Ultra-high-grade neodymium‑iron‑boron (Nd-Fe-B) magnets are essential components of clean energy technologies, including electric vehicles and wind turbines. However, they often contain dysprosium (Dy), a heavy rare earth whose extraction poses environmental, social and supply risks, and which is classified as a critical raw material by the European Union. Reducing Dy content while maintaining the magnetic performance required for these applications, enabled by material efficiency techniques, is therefore a critical step toward a sustainable energy transition. This study quantifies the life cycle environmental impact and raw material costs of manufacturing Nd-Fe-B magnets containing 1–8 wt% Dy. We modelled Nd and Dy as distinct upstream supply chains in the magnet life cycle. The results reveal that despite its low content, Dy contributes disproportionately to environmental impacts (e.g., 78% of freshwater ecotoxicity, 75% of marine eutrophication, and 78% of land use, for a baseline magnet composition with 4 wt% Dy), as well as magnet materials costs (25–44%). Reducing Dy content from 4 to 1 wt% would prevent the leaching of 480–840 kg of ore, avoid 12–17 kg CO₂-eq, reduce the fifteen other environmental impacts assessed by 11 to 64%, and save around €10 per kilogram of magnet. At the system level, without material efficiency (baseline 4 wt% Dy), cumulative Dy demand for magnets in a net-zero emissions scenario would be equivalent to two-thirds of present reserves by 2050. Limiting Dy content in magnets, whose technical feasibility has been demonstrated, represents a crucial solution to mitigate resource depletion, reduce environmental impacts and lower costs. This work has been funded by the European Union under the Horizon Europe grant 101129888 (GREENE). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission and its Health and Digital Executive Agency (HaDEA) – hereinafter referred to as the granting authority. Neither the European Union nor the granting authority can be held responsible for them.