Circular strategies of prolonging and closing resource flows within the economy hold the potential of avoiding greenhouse gas emissions and reducing other environmental impacts. There are several circularity indexing systems that account for material circularity in buildings. However, there is a lack of harmonized benchmarking systems, meaning that circular buildings are mostly compared with alternatives of their own design. This study addresses these research gaps by exploring a mass-based circularity indexing system and identifying current and future references for the circularity performance of buildings, for use in FutureBuilt, a Norwegian green building innovation programme. The FutureBuilt Circularity Index system rates a building’s circularity performance between 0% and 100%. The indexing system discerns between materials and masses used during construction, and materials available at the end of life of the building. A weighting system is introduced to emphasize the importance of reusing materials at the highest level of functional quality, and to emphasize the importance of current resource circularity versus circularity at the building’s end of life. The circularity index is tested on four Norwegian buildings that have a high focus on circularity, achieving indexing levels from 44% to 69%. A mixed methods approach using statistics, environmental product declarations, and interviews is applied to establish the reference baseline for Norwegian buildings (i.e. 7% circularity) and scenarios for future circularity in 2050 (from 35% to 80% circularity). Actors in the Norwegian construction industry can evaluate the level of ambition for circular buildings by using this novel indexing system and benchmarking approach, which can also be adopted to other national or regional contexts.