The construction industry is a major contributor to resource consumption and waste generation in the EU, making the transition to circular construction essential. Reuse of building products is an important circular strategy, but large-scale implementation is hindered by limited market maturity and uncertainty regarding product availability. This study aims to investigate the gap between reported and estimated quantities of reusable building products in demolition projects, using data from 57 demolition permits for preschools and multifamily buildings in Greater Gothenburg. A bottom-up method was applied to estimate quantities of six product types: windows, radiators, façade, roofing, framework, and gypsum boards. Independent desktop estimations were compared with reported data (pre-demolition audits, waste plans, and waste quantities) to assess the potential supply of reusable products. Results reveal significant inconsistencies: desktop estimates often exceeded reported quantities – e.g., wooden windows in preschools were nearly triple the reported amount. Conversely, reported waste quantities for timber frameworks in multifamily buildings were nearly seven times higher than desktop estimates. These discrepancies highlight systemic deficiencies in demolition permit documentation, which risk distorting reuse potential. To address this, the study introduces scalable key performance indicators (KPIs) that estimate product quantities per building footprint area, offering a practical tool for contractors and municipalities to estimate product availability and validate pre-demolition audits. The study underscores the need for standardised and digitalised product-level reporting in demolition permits. By advancing a methodological approach for quantifying reusable products, this research supports the development of a large-scale reuse market and informs policy strategies for circular construction