Gas sorption can lead to the volumetric swelling of the shale matrix and reduction of the effective pore volume, which further impacts the gas transportation in micro- and nanopores in shale. At present, it is very challenging to directly measure the pore volume shrinkage (i.e., pore volumetric strain) in the laboratory. In this study, an innovative method is proposed to quantify the pore volumetric strain resulting from the sorption-induced matrix swelling in shale. More specifically, Gibbs methane sorption capacities of the Barnett and Eagle Ford shale core samples were determined via volumetric and gravimetric methods, respectively. Meanwhile, the bulk volume swelling of the shale core sample was also measured during the gas sorption process. Correlations between the sorption-induced bulk and pore volumetric strains were developed to calculate the pore volume shrinkage ascribed to the gas sorption, which was further validated with the measured gas sorption capacities. It is found that the pore volume shrinkage is 3.7-4.8 times greater than the bulk volume swelling during the gas sorption process for the Barnett shale, while such a ratio reaches as high as 59.8-67.1 times for the Eagle Ford shale. In addition, the sorption induced bulk and pore volumetric strains follow a power law relationship for both Barnett and Eagle Ford shales, which yields a nearly identical absolute gas sorption isotherm with the ones determined using volumetric and gravimetric methods. The results of this study give insight into the feasibility of characterizing the sorption-induced pore volume shrinkage in shales and illustrate the benefits of applying both gravimetric and volumetric methods to evaluate the gas sorption behaviors.