Previous studies have investigated capital and operating cost comparisons between membrane bioreactors and conventional treatment technologies for particular treatment scenarios. This study expands on prior work with the development of an Excel and Biowin based tool to compare the capital and operating costs of equivalent size greenfield wastewater treatment facilities using membrane bioreactor (MBR) vs. conventional activated sludge (CAS) processes based on identical influent loading conditions, variable temperature and flow peaking factors, and varying levels of effluent treatment objectives. The resulting capital and operating cost tables developed using this tool indicate which system is most cost-effective for each scenario evaluated. The tool can also be used to easily compare the 20-year Present Worth cost impacts of changing input variables such as the cost of concrete, land, chemicals, or electricity.