A relatively cheap, short cut method for prediction of the form and location of the gas core, and the residual plastic wall thickness in gas assisted injection molding (GAIM) is described. The basis is a steady state, single phase solution for flow of the polymer melt through the channel of interest, without the need to model the gas penetration. The gas-polymer interface position is predicted by an appropriately chosen isovel of the flow. For a prismatic or slowly varying channel, only a two-dimensional developed flow solution is required. For more sharply varying cross sections, and where bends are present, a steady three-dimensional (3D) solution is necessary. When a gas delay is used, during which polymer cools to the cavity walls, a solution for transient conduction in the static melt is carried out before the flow solution. By comparisons with the results of full 3D, transient, two-phase simulations of GAIM, and with experimental results, the short cut method is shown to provide reasonable approximations, and in contrast to other currently used approximate methods, captures thickness variations around the circumference of noncircular channels, The asymmetric gas core location in bends is reproduced, as is the increased plastic wall thickness resulting from cooling during a gas delay. While the full analysis will still be required for complex parts and when high accuracy is required, the described short cut method is likely to prove useful in many other cases.