Measurements and simulations have found that water moves through carbon nanotubes at exceptionally high rates owing to nearly frictionless interfaces(1-4). These observations have stimulated interest in nanotube-based membranes for applications including desalination, nano-filtration and energy harvesting(5-10), yet the exact mechanisms of water transport inside the nanotubes and at the water-carbon interface continue to be debated(11,12) because existing theories do not provide a satisfactory explanation for the limited number of experimental results available so far(13). This lack of experimental results arises because, even though controlled and systematic studies have explored transport through individual nanotubes(7-9,14-17), none has met the considerable technical challenge of unambiguously measuring the permeability of a single nanotube(11). Here we show that the pressure-driven flow rate through individual nanotubes can be determined with unprecedented sensitivity and without dyes from the hydrodynamics of water jets as they emerge from single nanotubes into a surrounding fluid. Our measurements reveal unexpectedly large and radius-dependent surface slippage in carbon nanotubes, and no slippage in boron nitride nanotubes that are crystallographically similar to carbon nanotubes, but electronically different. This pronounced contrast between the two systems must originate from subtle differences in the atomic-scale details of their solid-liquid interfaces, illustrating that nanofluidics is the frontier at which the continuum picture of fluid mechanics meets the atomic nature of matter.