Disclosed are techniques for determining shroud size of a fan. The techniques receive by a computer processing system digital data of a three-dimensional representation of a shroud of an axial fan, partition the received data into a first partition corresponding to a shroud segment and a second partition corresponding to a fan segment, determine a shroud boundary ring for the shroud segment and a viewing angle of the shroud boundary ring, apply to an image of the first partition a beam shooting process to determine the shroud diameter, determine if there are pixels in the image, which have values that produce signals indicating that the pixels are coincident with portions of the shroud and when signal is detected, calculate the shroud diameter. One aspect includes using the determined should size opening for performing a flow simulation.