urrent satellite altimeters map sea surface height (SSH) with an effective spatial scale of O(100 km) and, as a result, surface ocean velocity can be appropriately estimated from merged SSH fields by assuming geostrophic equilibrium. The validity of the geostrophic assumption down to the spatial scale of O(10 km) that will be newly resolved by the next generation of satellite altimeters, such as the Surface Water Ocean Topography (SWOT) mission, remains unknown. In this study, the accuracy of geostrophy for the estimation of surface currents from a knowledge of instantaneous sea level is quantified using the hourly fields from a tide- and eddy-resolving global numerical simulation. Geostrophic balance is found to be the leading-order balance in frontal regions characterized by large kinetic energy, such as the western boundary currents and the Antarctic Circumpolar Current. Everywhere else, the ageostrophic flow is of comparable or larger amplitude than the total flow. As expected, the validity of geostrophy is shown to improve at low frequencies (typically <0.5 cpd). Global estimates of the horizontal momentum budget reveal that the tropical and mid-latitude regions where geostrophic balance fails are dominated by fast (e.g., semidiurnal and supertidal) unbalanced motions and turbulent stress divergence terms rather than higher-order geostrophic terms. These findings indicate that the estimation of velocity from geostrophy applied on SWOT raw sea level maps may be challenging away from energetic areas.