Conventionally, the most common units used for concentrations in this type of measurements is g/dl (grams per deciliters, which is rather uncommon units in other fields), so [η] is usually expressed as dl/g (units of inverse concentration according to Equation 17.14, since ηsp is dimensionless). The intrinsic viscosity reflects the average interactions of single polymer molecules with the solvent and, if the molecule is considered to be spherical, [η] is proportional to the volume of the molecule. In most of the cases, the configuration of polymer molecules in dilute solution roughly resembles a ball, whose size is characterized by the radius of gyration Rg. For polymers having a randomcoil configuration, it is calculated that Rg ∝ M1/2, where M is the molar mass of the polymer. Thus, the relationship between intrinsic viscosity and molar mass is given by