U

"U", or less commonly known as Rack Unit, or "RU", is a unit of measure used to describe the height of equipment intended for mounting in a 19-inch rack or a 23-inch rack (the dimension refers to the width of the equipment mounting frame in the rack). One rack unit ("U") is 1.75 inches (44.45 mm) high.

The height of a piece of rack mounted equipment is frequently described as a number in "U". For example, equipment which is 1¾" high is referred to as 'one rack unit' or "1U"; 3½" high is referred to as 2 rack units or "2U" and so on.

Each U on a rack's mounting rail consists of three (3) holes. The hole spacing alternates at: ½"– ⅝" – ⅝" and repeats. The start and stop of the "U" space is centered between the ½"spaced holes.