It is crucial to accurately characterize methane absolute adsorption in kerogen nanoporous media for gas -in-place evaluation and well productivity prediction. Assuming that methane forms a single-layer adsorption in kerogen nanopores, a large number of approaches have been reported to convert the experimentally measured excess adsorption to the absolute adsorption. Recently, we have shown that methane adsorption behavior depends on the pore size and may be very different from the single-layer adsorption model, such as the Langmuir model. Thus, it is necessary to explicitly consider the pore size distribution (PSD). While these conversion methods have been extensively used, their validity in the characterization of methane absolute adsorption in nanoporous materials, such as kerogen, has not been systematically assessed. As in our previous work, we used model kerogen with varying PSDs and grand canonical Monte Carlo simulations to model methane adsorption up to 500 bar to assess various commonly used methods converting excess adsorption to absolute adsorption. We find that the predetermined density methods using 373 or 424 kg/m(3) may show unphysical phenomena and Langmuir as well as SDR models can largely overestimate the absolute adsorption. On the other hand, the Ono-Kondo (OK) lattice model with PSD can accurately characterize the absolute adsorption in nanoporous media. Interestingly, Langmuir and SDR models coupled with PSD can provide comparable predictions to OK with PSD. In addition, we also suggest to use the high-pressure excess adsorption data (up to 500 bar), instead of the commonly used low-pressure excess adsorption measurements (up to 150 bar). Our work also calls for the accurate characterization of PSD in nanoporous materials to obtain their absolute adsorption capacity.