What the count is actually counting
Box fill is not a measurement of how much copper is in the box. It is a bookkeeping exercise where things are assigned volume allowances by the size of conductor involved, and the allowances are deliberately generous compared to the physical wire, because the wire is not the problem. The problem is the bend radius, the splice caps, the body of a device, and the fact that everything has to be pushed back into the box without pinching insulation against a metal edge.
The conventional rules run like this. Each conductor that enters the box and terminates or splices inside it counts as one allowance for its size. A conductor that passes through unbroken counts once, not twice. A device — a switch, a receptacle, anything on a yoke or strap — counts as two allowances at the size of the largest conductor in the box, regardless of the size of the wires actually landing on it. All the grounding conductors together count as one allowance, no matter how many there are, and a second separate group such as an isolated ground adds one more. Internal cable clamps count as one allowance in total, again at the largest conductor size, and clamps that live outside the box count for nothing. Fixture studs and hickeys count one each.
The things that count for nothing are worth knowing too: wire connectors, small fittings such as locknuts and bushings, and conductors that originate inside the box and do not leave it, such as a fixture pigtail entirely contained in the enclosure.
The allowances, and why they are inputs here
The volume allowance per conductor size is a published table, and the figures below are the ones in common use. They are shown in this calculator as editable fields rather than baked in, because the table that binds you is the one in the edition your jurisdiction adopted.
| Conductor | Common allowance | A device costs |
|---|---|---|
| 14 AWG | 2.00 cu in | 4.00 cu in |
| 12 AWG | 2.25 cu in | 4.50 cu in |
| 10 AWG | 2.50 cu in | 5.00 cu in |
| 8 AWG | 3.00 cu in | 6.00 cu in |
| 6 AWG | 5.00 cu in | 10.00 cu in |
Notice how fast a switch loop with two cables fills an 18 cubic inch box. Two 12 AWG cables bring six conductors: 13.50 cubic inches. One device: 4.50. Clamps: 2.25. Grounds: 2.25. That is 22.50 cubic inches in a box that holds 18, and it is a completely ordinary arrangement that appears in kitchens everywhere.
Where remodels go wrong
New work rarely fails this test, because the box gets chosen after the conductor count is known. Remodels fail it constantly, because the box was chosen decades ago for a load of two cables and one device, and now somebody wants a smart switch, a neutral that was never pulled, and a third cable to feed the island.
The failure mode is specific. Old metal boxes are often small — some are 12 cubic inches or less — and they frequently have no volume marking, so the volume has to be worked out from the physical dimensions or looked up for that box type. Add to that the habit of the era, which was to leave three inches of conductor in the box rather than the six that makes a splice workable, and a remodel in a small metal box turns into a box replacement more often than anyone plans for.
The available fixes are box extenders, which add a marked volume and are listed for the purpose, deeper boxes, and old-work boxes that trade some depth for an easier install. Which of those is appropriate is a site question.
What the number does and does not settle
Passing the fill count says the enclosure is large enough for what is in it. It says nothing about whether the circuit is correct, whether the splices are made with listed connectors suited to the conductor combination, whether the box is properly supported and bonded, or whether the cables are secured where they enter. A box that passes fill and fails on any of those is not a safer box.
Every threshold on this page — fill percentages, volume allowances, ampacity, the continuous-load factor, derating for ambient temperature and for the number of current-carrying conductors — is shown as an input with a common working default. The figure that governs your installation is the one in the code edition your jurisdiction has adopted, with whatever local amendments came with it, as your Authority Having Jurisdiction reads it. Check the defaults before you rely on them. Electrical work on a building generally needs a permit and an inspection, and many jurisdictions restrict who is allowed to do it at all. An error in this part of a house burns houses down and kills people. This page sizes and estimates. It does not replace the judgement of a licensed electrician, and it gives no instruction for opening equipment or making connections.
Work out the raceway side with the conduit fill calculator, the circuit total with the circuit load calculator, and the run itself with the voltage drop calculator.
Questions people ask
Do wire nuts count toward box fill?
No. Wire connectors, along with locknuts, bushings and similar small fittings, are conventionally excluded from the count. The reasoning is that the conductor allowances are already generous enough to absorb them — a 12 AWG conductor is nowhere near 2.25 cubic inches of copper, and the difference is exactly the space the connector and the bend occupy. This is one of the cases where knowing what is excluded is as useful as knowing what counts, because people who add the connectors in end up buying boxes larger than they need.
How do I count a wire that passes straight through the box?
Once. A conductor that enters the box and leaves without being spliced or terminated counts as a single allowance for its size, the same as a conductor that ends there. It does not count twice for entering and leaving. The one adjustment to know about is that a conductor looped through the box unbroken is sometimes counted as two if the loop is long enough to be a deliberate slack allowance rather than a pass-through, and the threshold for that is in the adopted edition. If your loop is generous, check it.
Why does a receptacle count as two conductors?
Because the yoke and the body of the device physically displace roughly that much room, and because that space cannot be used for anything else. The allowance is taken at the size of the largest conductor present in the box rather than at the size of the conductors landing on the device, which catches the case where a large feeder passes through a box that also holds a small switch. A duplex receptacle counts as one device, not two, because it is one yoke. A two-gang device on a single strap is still counted by the strap.
How do I find the volume of an old metal box with no marking?
Boxes manufactured to a standard size have published volumes for their type and dimensions, so the practical route is to identify the box — a 4 inch square by 1-1/2 inch deep, a 3 by 2 by 2-1/2 device box, and so on — and look up the figure for that description. Measuring the inside and multiplying gives a number close to the published one but not identical, because the published figure accounts for the internal geometry. If the box has no marking and no identifiable type, treat that as a sign to replace it rather than to estimate, particularly since unmarked boxes tend to be old ones that are small.
What happens if a box is over-filled?
Nothing, on the day. That is what makes it dangerous. The immediate consequences are mechanical: conductors forced into position get their insulation nicked on the box edge or crushed against a terminal screw, splices get compressed until a connector loosens, and the device sits proud of the wall because there is nowhere for the conductors to go. Over years, a loose splice under mechanical stress is exactly the condition that develops resistance, heats, and starts a fire behind a wall. An inspector will fail it on sight, and that is the cheap version of finding out.