In 2018 the General Conference on Weights and Measures plans to redefine four of the base units of the International System (SI) – the kilogram, ampere, kelvin and mole – by fixing the numerical values of four fundamental constants. This change is meant to release the uncertainties of metric measurements from their dependence on the idiosyncrasies of particular material artefacts. And yet the planned redefinition raises an epistemological difficulty: in the absence of concrete standards, what does it mean for a measurement outcome to be more or less accurate? This puzzle is solved, I argue, by acknowledging that measurement uncertainty is a special case of predictive uncertainty, that is, the uncertainty involved in predicting the behaviour of a measurement process with a theoretical and / or statistical model of that process. In particular, the uncertainty assigned to a measuring system in the new SI reflects scientists' ability to use fundamental physical equations to predict the behaviour of that system. Viewed in this light, the planned redefinition of SI units implicitly promotes a new economy of uncertainty in the physical sciences, i.e. a new set of principles for the management of scientific uncertainty that treats measurement as the approximation of ideal theoretical relations. I explore some of the counterintuitive epistemological consequences of this shift.