To be precise, 1U equals 1.75-inches (44.45mm) of rack height. Therefore, a 2U rack mount height would be 2 x 1.75", which equals 3.5-inches high. A 3U height would be 3 x 1.75" = 5.25-inches. It should be noted that the 1U, 2U, 3U, etc., heights are maximum dimensions.What is 1U rackmount?
1U is the standard unit of measure for designating the vertical usable space or height of racks/cabinets. It is the smallest rack mount size and equal to 1.75 inches height.