In this study, we present an analytical equation for describing the memory window of ferroelectric field-effect transistors (FeFETs). The analytical equation is derived based on the effect of oxide charge on the threshold voltage shift of the field-effect transistor and can be expressed by simpler parameters, such as the quantity of polarization switching and trapped charge. We demonstrate that the derived equation is in quantitative agreement with the results of the numerical calculations using a technology computer-aided design simulation tool, which confirms the validity of the equation. Our results show that the analytical equations provide an accurate and practical description of the memory window for FeFETs with various structures.