Thermal models of mid-ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust. These models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast-spreading ridges. The rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros. Successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge, with ∼50–70 kW of hydrothermal cooling for every 1 m of ridge axis at crustal ages of <0.1–0.4 Ma. These estimates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges.