Functional Unit (FU)

The quantified function a system delivers, used as the reference to which every input, output and impact is related — for example the production of 1 m³ of reclaimed water of a stated quality. Two options can only be compared fairly if they deliver the same functional unit. Choosing it well is one of the most consequential decisions in a study: “1 m³ treated” and “1 m³ reused for irrigation” can rank the same technologies differently.