Standards and Technology. Data for the extension of the calibration range beyond that of spinning rotor gages will be presented. An analysis of uncertainties inherent to the flowmeter is also presented.