This paper uses the work of John Richardson, an eighteenth-century brewing theorist, to explore the view that physical quantities, though they appear ‘natural’ or ‘given’, are actually contingent entities constructed to serve particular aims. It focuses on the pounds-per-barrel extract, a brewery-specific quantity which, in a reversal of the familiar position, seems self-evidently constructed to the general reader yet came to be accepted as ‘natural’ among its users. Central to Richardson's work in achieving this acceptance was an instrument, the saccharometer, which, by providing measurements of the quantity, legitimated it. Richardson presented both instrument and quantity as tailored to serving the particular needs of his fellow brewers, at the same time emphasizing their separateness from parallel work in distillery assessment, which had made Richardson's innovation possible but now threatened his projected consensus. Richardson's overall project encompassed the direct proportionation of material costs, retail prices and Excise duties to extract values as defined by the saccharometer, which he sought to monopolize. The scheme was not wholly successful, yet Richardson's quantity remained in brewery use into modern times. The end result, I contend, shows how a quantity, by becoming naturalized, may survive the loss of its initial theoretical underpinning.