The Eurocard packaging system is a complex mixture of English and metric dimensions. Although this may seem confusing, widespread conformance to the standard dimensions means that users are not troubled by these issues.
Eurocard subracks have standardized sizes in all three dimensions. Height is specified by the unit ‘U’ (which stands for ‘Unit’), with 1 U being 1.75 inches. Width is specified by the unit ‘HP’ (which stands for ‘Horizontal Pitch’), with 1 HP being 0.20 inches. Subracks usually range in height from 1U to 9U.
The height of a eurocard is less than the height of rack by 33.35 mm to allow space for panels and card guides. The height of the card in a 3U rack is therefore 100 mm. As two stacked 3U cards are the same height as a 6U card, this scheme allows racks to be constructed which mix 3U and 6U cards. Front panels are also slightly smaller than the rack size, and the typical panel height for a 133.35 mm 3U rack is 130 mm. Eurocards come in modular depths that start at 100 mm and then increase in 60 mm increments.
The standard allows for a vast number of permutations, but in practice there are only comparatively few sizes in use. Heights are commonly 3U or 6U, and only occasionally 9U. The 160 mm depth is the most common today, followed by 220mm. However standard hardware is also available to accommodate depths of 100 mm, 280 mm, 340 mm, and 400 mm.
A 3U high subrack is 133.35 mm (5.25 inches) high and accepts a 3U Eurocard which is 100 mm high.
A 6U high subrack which is 266.7 mm (10.5 inches) high and accepts 6U Eurocards which are 233.35 mm high.
More information about VMEbus history and specificiations can be found at the VITA website.