This paper generalizes existing BRDF fitting algorithms presented in the literature that aims to find a mapping of the parameters of an arbitrary source material model to the parameters of a target material model. A material model in this context is a function that maps a list of parameters, such as roughness or specular color, to a BRDF. Our conversion function approximates the original model as close as possible under a chosen similarity metric, either in physical reflectivities or perceptually, and calculates the error with respect to this conversion. Our conversion function imposes no constraints other than that the dimensionality of the represented BRDFs match.