Temperature-dependent small-angle and wide-angle X-ray scattering (SAXS/WAXS) measurements on a series of chemically well-defined and highly regioregular poly(3-hexylthiophenes) were analyzed to determine absolute values of the crystallinities. The analysis is based on the evaluation of the scattered intensity from the amorphous regions providing an easy and fast method for the determination of the crystallinity in the class of side chain substituted polymers. The resulting values are in the range of 68-80% at room temperature depending on the molecular weight. Based on these values, an extrapolated reference melting enthalpy of a 100% crystalline material was determined (Delta H-m(infinity) = 33 +/- 3 J/g) for use in DSC measurements. For higher molecular weights a decrease of the crystallinity was observed which can be explained by the onset of chain folding as deduced from the analysis of the SAXS patterns. An in-depth analysis based on Ruland's method showed that the crystalline regions of P3HT exhibit a large amount of internal disorder.