A new methodology is developed to determine the shape distribution of gold nanoparticles (NPs) from optical spectroscopic measurements. Indeed, the morphology of Au colloids is deduced by fitting their absorption spectra with an effective medium theory which takes into account the nanoparticle shape distribution. The same procedure is applied to ellipsometric measurements recorded on photoresist films which contain Au NPs. Three spaces (L2, r2, P2) are introduced to interpret the NPs shape distribution. In the P2 space, the sphericity, the prolacity and the oblacity estimators are proposed to quantify the shape of NPs. The r2 space enables the determination of the NP aspect ratio distribution. The distributions determined from optical spectroscopy were found to be in very good agreement with the shape distributions obtained by transmission electron microscopy. We found that fitting absorption or ellipsometric spectra with an adequate effective medium theory, provides a robust tool for measuring the shape and concentration of metallic NPs.