Natural fluids approximated by the H 2O–NaCl–CaCl 2 system are common in a wide range of geologic environments, including sedimentary basins associated with hydrocarbon occurrences and MVT deposits, submarine hydrothermal systems, and other metamorphic, magmatic and hydrothermal environments. We present a comprehensive numerical model and Microsoft® Excel©-based computer program to determine the compositions of fluid inclusions in the H 2O–NaCl–CaCl 2 system based on microthermometric and microanalytical data. The model consists of six polynomial correlation equations that describe liquid salinity as a function of NaCl/CaCl 2 ratio and melting temperature on each of the ice, hydrohalite, halite, antarcticite, CaCl 2·4H 2O and CaCl 2·2H 2O vapor-saturated liquidus surfaces. The cotectic and peritectic boundaries are determined from the intersections of the liquidus surfaces. The model is implicitly internally consistent and topologically correct. The model expands upon the compositional range of applicability and the data types that can be used for compositional determination. It reproduces experimental data for all compositions that lie within the H 2O–NaCl–CaCl 2·4H 2O compositional triangle in the H 2O–NaCl–CaCl 2 system and yields accurate reproductions of the H 2O–NaCl and H 2O–CaCl 2 binaries. Furthermore, in comparison to previously published models, the one presented here eliminates systematic errors, wavy isotherms and cotectic and peritectic curves with local “bumps.”