<p>Despite rapid growth in corporate environmental, social and governance reporting, water use sustainability reporting remains inadequate. Current reporting combines qualitative evaluations of investment pledges to improve sustainability with limited quantitative metrics. Unfortunately, these semi-quantitative metrics differ substantially across reporting entities, and their algorithms may not be publicly available. Their non-uniform and opaque nature raises the potential for greenwashing and hinders identifying cost-effective investments to improve sustainability. To address this deficiency, we propose a transparent and quantitative ‘water sustainability index’ (WSI) as a novel metric. The WSI considers the volume and source water type for watershed withdrawals, the volume and quality of wastewater discharges, the volume of water consumed, and the extent to which facilities reuse water. Weighting factors adjust for operations in stressed watersheds while being tuned to encourage the adoption of sustainable facility improvements. WSI’s quantitative and transparent nature helps corporations identify cost-effective investments to improve water sustainability.</p>