In this paper, we present a compact model for charge density and gate capacitance for low effective mass channel material based quadruple-gate FETs (QGFETs). The proposed model accounts for the effect of quantum capacitance and conduction band non-parabolicity, which are important in FETs comprised of the low effective mass channel material. In modeling of QGFET, we propose and use a new form of Fermi–Dirac integral of order $-$ 1/2, which matches closely with the numerical data. Our model for the charge density and gate capacitance is compared with three-dimensional technology computer aided design (TCAD) simulations data, and shows excellent match. The proposed explicit compact model can be easily employed in efficient exploration of circuits based on the low effective mass QGFET nanowires.