In this paper, we present a study of Lithium (Li)-air battery capacity by accounting for the voltage loss associated with the electrode passivation and transport resistance caused by insoluble product formation. Two regimes are defined, in which approximate formulas are developed to explicitly evaluate the battery capacity, along with extensive validation against experimental data of various cathode properties and materials from our and several other groups. The dependence of battery capacity on the surface coverage factor, tortuosity, and Damköhler numbers (Da) is explicitly expressed and discussed. The formulas provide a guideline for experimentalists and practitioners in air cathode design, analysis, and control.