Scope 3 emissions, which encompass both direct emissions and induced emissions along upstream supply chains, have emerged as an important metric for evaluating corporate responsibility toward climate change. Scope 3 targets are being evaluated as a core element of decarbonization plans in many economically important jurisdictions. Yet robust Scope 3 reporting has been challenging due to varying system boundaries, diverging emission factors used for estimation, and the general absence of compliance audits. To overcome these limitations, and understand the efficacy of voluntary reporting, here we establish a consistent, independent estimate of Scope 3 emissions at the firm level using a new enterprise-level global supply chain database. We find that, collectively, company self-reported Scope 3 emissions may be underestimated by nearly 50%, or 0.75 Gt C, compared to our estimate using a harmonized approach.