We present a minimal model for proton exchange membrane unit cells with straight channels in steady state conditions capable of predicting: current density profiles, local and total water crossover, effects of inlet humidity, and voltage sensitivities. The model is capable of describing co-flow and counter-flow operation. In the model, channel flows are represented by average molar fluxes. The membrane is represented in some detail, with proton concentration determined by a local chemical equilibrium, and nonlinear water and proton diffusivities. Cathode electrochemical losses are considered based on standard Tafel relationships. The model contains only three main fit parameters: exchange current density, a parameter representing oxygen transport losses from the channels to the catalyst sites, and a parameter representing limitations on water transport from the catalyst to the channels. Techniques for effectively fitting these parameters to experimental data are outlined. Extensive comparison to data from the Ballard Mk9 hardware has been conducted.