The spin coating process was investigated using solutions of polystyrene dissolved in toluene. The residual film thickness depends not only on the spinning velocity and concentration, but also on molecular weight. Specific scaling exponents were determined. The molecular weight dependence was investigated in detail to reveal the type of molecular weight average. This enables a fast determination of the molecular weight by use of spin coating. The use of model molecular weight distributions yields a relation to number M(n) and weight M(w) average molecular weight.