Water reclamation will be the basis of the closed loop Water Recovery Management (WRM) system on Space Station Freedom (SSF). The life support loop is considered “closed” in the sense that water will not be resupplied to SSF, only food with sufficient water percentage to make-up for inefficiencies in recovery, will be resupplied every 90 days. Oxygen will be generated from water. Mass balance for water, atmosphere and solids pertaining to Environmental Control and Life Support Systems (ECLSS) on SSF has been modeled on a Lotus 123 spreadsheet. Pictoral and numerical representations of the mass balance is shown in Figures 1, 2 and 3 for Manned Tended Configuration (MTC), Permanently Manned Configuration (PMC) and Eight Man Capability Configuration (EMCC), respectively. Parameters are varied to analyze the sensitivity of the mass balance to different hardware combinations, crew configurations and metabolic rates. This program has been used by Boeing ECLSS design to estimate capacity, tank sizing and system integration of ECLSS hardware.