Nuclear magnetic resonance (NMR) transverse relaxation time (T-2) measurements of fully water-saturated rock samples can provide a reference for pore size distribution (PSD). Commonly, surface relaxivity of the rock must be known before obtaining an absolute PSD sample using NMR data. There are many reports concerning the surface relaxivity of sandstones, shales, and carbonate rocks, however, little research has been performed on the surface relaxivity of coals, which limit the application of the NMR in PSD evaluation of coals. To obtain a standard surface relaxivity for PSD measurements of coals, we performed NMR, low-temperature N-2 adsorption (LTNA), and mercury intrusion porosimetry (MIP) measurements for 15 bituminous and anthracite coals, whose R-o. range from 0.52% to 3.07%. The results show that the values of surface relaxivity from LTNA (rho 2. svR) are inconsistent with those from MIP (rho 2.(MIP)) for all samples. Moreover, the applications of rho 2.(SVR) and rho 2.(MIP) for PSD conversions are valid only for the smaller pores and larger pores, respectively. Evidently, both these two surface relaxivities cannot be used individually to calculate full-scale PSD. To obtain the actual surface relaxivity (rho 2) for full-scale PSD, we rebuilt a full-scale PSD by combining the pores smaller than 25 nm from LTNA and the pores larger than 25 nm from MIP. The results indicate that the calculated rho 2 provides excellent function for PSD transformation for all coal samples. Finally, the references of rho 2 for different coals are provided, i.e., values of 2.1 mu m/s for sub-bituminous coal (low-rank coal), 3.0 mu m/s for bituminous coal (medium-rank coal) and 1.6 mu m/s for anthracite coal (high-rank coal). Using these referential surface relaxivities, a T-2 distribution from NMR measurement can be converted to a PSD, which is applicable not only in the laboratory but also in field applications such as well logging.