Measure the route the cable takes, never the distance between the rooms
The number that matters is the length of the path, and the path is longer than the building suggests. A cable that goes from a cupboard, up into a ceiling void, along a joist run, down a wall cavity and out at a faceplate has travelled twice the diagonal between the two rooms — sometimes more.
Walk it with a length of string or a long tape and add up the segments. Then add the vertical drops at each end, which are the parts people forget, and a working allowance at both terminations so that the panel or the faceplate can be pulled out into your hands.
Write down the total for each run separately. Two runs to the same room are two numbers, not one number doubled, because they rarely take the same route.
The hundred-metre figure is a channel, and your patch leads live inside it
Structured cabling is specified as a channel: the fixed run in the wall plus the flexible leads at each end. The published limit for a copper channel in the categories used in homes is one hundred metres, and it is divided — ninety metres for the permanent link between the panel and the faceplate, ten metres in total for the patch cords at both ends.
That division is where domestic installations go wrong. Two long leads from a cabinet to a switch, and another long lead from a faceplate to a desk, eat into the allowance that the run in the wall was measured against. If the fixed part of a run is already near ninety metres, the lead at each end has to be short, and there is nothing in the standard that gives the length back.
Below the categories, the same figure governs. It is a property of the channel, not of any particular rate.
Count the ports twice, then count the ones you will want in two years
Two counts, and they answer different questions.
The first is what plugs in today: every fixed device in every room, plus the wireless access points, plus the uplink back towards the router. Write it out room by room rather than as a total, because the count that matters at a switch is the count in that room.
The second is the count you would have to replace the switch to reach. A five-port switch with four devices and one uplink is a full switch, and the fifth device arrives eventually. If a run is going into a wall for the next fifteen years, terminate more pairs than you presently need — the labour is the expense, and a spare socket costs almost nothing while the wall is open.
Where any of those ports will feed a device powered over the cable, count separately how many, and read the total budget the switch publishes rather than its per-port figure. Per-port wattage says what one port may offer; the budget says how many ports can offer it at the same time.
Measure a cabinet in three dimensions, and give depth the most attention
Width is the least interesting of the three, because the mounting rails are standardised at nineteen inches and every rack-mounted product is built to it. Height is published in rack units, one of which is 44.45 mm, and a unit’s height is the count of those.
Depth is where the orders go wrong. The figure to compare is not the cabinet’s external depth but its usable depth behind the mounting rails, and the thing that has to fit inside it is the equipment plus its power lead, plus the patch leads coming out of the front, plus the bend radius those leads need before they can turn. A short wall-mounted cabinet swallows a patch panel and a compact switch and refuses a desktop-style unit laid on a shelf.
Measure the doorway and the stairwell too, if the cabinet is large. That is not a joke about furniture; a full-height cabinet is delivered assembled.
Write down the rack units before you write down the equipment
Add up the units your equipment occupies, then add the parts that occupy units without being equipment: a shelf for anything that is not rack-mountable, a blanking panel, a patch panel per bundle of runs, and a spare unit above anything that runs warm.
A patch panel’s port count is chosen from the second of the two counts above, not the first, and it is the item most worth over-specifying, because adding one later means re-terminating.
Measure the drive bays before you count the terabytes
Capacity is the number people shop on and the one that never causes a physical problem. The measurements that do are the form factor, the height and the interface.
Form factor is the 3.5-inch or 2.5-inch designation, and an enclosure states which its bays take — many take both, with different mounting holes for each. Height is the measurement that catches people out on the smaller size, where 7 mm and 15 mm are both sold and a 15 mm drive does not go into a bay built around a 7 mm one. Interface is the connector and the protocol behind it, and they are separate questions: a socket shaped for one card can be wired for a different bus.
On solid-state cards, the four digits of a designation such as 2280 are a measurement and nothing else — the width in millimetres, then the length. A card of the right length can still be keyed for a socket that will not speak to it, so the length is the first check rather than the only one.
Capacity, when you get to it, is quoted by the maker in its own units, and a machine displaying in units of 1024 shows a different figure for the same drive. Both are correct descriptions of the same hardware. Write down the maker’s figure, in the maker’s unit, and compare like with like.
Two measurements that are not yours to take
Anything involving the fixed electrical installation — a socket added inside a cabinet, a spur for a rack, the earthing of anything metal, or the mains side of an uninterruptible supply — belongs to the manufacturer’s instructions for the equipment and to the wiring rules that apply where you live. Measure what the equipment needs, then hand those figures to somebody qualified to act on them.
The other is heat. A cabinet’s ventilation and the clearance a maker specifies around a unit are published figures, and a cabinet in a cupboard is a different thermal problem from the same cabinet in a hallway. Read the clearances before deciding where the cabinet goes, not after.
Keep the numbers in one place, and keep them after the build
One page, one column of names and one column of figures: each run and its length, the cabinet’s usable depth, the unit count and what occupies each, the port counts per room, the bay sizes and what is in them.
That page is what the next change to this network gets planned against. Without it, the next person measures the building again — and that person is usually you.
The questions that come up before an order
How much slack should I add to a measured cable route?
Enough to reach the termination point comfortably with the panel or faceplate pulled out of its position, plus whatever the route needs for corners it has to turn rather than cut. A run that arrives exactly at its socket cannot be re-terminated after a bad crimp, and re-terminating is normal.
Which cabinet dimension gets people wrong most often?
Depth, by a distance. Width is standardised, height is written on the box in rack units, and depth is the one that has to swallow the equipment plus its plugs plus the bend in the cable behind it. A unit that fits on the mounting rails can still be an inch too long to close the door.
Do I need a tester if I am only using ready-made leads?
Not usually. A tester earns its place where you have terminated something yourself or inherited runs of unknown history, because it tells you which pairs arrive where. On a factory lead the useful check is simply whether the link comes up at the rate you expected.
Last reviewed 10 September 2026
