The relation between the number of molecules in a polymer and their individual chain length. It is often also called molar mass distribution (MMD). Technical polymers are always polydisperse as a result of the nature of the catalyst and the polymerisation conditions. The MWD is often characterised by size exclusion chromatography (SEC), but it can also be calculated approximately from rheological data for purely linear polymers.

The distribution of molecular weight of molecules in a polymer sample. Distribution can be defined as weight average and number average.