Nanowire field-effect transistor (NW FET) biosensors are known to be highly sensitive devices that can detect extremely low concentrations of biomolecules. In this article, we present an analytical model alongside with numerical simulations to calculate the sensitivity of NW FET biosensors. The model accounts for biosensing dynamics as well as diffusion of ions in the solution and across the functionalized layer. The signal-to-noise ratio (SNR) is also estimated, which gives a lower limit in terms of sensitivity. The model is physics-based and is validated against COMSOL multiphysics simulations and experimental data. It predicts the biosensitivity down to the femtomolar concentration of biomolecules without any fitting parameter.