<p>This study introduces the MS Excel-based WWTP-CF calculator, a user-friendly tool for calculating carbon footprints (CFs) of wastewater treatment plants (WWTPs), identifying major greenhouse gas (GHG) emission sources, and developing effective mitigation strategies. The tool was applied to small- to medium-sized urban WWTPs (uWWTPs) and central industrial WWTPs (iWWTPs). The results showed that CFs in uWWTPs were not correlated with plant capacity but were significantly influenced by unit operations, particularly membrane filtration and sludge disposal through landfilling. Energy consumption and sludge drying were identified as major GHG emission sources. In iWWTPs, energy-related emissions dominated as plant capacity decreased. The higher specific CFs and energy consumption observed in iWWTPs highlight the role of wastewater organic load in driving GHG emissions. The findings emphasize renewable energy adoption, process optimization, and improved configurations, particularly anaerobic/anoxic/oxic (A²/O) systems, as key strategies for reducing GHG emissions, fostering more sustainable wastewater management. The proposed simple tool provided an effective means of assessing major plant-based emission sources and developing strategies for CF reduction. Adoption of the proposed MS Excel-based WWTP-CF calculation tool can support preliminary assessments of emission sources, helping to identify reduction strategies and advance sustainability goals in the wastewater sector.</p> Graphical Abstract <p></p>