The physical specification is based on the Eurocard standard of: 3U (5.06 inches or 128.5 mm), where height "U" is measured in rack units, rounded for a lip (nb: 3U in standard rack units would be 5.25 inches or 133.3 mm). 1HP (0.2 inches or 5.08 mm), where width "HP" is measured in horizontal pitch units.