A simulation of slit dies in operation is described. Two-dimensional finite element analyses are used for the pressure field in the die (based on Hele-Shaw flow assumptions), and for the deflection of the die body resulting from this internal pressure (using Mindlin thick plate theory). These analyses are coupled in an iterative scheme, which converges to give results for the dynamic operating performance of the die. Predictions of deflections and pressure drops agree closely with experimental data. Post-processing of primary pressure results yields the stream function, melt residence time, total melt strain, and wall shear stresses. The calculation routines are enclosed within a graphical user interface in the MS-Windows operating environment, running on PCs, and providing a powerful and convenient computational aid for the designer and process operator.