RU totals, a spacing allowance for fans and blanking, and the standard enclosure that fits. Free, no signup, using the same preset heights as the WireFlow rack builder.
Preset heights for patch panels, switches, PDUs, UPS, shelves, and blanks
Spacing allowance so the answer leaves room for fan panels and service access
Recommends the smallest standard enclosure that fits, 12U through 45U
Flags when a build needs more than one rack, and how many
Adding up RU heights gives you a number that is always too small. A rack filled to exactly its gear total has no fan panel, no blanking to stop hot exhaust recirculating to the intake side, no spare U for the thing that gets added in week three, and no room to get a hand behind a connector. The spacing allowance in this calculator exists so the number you take to a supplier is the number you can actually build in.
Twenty percent is a reasonable place to start. It is not a standard, and it is not a substitute for thinking about what your gear dissipates: a rack of passive patch panels needs almost nothing, and a rack of amplifiers needs considerably more than twenty percent plus a plan for where the air goes.
Rail pitch is not panel height
One rack unit is 1.75 inches, or 44.45 mm, of rail pitch under EIA-310. That is the hole-pattern spacing, not the height of the panel that bolts to it. A 1U panel is nominally slightly shorter, and that difference is the clearance that stops adjacent gear binding when everything is torqued down.
The distinction matters exactly once, and it matters a lot: when you are checking whether a shelf clears the box above it. Multiply rail pitch and you will be a couple of millimetres optimistic. Read the chassis drawing instead.
Frequently asked questions
How many inches is a rack unit?
1.75 inches, or 44.45 mm, of rail pitch. That is fixed by EIA-310, and nearly all rackmount gear is built in whole-U multiples. Fractional and oversize chassis exist, so check the datasheet when a height looks unusual.
How much spare space should a rack have?
Enough for fan panels, blanking, cable service loops, and whatever gets added after the design is signed off. Twenty percent is a common planning figure rather than a rule, and a rack full of amplifiers needs more thought than a rack full of patch panels.
What are the standard rack heights?
Common shells are 12U, 24U, 42U, and 45U, and this calculator recommends the smallest of those that fits. Touring and shock racks come in their own sizes, and a shock rack loses usable U to the suspension frame, so confirm the internal height rather than the case height.
Do blanking panels really matter?
In a front-to-back airflow rack, yes. An open U is a leak that lets hot exhaust recirculate to the intake side, so solid blanks force air through the gear instead of around it. In a convection-cooled rack with side-vented equipment the calculus is different, which is why the airflow scheme belongs on the drawing.