ENGINEERING NOTE · RACK HEIGHT
42U vs 47U: choosing rack height
Between 42U and 47U the outside of the cabinet grows by 200 mm. The usable rail space grows by 222.25 mm. Those two numbers cannot both be right — and the difference is where the decision gets made.
42U = 2000 mm · 47U = 2200 mm · 1U = 44.45 mm
Outside height, 42U
Outside height, 47U
One rack unit
The gap you are actually deciding about
The subtraction
Five units in, half a unit out.
| Item | 42U | 47U | Difference |
|---|---|---|---|
| Outside height | 2000 mm | 2200 mm | +200 mm |
| Usable rail space (U count × 44.45 mm) | 1866.9 mm | 2089.15 mm | +222.25 mm |
| Height left over for everything that is not rail | 133.1 mm | 110.85 mm | −22.25 mm |
| That leftover expressed in rack units | 2.99 U | 2.49 U | −0.5 U |
| Front door mesh area | not less than 75% | not less than 75% | — |
| Rear door mesh area | not less than 60% | not less than 60% | — |
| Static load, standard rating | 1000 kg | 1000 kg | — |
| Static load, reinforced-frame models | 1500 kg on reinforced-frame models only | 1500 kg on reinforced-frame models only | — |
| Standard width × depth options | 600 × 1000 / 600 × 1100 / 600 × 1200 / 800 × 1200 mm | same | — |
- 42 × 44.45= 1866.9rail at 42U
- 47 × 44.45= 2089.15rail at 47U
- 2000 − 1866.9= 133.1non-rail at 42U
- 2200 − 2089.15= 110.85non-rail at 47U
- 133.1 − 110.85= 22.25the difference
- 22.25 ÷ 44.45= 0.5units given up, rounded
The 22.25 mm is the point of this table. Five units of rail are bought with half a unit of the space you were using for cable entry, a plinth, castors, or simply clearance to the ceiling. On a 42U cabinet that leftover is 2.99 U. On a 47U cabinet it is 2.49 U — and it is the first thing to disappear when a design adds a top-mounted fan tray.
Drawn to one scale
The two cabinets, side by side, both on the same floor line.
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1
Top clearance. Cable entry, a fan tray, or nothing at all. This band absorbs the first change a design makes.
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2
Rail space. 1866.9 mm at 42U, 2089.15 mm at 47U. This is what you are buying.
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3
Bottom clearance. Plinth, castors, bottom cable entry. Once this is spent on a plinth it does not come back.
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4
The 0.5U difference. Not a manufacturing tolerance — it is arithmetic: five units in, half a unit out of the margin.
Drawing note — the two hatched bands are drawn schematically so both cabinets read clearly. The arithmetic gives their combined height, 133.1 mm at 42U and 110.85 mm at 47U; it does not give the split between top and bottom. The measured figures on this drawing are the two outside heights and the two rail spaces.
What five units buy
5U = 222.25 mm. Five ways to spend it.
| Option | Rail used | Space left in the 5U | Note |
|---|---|---|---|
| Five 1U servers, no gaps | 5U | 0 | Densest, and the hottest. Every gap you remove is a gap the air needed. |
| Four 1U servers + 1U of cable management | 5U | 0 | The realistic version of the row above. |
| One 4U chassis + 1U of cable management | 5U | 0 | Cable management above a 4U chassis usually wants more than 1U. |
| Three 1U servers + 1U blanking panel + 1U spare | 5U | 0 | Blanking panels are the cheapest airflow spend on the list. |
| Keep 5U empty and fit nothing | 0 | 5U | Valid. Growth room costs nothing to install and nothing to cool. |
The arithmetic above is easy and it is not the hard part. The hard part is that every option except the last one reduces the free area the air has to cross, and a taller cabinet with the same door mesh turns into a taller obstruction. If the plan is to fill the extra 5U immediately and completely, read the airflow guide before the rack schedule is signed — and read the next table before choosing 47U at all.
Where 47U is the wrong answer
Four things that bind before height does.
1 — Depth binds before height.
Height is rarely the first dimension to run out. Deep servers set the depth first, and the standard depths on this range are 600 × 1000, 600 × 1100, 600 × 1200 and 800 × 1200 mm. Decide the depth before you decide the U count: a taller cabinet does not help a server that does not fit front to back.
2 — The top and bottom bands are not spare space.
They carry cable entry and they carry the plinth. If the design enters cables from the bottom, the bottom band is already spent before any decision about height is made.
3 — The containment roof has a ceiling, and it is in a test report.
The cold aisle roof system on this range was inspected at two states: closed, not more than 2450 mm overall; open, not more than 2600 mm. In working state it measured 150 mm above the cabinet top. That figure is what a taller cabinet eats into.
- 2000 mm cabinet + 150 mm= 2150 mm
- 2200 mm cabinet + 150 mm= 2350 mm
- closed-state limit= 2450 mm
Run that addition before you choose. Both totals sit under the 2450 mm closed-state figure — the second one by 100 mm. If your hall has a lower ceiling or a deeper roof build-up, that 100 mm is the whole margin.
4 — The load rating is per model, not per height.
1000 kg static load is the standard rating. 1500 kg applies to reinforced-frame models only. A taller cabinet loaded to the top is a different case from a 42U cabinet loaded to the middle — if that is the plan, ask specifically about the reinforced-frame option rather than assuming the standard rating covers it.
A U count, a width and depth, a load figure and the cabinet count is enough. We will come back with the model, the drawing and the lead time.
What arrives on paper
The tolerances, as one inspection recorded them.
| Dimension | Tolerance in the inspection | Measured | Verdict |
|---|---|---|---|
| Width | 600 ± 1.0 mm | 600.0 mm | Pass |
| Depth | 800 ± 1.2 mm | 800.0 mm | Pass |
| Height | 2000 ± 1.5 mm | 2000.0 mm | Pass |
These tolerances are from a third-party inspection of one 600 × 800 × 2000 mm IT cabinet. Ask for the equivalent table for the model and size you are ordering — do not assume it transfers.
Before the drawing is signed
Four steps, in this order.
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Step 1
Fix the depth from the equipment. Pick from 600 × 1000 / 600 × 1100 / 600 × 1200 / 800 × 1200 mm.
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Step 2
Add up the rail you actually need, including cable management and blanking panels, then add one unit of growth.
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Step 3
Subtract the top and bottom bands your design spends: plinth, castors, bottom entry, fan tray.
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Step 4
Only now pick the U count. If the answer lands between 42U and 47U, choose 47U only after checking the roof clearance and the load rating on the model you are specifying. Above 47U the cabinet is a custom build, built to order.
Rack schedule in, model and drawing out
Send us your rack schedule.
A U count, a width and depth, a load figure and the cabinet count is enough. We will come back with the model, the drawing and the lead time.
