The development of a Microsoft® Excel-compatible Environmental Control and Life Support System (ECLSS) Sizing Analysis “Tool” for conceptual design of Mars human exploration missions makes it possible for a user to choose different possible technologies in a corresponding subsystem. This tool estimates the mass, volume, and power requirements of technologies in a subsystem and the system as a whole. Furthermore, to verify that a design sized by the ECLSS Sizing Tool meets the mission requirements and integrates properly, a tabulated Integration Interface Module has been developed for quick referencing. Mass balance models that solve for component throughputs of ECLSS systems as the Water Recovery System (WRS) and Air Revitalization System (ARS) are being developed for the sizing of the subsystems. The ARS Mass Balance Model will be discussed in this paper.