Experimental GPC calibration data for polystyrene, poly(methyl methacrylate), and poly(dimethyl siloxane) in chloroform is reported. These polymers have similar hydrodynamic volumes, since their unperturbed dimensions and extensions in chloroform, due to polymersolvent interaction, are very similar. As a result, the same molecular weight calibration curve is obtained for the three polymers. Recently published GPC data is examined on a plot of logarithm of unperturbed end-to-end distance against elution volume. This procedure gives a universal calibration for linear polymer molecules, providing the column combination, solvent, and temperature do not change. For the data of Meyerhoff, use of the unperturbed dimensions is superior to the plot of η suggested by Grubisic, Rempp, and Benoit. A possible explanation, based on a model for the polystyrene gel pores, is proposed for the use of the unperturbed dimensions.