Molecular weight is a key parameter of any polymer. Characterizing the molecular weight of conjugated polymers is often nontrivial due to their semiflexible backbones and poor solubilities. Perhaps the most used technique for measuring the molecular weight is gel permeation chromatography (GPC), and results are often calibrated relative to flexible polymer standards. This mismatch between chain flexibilities of samples and standards, combined with poor sample solubility in the mobile phase, leads to inaccuracies in many GPC measurements of conjugated polymers. In this work, we use a universal calibration combined with in-line concentration measurements to yield reliably accurate results for polymers of various stiffnesses. Accuracy of results is verified with absolute molecular weights obtained from static light scattering. We show that measuring the refractive index increment is key to confirm full recovery of the polymer and ensure accurate values of the absolute molecular weight from GPC.