In this letter, we propose to introduce the notion of equivalent capacitance and to generalize the so-called equivalent-thickness concept to model arbitrary shapes of lightly doped nonplanar multigate MOSFETs, without the need to introduce any unphysical parameter. These definitions, which merely map a multigate geometry into the symmetric double-gate (DG) MOS-FET topology, have been validated by extensive comparison with 3-D numerical simulations of quadruple-gate, triple-gate (TG), triangular gate, cylindrical gate-all-around, and DG Fin Field Effect Transistors (FinFETs). Based on this modeling approach, any multigate architecture inherits of the fundamental relationships that have been developed for planar DG MOSFETs, including the normalization of all electrical quantities that considerably simplifies its analysis. In addition, considering a constant mobility, we find that the model can predict electrical characteristics of FinFETs from 275 to 425 K, without the need for any additional parameters. Finally, we were able to predict electrical measurements of a TG MOSFET, making of this generic model an interesting candidate for a design-oriented compact model for arbitrary multigate MOSFETs geometries.