This paper presents a novel method for determining the power available to a passive radio-frequency identification (RFID) tag, the Power Conversion Gain (PCG). The power conversion gain of a tag combines the tag's antenna pattern with its charge pump efficiency to give the system designer the tag's output power as a function of angle and maximum received input power. This metric is more useful for system designers because it helps to determine the tag's sensitivity to orientation which is exaggerated by the non-linearity of the charge pump. This paper demonstrates the use of PCG using measured results from a 4-stage Dickson charge pump operating at 5.8 GHz and a directional antenna as a worst case scenario.