Estimate the probability that a particular moiety of a side chain bears a positive or negative electrical change. This will depend on the stoichiometry of the reaction to produce the polymer as well as the chemical environment in which the finished polymer currently exists. For example, consider an amine or carboxylic acid moiety in a side chain that can have a different charge depending on the pH of the environment.
Estimate the distance between individual monomer units in the finished polymer. This will depend on the identity of the monomer units and the thermodynamic conditions in which the finished polymer exists. At a given temperature, sum the length of a monomer conformation with the thermodyamic probability that the monomer exists in that conformation to calculate an average monomer length.
Calculate the charge density per unit length of the polymer summing the probability that a side chain is charged over the length of the polymer. For a polymer containing only a single monomer group, sum the product of the charge magnitude and probability for each monomer, and divide by the length between monomers in the polymer.
Extend charge density to a volumetric measurement by considering the distance of the charge along the length of the polymer backbone as well as the distance of the charged moiety along the polymer side chain. Most polymers can be approximated by planes or cylinders, rather than a one-dimensional chain. A more accurate estimate of charge density can be calculated by considering the 2-D or 3-D coordinates of potentially charged moieties and summing along the area or volume of a plane or cylinder, respectively.