We demonstrate a method to determine absolute crystallinity in high molar mass poly(3-hexylthiophene) (P3HT), as used in commercially relevant organic photovoltaic devices, using enthalpy of fusion and melting temperature values derived from differential scanning calorimetry (DSC) and C-13 CPMAS NMR. By studying P3HT with molar masses ranging from 3.6 to 49 kg/mol and using recent work on oligomeric 3-hexylthiophene, we demonstrate several critical items. First, that proper extrapolation to infinite chain lengths, i.e., crystal size, yields values for the equilibrium melting temperature T-m(0) of 272 +/- 6 degrees C and the enthalpy of fusion per crystalline repeat unit Delta H-u of 49 +/- 2 J/g for Form I crystals of P3HT. Second, that a simple correction for crystal size using T-m(0) is critical for determining an accurate degree of crystallinity from enthalpy measurements because of finite crystal size effects. Furthermore, our results demonstrate that the ordered fraction of P3HT measured from C-13 NMR is indistinguishable from the DSC-determined crystalline fraction, once crystal size corrections are properly implemented. The connection between crystal size and melting temperature is affirmed by successive self-nucleation and annealing (SSA) measurements, which, when performed as a function of molar mass, allowed us to identify the molar mass at which chain folding occurs in P3HT in the melt, approximate to 11.5 kg/mol.