The 42U cabinet that holds 36
Twelve cabinets at 42U is 504U of rails on the purchase order. Take six U a cabinet for patch panels, horizontal cable managers, a horizontal PDU and the space at the top nothing reaches, and 72U of that never holds a server. What is left is 432U, and with 288U in use the floor is 66.7 percent full with 144U free.
Six U a cabinet is a guess until somebody counts one. It is worth doing properly, because at this scale it is the equivalent of nearly two whole cabinets, and the number is stable — whatever the first cabinet does, the rest of the row does the same.
Two runways, and they are not the same length
At 9U a month, 144U of free space is 16 months. The power side runs on different arithmetic: 4.6 kW a cabinet metered against a 6.5 kW ceiling leaves 1.9 kW a cabinet, 22.8 kW across the twelve, and installs are adding 0.81 kW a month. That is 28.1 months.
So space empties first, twelve months ahead of power. That single fact decides what to buy: cabinets and floor, not switchgear. Change the equipment mix — swap the disk shelves for GPU chassis and put the kW per U up by a factor of four — and the power runway collapses to seven months while the space runway does not move at all. The order of the two answers flips, and so does the purchase.
Free space that is not free
The number this page prints is arithmetic on totals, and totals are optimistic about rack space in a way they are not about power. Kilowatts pool; rack units do not. Four single-U gaps in four cabinets are four U on the report and zero U to anyone holding a 4U chassis, and the gaps left by a decommissioning round are almost always exactly that shape.
The other half of the problem is that a gap needs more than rails. It needs a port on the top-of-rack switch, an outlet on the PDU with the right connector, and enough power in that specific cabinet. A cabinet with 8U free and no outlets left is full. If the inventory system can report largest contiguous free block per cabinet, that number is worth more than the total this page gives.
The horizon, and what it costs
Twenty-four months at 9U a month is 216U more equipment, taking the total to 504U against 432U of usable space. That is 72U over, which is two more cabinets at 36 usable U each and 35 square feet of floor including the aisles.
Those two cabinets bring the load to 74.6 kW across fourteen cabinets, which is 5.33 kW each against a 6.5 kW ceiling. Adding cabinets solves the space problem and improves the power figure per cabinet at the same time, because the load spreads. That is the ordinary case. The awkward case is when the new equipment goes into existing cabinets rather than new ones, and then the per-cabinet ceiling arrives long before the floor runs out of room.
Questions people ask
How many U are usable in a 42U rack?
Whatever is left after your own patch panels, cable managers, horizontal PDUs and blanking, which is a number to count rather than look up. Six U a cabinet is a common enough allowance to use as a placeholder and it is wrong for plenty of installations — a cabinet with vertical PDUs and no horizontal patching gives back nearly all of it, and a network cabinet with a panel above every switch gives back much less. Count one and use that.
Will I run out of space or power first?
It depends entirely on your install rate and the kind of equipment being installed, which is why the page computes both runways rather than asserting one. At the defaults space runs out at 16 months and power at 28. Multiply the kW per U by four, which is roughly what moving from storage to dense compute does, and power arrives at seven months while space is unchanged. There is no general answer and the two orders lead to completely different purchases.
Does free rack space mean I can install a server there?
Not on its own. Rack units have to be contiguous for a chassis of any depth, and a floor with plenty of free U often has it as single-U gaps left by decommissioning. Beyond the rails there has to be a switch port, a PDU outlet with the right connector and enough power in that cabinet. Every one of those runs out independently, and the smallest of them is the real capacity.
What growth rate should I use?
Your own, from the change log, net of removals, over at least twelve months. A plan is a statement of intent and installs are a measurement, and the two rarely match. If the history is not available, count what went in over the last quarter and treat the answer as provisional until there is a year of it.
Should decommissioned equipment count as in use?
Until it physically leaves the rails, yes, because it is occupying the space and often the power outlet as well. Kit that is powered off but still mounted is one of the more common reasons a floor reports itself fuller than it is, and a decommissioning sweep is usually cheaper than a new cabinet. Count what is bolted in, then run the page again after the sweep.