A precise modeling framework for short-channel nanoscale double gate (DG) and gate-all-around (GAA) MOSFETs is presented. In the subthreshold regime, the modeling of the electrostatics of the DG MOSFET is based on a conformal mapping analysis. This analytical 2D solution of Laplace's equation gives the inter-electrode capacitive coupling. The GAA MOSFET is a 3D structure to which the 2D conformal mapping technique is not directly applicable. However, due to the structural similarities, the DG calculations can also be applied with a high degree of precision to the cylindrical GAA MOSFET by performing a simple geometric scaling transformation. Near and above threshold, self-consistent procedures invoking the the 2D/3D Poisson's equation in combination with boundary conditions and suitable modeling expressions are used to model the electrostatics of the two devices. The drain current is calculated as part of the self-consistent treatment, and based on the precise modeling of the 2D/3D electrostatics the intrinsic capacitances can also be extracted.