The bound model to determine coating toughness limits by nanoindentation tests was originally proposed by Toonder et al. However, the assumptions in this method conflict with each other. It provides lower and upper toughness limits for load control tests; however, it is unable to give the lower toughness limit for displacement control tests. In this paper, a modified model is presented by more comprehensive analysis of the unloading curves at the points where the crack starts and stops respectively. This modified method equally provides the lower and upper limits of fracture dissipated energy for both load control and displacement control. The model developed here has been successfully applied to fullerene like CNx coatings deposited on various substrates such as Si, SiC and Al2O3. It is also applicable to other coated systems provided that the through thickness fracture induced excursion is observed in the load–displacement curve.