An expert system based Environmental Control and Life Support System (ECLSS) trade study tool is under development which calculates resource requirements and penalties for given system configurations and mission definition parameters. The user friendly, graphical software application allows important ECLSS resources such as power, mass, volume, resupply mass (consumable and expendable), heat rejection and ultimately cost to be analyzed in an efficient hierarchical manner. Hardware resources are calculated using scaling algorithms specific to each technology, based on existing hardware where possible. Fluid mass balances are tracked and summarized as fluids input into the system and waste output leaving the system. This tool will aid in technology selection and optimization of transportation vehicle or surface habitat designs.