This work deals with the simulation of the viscous flow of polymers in flat film coextrusion regarding asymmetrical films as well as slip at the wall and slip at the polymer-polymer interface. A one dimensional finite volume approach is used to calculate complex flows of non Newtonian materials using numerous damped numerical solvers. This iterative approach is neither depending on the numbers of layers used, nor on the mathematical models of the viscosity, the wall slip or the interfacial slip. Furthermore the work shows how this one-dimensional simulation can be done using standard spreadsheet software, so the work should help dimensioning feed blocks for coextrusion. The first step of doing this simulation is defining the following terms: overall geometry of the flow channel, required volumetric flow of the single layers, models and parameters needed to calculate the viscosity of all layers, models and parameters needed to calculate the sliding velocity at the wall and that at the polymer-polymer interface, starting conditions for the iterative simulation, precision of the calculation; The second step is the iterative solution of the following terms: 1. Asymmetry of the shear stress (in case of asymmetrical layer construction) 2. Pressure gradient to achieve the predefined overall throughput 3. Thickness of the single layers to achieve the predefined throughput of the single layer Results of the simulation of a 7 layer Film are shown and compared to simulations done with the software “Open Foam”.