Estimates of the world's mineral resources of numerous by-product metals remain highly uncertain at best, despite the high criticality of many of these elements to society. This stems from the limited reporting of the concentrations of these elements within mineral deposits by the mining industry, meaning that we require methods to estimate the availability of these resources that overcome this limitation. Here, we present a method for quantifying poorly reported mineral resources of by-product metals that builds upon deposit-by-deposit approaches to global resource estimation, arguably the best-practice approach for well-reported commodities, but also adds the use of proxies for by-product grade estimation. This proxy method allows for deposits with known or inferred by-product metals to also be incorporated within global resource estimates and provides a greater basis for assessing future supply potential. We demonstrate the application and verification of this methodology with indium, a critical metal for which < 1% of identified zinc, tin, and copper deposits potentially hosting indium mineralisation report grades using CRIRSCO (or equivalent) mineral resource reporting codes. The use of the method outlined in this manuscript will allow the global resources of any metal commodity, especially the often under-reported by- and co-product metals that are becoming increasingly essential to modern life, to be quantified to a significantly greater level of accuracy and precision than is allowed by other approaches.