The work of fracture of refractories is commonly calculated from Crack Mouth Opening Displacements (CMODs) in Wedge Splitting Tests (WSTs). This paper proposes a methodology for estimating the fracture energy from Notch Opening Displacement (NOD) measurements, which is useful for setups where CMOD is not accessible. NODs and CMODs are calculated for both faces of two WSTs experiments on a castable refractory via Digital Image Correlation (DIC) and finite element simulations. A quadratic function fits well the non-linear CMOD vs. NOD behavior in the crack initiation regime, while an affine trend describes the propagation regime. Although the nonlinearity associated with crack initiation is more complex, the crack propagation energy can easily be estimated from NOD data when CMODs cannot be measured.