This work presents a simplified mathematical method to capture the k.p-based band structure modifications with confinement and device substrate/transport orientation in the compact model of quantum confined Nanosheet FETs. The change in effective mass with confinement is captured in terms of non-parabolic sub-bands. The estimated sub-bands are used to compute inversion charge density and gate capacitance using a bottom-up scalable compact model for different device dimensions and substrate/channel orientations. The accuracy of the proposed method is confirmed using k.p simulation in Global TCAD Solutions (GTS).