IsoFATE (Isotopic Fractionation via ATmospheric Escape) models mass fractionation resulting from diffusive separation in escaping planetary atmospheres and numerically computes atmospheric species abundance over time. The model is tuned to sub-Neptune sized planets with rocky cores of Earth-like bulk composition and primordial H/He atmospheres. F, G, K, and M type stellar fluxes are readily implemented. IsoFATE has two versions, the first of which simulates a ternary mixture of H, He, and D (deuterium); the second version is coupled to the magma ocean-atmosphere equilibrium chemistry model Atmodeller.