This paper discusses the process of estimation of the effective relative permittivity for 3D-printed dielectric slabs with varying infill densities. The dielectric slabs with varying infill densities were placed inside rectangular waveguides with varying dimensions that correspond to different frequency ranges to determine the relative permittivities of the dielectric slabs. The results from the measurement of relative permittivity for these dielectric slabs with varying infill density indicated that the relative permittivity of 3D-printed dielectric slab decreases as the infill density decreases. This reduced relative permittivity of the slab has been coined as effective relative permittivity (εr.eff) throughout the paper. The relationship between the εr.eff and the infill density which was found to be linear in our previous simulation study was validated for higher infill densities through extensive measurements conducted in this study.