Universal calibration of retention volume in size exclusion chromatography (SEC) is examined using Gaussian chain models for linear and branched chain polymers. The question is which molecular dimension, the radius of gyration R-g or the hydrodynamic radius R-H, determines the partitioning of polymers of different architectures. The partition coefficient K is calculated for asymmetric star polymers, two-branch-point polymers, and comb polymers with a pore of slit geometry of opening d. The plots of K as a function of R-H/d for these polymers nearly overlap with each other, whereas the agreement is poorer for the plots of K as a function of R-g/d. The experimental data of the SEC retention volume recently obtained by Sun et al. for linear and branched polyethylene [Macromolecules 2004, 37, 4304] are used to demonstrate that the quality of the calibration curve using R-H is superior to the one using R-g and comparable to the one using hydrodynamic volume widely practiced in the community. The universal calibration by R-H Will facilitate inferring the branching structure of a given chain polymer from its SEC retention volume, since calculation of R-H is much easier for any chain architecture compared with the hydrodynamic volume.