<p>Life cycle assessment (LCA) databases have focused on capturing the links between unit processes to calculate the direct and indirect impacts of supplying individual products or services. These databases rarely record sufficient properties of flows to properly evaluate the respect of mass balance. Mass balance can help ensure data quality by verifying correctness and completeness and opens up potential re-use of inventories in other fields. We propose a reconciliation of unallocated LCA databases with conservation of mass. We formulate an optimization problem for a large database, like ecoinvent, to comply with conservation of mass while changing the mass content of products and elementary flows as little as possible. We find that we can estimate the mass imbalance in the database (36.79% imbalance for ecoinvent v3.10) and reduce it to 10.76% imbalance through the automized application of four steps. We disaggregate heterogeneous flows, add obviously missing flows, include capital end-of-life flows when capital products are used and optimize the mass content of products and elementary flows. The remaining imbalance (10.76% imbalance for ecoinvent v3.10) can be attributed to missing flows and needs some manual work to resolve. This is the first step towards a transformation of LCA databases into technology-rich material flow analysis (MFA) databases and enables the calculation of purity and composition of all products and elementary flows.</p>