Green Footstep is an online assessment tool for reducing carbon emissions from building construction projects and is publicly available at greenfootstep.org. The Green Footstep tool guides the user through the process of life cycle carbon assessment, and then reveals how users can theoretically reduce the net carbon emissions. Some description of the calculation method is provided on the website; however the calculations and data sources are not provided in detail. This paper provides these calculations and data sources in order to make the Green Footstep methodology completely accessible to the general public.