The Costs and Benefits Appraisal Tool for Transit Buses aims to inform bus operators and city officials of the costs, emissions, and social benefits associated with bus fleets using different fuel types. The Tool’s outputs can help bus operators make the most cost-efficient decisions when making a clean bus upgrade, allow transit agencies to validate information provided by bus operators, and inform city officials of the social benefits of a low-carbon transit fleet. Compared with the previous version, this tool includes more default data on electric bus costs and operational information with wider geographic coverage and more bus types; provides social cost and benefit calculations; and includes simplified cost functions. Users are encouraged to input their own data to reflect their local situations. The Tool is published as a webtool to increase user friendliness, and allows users to compare basic procurement and business model scenarios.