A method is presented for computation of performance of two-dimensional (planar) diffusers with steady turbulent inflow of an incompressible fluid. Previous methods can predict one regime of flow. The present method gives accurate predictions covering three flow regimes: unstalled flow, transitory stall, and fully developed stall. The method is a considerable extension of the procedure given by Ghose and Kline [5]; it also uses some ideas from the method for fully stalled flows given by Woolley and Kline [4]. The flow model is zonal and steady. It uses a one-dimensional flow model for the potential core. A momentum integral equation and an entrainment equation are employed for the boundary layer zone. Simultaneous solution is employed to model the different zones where the flow is separating or separated. Improved correlations of flow detachment and of the boundary layer flow state approaching detachment are presented as part of the work and employed in the computations. These will be reported more fully in a separate paper. This model is too simple for the full representation of the physics of transitory stall, which is not symmetric, steady, or one-dimensional in the core. Despite this, the main features of the mean flow, including wall pressure as a function of streamwise location, are accurately represented with very modest computation times, typically tenths of a second on an IBM 3033. The results again indicate that the key features in modeling separated flows are: • correct representation of blockage of shear layers and stalled zones, • adequate modeling of the interaction of potential and viscous zones.