By analyzing phonon dispersion, we have evaluated the average Young’s modulus and Poisson’s ratio in graphite and in graphene grown on Ru(0001), Pt(111), Ir(111), Ni(111), and BC3/NbB2(0001). In both flat and corrugated graphene sheets and in graphite, we find a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. The unique exception is graphene/Ni(111), for which we find different values because of the stretching of C-C bonds occurring in the commensurate overstructure (0.36 and 310 N/m for the Poisson’s ratio and Young’s modulus, respectively). Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. The high crystalline quality of graphene grown on metal substrates leads to macroscopic samples with high tensile strength and bending flexibility for use in technological applications such as electromechanical devices and carbon …