The weight average molecular weight is a way of determining the molecular weight of a polymer. Polymer molecules, even if of the same type, come in different sizes (chain lengths, for linear polymers), so we have to take an average of some kind. For the weight average molecular weight, this is done as follows: weigh a number of polymer molecules, add the squares of these weights, and then divide by the total weight of the molecules.

Intuitively, if the weight average molecular weight is w, and you pick a random monomer, then the polymer it belongs to will have a weight of w on average.

The weight average molecular weight can be determined by light scattering, small angle neutron scattering (SANS), and by sedimentation velocity.

An alternative measure of molecular weight for a polymer is the number average molecular weight; their ratio is called the polydispersity index.