The performance of showerheads used in semiconductor processing applications has been examined over a wide range of Knudsen number and Reynolds number. Both computational fluid dynamics and direct simulation Monte Carlo techniques have been used to characterize the showerhead flow with the intent of developing correlations between the upstream velocity and the pressure drop across the showerhead. Empirical correlations developed to account for entrance effects and rarefaction are also examined. Recommendations are made concerning boundary conditions and, when appropriate, correlations for given pressures and flow rates.