The common 5U modules are exactly 8.75" (222.25 mm) high, although non-American manufacturers may prefer 220 mm or 230 mm. The format is called 5U because 8.75" is equal to five vertical units in a standard 19" equipment rack.
The width of a single width 5U modules varies. Some manufacturers use the width of the original Moog modules (1U = 2.125"), and as such, these fall into a sub-format called MU (for Moog Unit). MU module manufacturers include Synthesizers.com – the format is also sometimes called Dotcom, Club Of The Knobs, Mos-lab, Moon Modular , STG Sound Lab, and ModuSonics. Other 5U manufacturers, like Synthesis Technology's MOTM, Oakley Sound Systems and Modcan B-series, use a slightly narrower width of 1.75", which is sometimes referred to as MOTM format.
Most 5U modules use a +/- 15v power supply, some also need 5v.