The correlation of hydrodynamic radius with protein molecular weight (Mw) is dependent on protein conformation. If proteins are globular, a strong correlation between Rh and Mw is observed. If proteins are unstructured or elongated the measured Rh will be larger than that of a globular protein of identical Mw. Consequently, comparison to a calibration curve of globular proteins will yield a good approximation of Mw for a globular protein of unknown Mw. This principle is used to calibrate SEC columns using samples of known molecular weights as the calibration standards.