Metal production consumes around 10% of all global energy, so is a significant driver of climate change and other concerns about sustainability. Demand for metal is rising and forecast to double by 2050 through a combination of growing total demand from developing countries, and ongoing replacement demand in developed economies. Metal production is already extremely efficient, so the major opportunities for emissions abatement in the sector are likely to arise from material efficiency – using less new metal to meet demand for services. Therefore this paper examines the opportunity to reduce requirements for steel and aluminium by lightweight design. A set of general principles for lightweight design are proposed by way of a simple analytical example, and are then applied to five case study products which cumulatively account for 30% of global steel product output. It is shown that exploiting lightweight design opportunities for these five products alone could reduce global steel requirements by 5%, and similar savings in aluminium products could reduce global aluminium requirements by 7%. If similar savings to those in the design case studies were possible in all steel and aluminium products, total material requirements could be reduced by 25–30%. However, many of these light-weighting measures are, at present, economically unattractive, and may take many years to implement.