This method involves measuring the flow times of a polymer in solution as it passes through a Ubbelohde viscometer and then determining the difference between the polymer solution and the solvent.

Taking into account the mass concentration of the polymer solution, the viscosity number can then be calculated.

The viscosity number can be correlated with a polymer’s molar mass. The result when polymers are subjected to major stress, during processing for example, is frequently molecular decomposition. This type of effect can be quantified by calculating the viscosity number. Molecular changes due to immersion in media can also be detected via this method (see figure).