A simple approach based on effective medium theory is proposed and applied to evaluate and design a polarization–insensitive antireflection metagrating for terahertz waves. The period of the grating is subwavelength such that there is only one propagating mode within the grating region and high–order diffraction orders do not exist. Thus, the grating region is treated as a homogeneous medium and the whole problem can be modelled as a Fabry–Perot resonator, whose thickness then determines the transmittance. The transmittances of the fabricated device for TE and TM waves at 0.87 THz are measured to be 84% and 95% for an air–silicon surface, respectively. This simple metagrating design will find important applications in antireflection scenarios in the terahertz frequency range.