Analysis of molecular flow in simple geometries is commonly approached using methods such as Monte Carlo or view factor analysis. These methods are applicable to a generalised geometry but require significant rework for even small geometry changes, rendering them impractical for rapid engineering design analysis. A method is developed, drawing on the strong analogy between Knudsen cosine reflection for molecules and Lambert diffuse reflection for radiation, where analysis is conducted using the heat transfer facilities of a commercial code. Test cases establish the accuracy of the method, and practical examples illustrate its use in arbitrary topologies.