This article describes the development and application of a numerical model of vacuum flow with outgassing. The mass conservation equation for a vacuum tube is formed with three outgassing terms : a 1/t term, a 1/t0.5 term, and a constant term. Each term arises from a distinct sorption process at the walls of the containing vessel. All of these outgassing terms include a linear pressure dependence for a more realistic characterization during the initial stages of pumpdown. Specific conductance equations for the entire range of vacuum flow, from viscous to molecular, are combined with the mass conservation equation to obtain a nonlinear partial differential equation for the pressure in a vacuum system. The resulting equation is then solved using an iterative finite difference scheme to give the pressures as a function of time and location. Pressure measurements from several pumpdowns of a long outgassing tube are then compared to the model results to evaluate outgassing rate parameters. The 1/t type outgassing was found to depend strongly on the humidity levels to which the tubes were exposed before evacuation.