units typically describe units that fit in a certain
number of rack units, but occupy only half the width
of a 19-inch rack (9.5 in or 241 mm). These are commonly
used when a piece of equipment does not require full
rack width, but may require more than 1U of height.
For example, a "4U half-rack" DVCAM deck would
occupy 4U (7 in) height × 9.5 in width, and in
theory, two 4U half-rack decks could be mounted side
by side and occupy the 4U space.
A front panel or filler panel in a rack is not an
exact multiple of 1.75-inches (44.45 mm). To allow space
between adjacent rack mounted components, a panel is
1/32 inch (0.031 inch or 0.79 mm) less in height than
the full number of rack units would imply. Thus, a 1U
front panel would be 1.719 inches (43.66 mm) high. If
n is number of rack units, the formula for panel height
is h = (1.750n - 0.031) inch = (44.45n - 0.79) mm.
Coincidentally, a rack unit is equal to a vershok,
an obsolete Russian length unit.
The rack unit size is based on a standard rack specification
as defined in EIA-310.