•Fracture energy of rocks is measured from size effect experiments.•Semi-circular notched specimens of various sizes are used. They easily are obtained from cores.•A fracture energy of 86 J/m2−90 J/m2 for shales, and of 8 J/m2 to 70 J/m2 is obtained for various limestones. An experimental technique based on size effect for the measurement of the fracture energy of rocks from core-based specimen is presented. It requires a simple apparatus and specimens may be easily obtained from cores. In order to obtain the fracture energy of the material, the non-dimensional energy release rate appearing in the size effect law, which is not available in the literature, needs to be computed first. This is done with the help of a continuum damage model. Computations have been performed for 4 different geometrically similar specimens and for 3 different values of the fracture energy. The non-dimensional energy release rate is back calculated from the fits of the size effect law. For the shale rocks tested, we obtain a fracture energy in between 86 J/m2 and 90 J/m2. For the limestones, we obtain a wider range, from 8 J/m2 to 70 J/m2, depending on their porosity.