We present a scalable, out-of-plane desalination approach using ion concentration polarization. A depletion boundary separates salt ions and purified water into distinct vertical layers. The out-of-plane design enables multiplexing in three dimensions, providing the functional density required for practical application. For membrane widths of 125-200 μm, and applied voltage of 5 V, the energy requirement is 4.6 Wh L(-1) for 20 mM solution, and 13.8 Wh L(-1) for 200 mM. Energy efficiency is found to be insensitive to flow rate as the depletion boundary adjusts to yield a commensurate volume of purified water. Scaled-up devices are presented, which have a 3-fold improvement in functional density over planar systems.