The sizing of rack mount systems is defined by the term “U”. Rack Unit or 1U is 1.75 inches (44.45 mm) high. So 3U racks = 5.25 Inches (133.35 mm). RS stock a range of 3U racks from leading suppliers such as nVent -Schroff, Metcase, Rittal, Hammond, OKW and RS Pro.