Optical ceramics are emerging materials for optical devices operating in the Mid-Wave Infrared (MWIR) spectral range. Thus determination of their refractive index is of huge importance but is still a challenging issue. In this work, a model is delivered, which highlights a simple mathematic relationship between the refractive index of an optical material and its transmittance spectrum measured by Fourier Transform Infrared Spectroscopy. Such model allows determining the refractive index of ceramics fabricated by hot-pressing of MgF2 nanoparticles. The reliability of this method has been verified on a silicon wafer taken as a reference (< 0.5%, discrepancy with data from literature on Si). Our approach can be applied to a variety of promising novel optical materials being developed.