Box fill calculator
Enter the cables you are putting in the box. The number stamped on it assumes the box is empty, and one receptacle and its ground can cost you a whole cable.
Every rule, and which wire sizes it
Four of these pick a "largest conductor" and each one picks it from a different set. Mixing them up is the second most common source of a wrong answer, after forgetting the device entirely, so each rule below says which wire it measures.
- (B)(1) Conductors
Every conductor that comes in from outside the box and lands or is spliced inside counts once, and so does every conductor that runs through the box without being cut. A conductor that never leaves the box, which is what a pigtail is, counts as nothing at all. A loop of uncut conductor counts twice once it is long enough to have been two conductors, which is twelve inches.
Sized on: Each conductor is counted on its own size, not on the largest in the box.
- (B)(1) exception Fixture wires from a light
Where a luminaire with a domed canopy sits under the box, its grounding conductor and up to four of its fixture wires smaller than 14 AWG are left out of the calculation entirely. The cable feeding the box is not covered by this: its own grounding conductor still counts.
- (B)(2) Internal cable clamps
One allowance covers every internal clamp in the box, however many there are. A connector whose clamping mechanism sits outside the box takes no allowance, which is most cable connectors on a metal box.
Sized on: The largest conductor present anywhere in the box.
- (B)(3) Fixture studs and hickeys
One allowance for each type of support fitting in the box.
Sized on: The largest conductor present anywhere in the box.
- (B)(4) Devices on a yoke
Each yoke or strap carrying one or more devices takes a double allowance. A device too wide to mount in a single two inch device box takes a double allowance for every ganged position it needs.
Sized on: The largest conductor connected to a device on THAT yoke, which is not necessarily the largest in the box.
- (B)(5) Equipment grounding conductors
Up to four grounding conductors or bonding jumpers entering the box share a single allowance between them. From the 2020 edition, each one beyond the fourth adds a quarter of an allowance. Before 2020 any number of them shared one allowance. A grounding pigtail that does not enter the box is not counted.
Sized on: The largest grounding conductor entering the box.
- (B)(6) Terminal blocks
One allowance for each terminal block assembly, from the 2023 edition. The whole assembly counts once rather than once per pole.
Sized on: The largest conductor landed on that assembly.
- (B) What takes no allowance
Small fittings such as locknuts and bushings are not counted, and neither are wire connectors: wire nuts, push-in connectors and lever connectors occupy no allowance of their own. The 2026 edition names splicing connectors explicitly to settle the question, which had been answered the same way without being written down.
- (A) What counts as the volume
The box itself plus any plaster ring, domed cover or extension ring, but only where that accessory is marked with its own volume or is itself made to one of the standard sizes. An unmarked ring adds nothing you may count. Where a barrier splits the box, the volume is divided between the resulting spaces, and an unmarked barrier is treated as taking half a cubic inch if it is metal and one cubic inch if it is not.
- (A)(2) Boxes that are not in the table
Every nonmetallic box, and every box of 100 cubic inches or less that is not one of the standard sizes, carries its volume marked by the manufacturer. That marking is the number to use. This tool will not guess it.
Volume required per conductor
The seven numbers the whole article rests on. Unchanged across every edition this tool covers.
| Conductor | 18 AWG | 16 AWG | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|---|---|
| Cubic inches | 1.50 | 1.75 | 2.00 | 2.25 | 2.50 | 3.00 | 5.00 |
Note where the jump is. A 6 AWG conductor is allowed 5.00 cubic inches against 2.50 for a 10 AWG, so a box carrying a couple of them fills far faster than the wire's own diameter suggests. Conductors of 4 AWG and larger have no allowance here at all: those boxes are sized by 314.28, which measures the pull rather than the volume, and this tool declines them rather than guessing.
Every standard box, and what the table says it holds
The conductor columns are not transcribed. Each one is the box volume divided by the allowance above, rounded down, computed when this page was built. That is exactly what the published table does, which is why these figures assume the box holds nothing else at all: no device, no ground, no clamp. Add a receptacle and its ground and the real capacity is three or four conductors lower.
| Box | Cu in | 18 | 16 | 14 | 12 | 10 | 8 | 6 |
|---|---|---|---|---|---|---|---|---|
| 4 x 1-1/4 round or octagonal | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 x 1-1/2 round or octagonal | 15.5 | 10 | 8 | 7 | 6 | 6 | 5 | 3 |
| 4 x 2-1/8 round or octagonal | 21.5 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 x 1-1/4 square | 18.0 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 x 1-1/2 square | 21.0 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 x 2-1/8 square | 30.3 | 20 | 17 | 15 | 13 | 12 | 10 | 6 |
| 4-11/16 x 1-1/4 square | 25.5 | 17 | 14 | 12 | 11 | 10 | 8 | 5 |
| 4-11/16 x 1-1/2 square | 29.5 | 19 | 16 | 14 | 13 | 11 | 9 | 5 |
| 4-11/16 x 2-1/8 square | 42.0 | 28 | 24 | 21 | 18 | 16 | 14 | 8 |
| 3 x 2 x 1-1/2 device | 7.5 | 5 | 4 | 3 | 3 | 3 | 2 | 1 |
| 3 x 2 x 2 device | 10.0 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 3 x 2 x 2-1/4 device | 10.5 | 7 | 6 | 5 | 4 | 4 | 3 | 2 |
| 3 x 2 x 2-1/2 device | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 3 x 2 x 2-3/4 device | 14.0 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3 x 2 x 3-1/2 device | 18.0 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 x 2-1/8 x 1-1/2 device | 10.3 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 4 x 2-1/8 x 1-7/8 device | 13.0 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 x 2-1/8 x 2-1/8 device | 14.5 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3-3/4 x 2 x 2-1/2 masonry box or gang | 14.0 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3-3/4 x 2 x 3-1/2 masonry box or gang | 21.0 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| FS, 1-3/4 deep cast, single cover or gang | 13.5 | 9 | 7 | 6 | 6 | 5 | 4 | 2 |
| FD, 2-3/8 deep cast, single cover or gang | 18.0 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| FS, 1-3/4 deep cast, multiple cover or gang | 18.0 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| FD, 2-3/8 deep cast, multiple cover or gang | 24.0 | 16 | 13 | 12 | 10 | 9 | 8 | 4 |
Volumes read 2026-09-08. A box is often larger than the table's minimum, and is then marked with its own figure, which takes precedence over anything here. Plastic boxes are never in this table: every one of them carries a moulded volume instead, and the calculator offers a few common ones as examples alongside a field for reading your own.
How the calculation actually works
Box fill is one addition and one comparison. Add up an allowance for everything in the box, compare it against the volume of the box, and the larger number wins. Every difficulty is in the first half: what counts, how many times, and measured against which wire.
Step one: what the box holds
A standard steel box has its volume in the published table. Every other box, which means every plastic box and any metal box that is not a standard size, carries its volume moulded or stamped inside it. That marking is the number. This tool will not guess it, because a guessed volume produces a confident wrong answer, and the answer here is one people close a wall over.
A plaster ring, domed cover or extension ring adds its volume only if it is marked with one or is itself a standard size. An unmarked mud ring adds nothing you may count. A barrier subtracts, and splits the box into separate calculations either side of it.
Step two: the allowance per conductor
Each wire size has a fixed figure: 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, and so on up to 5.00 for 6 AWG. Those seven numbers have not moved in any edition this tool covers. Everything else in the calculation is a multiple of one of them.
Step three: counting, which is where it goes wrong
Conductors entering the box count once each, on their own size. A 12/2 cable brings two of them. A conductor passing through uncut also counts once, and a loop of uncut conductor counts twice once it is twelve inches long, because it could have been two conductors.
A conductor that never leaves the box counts nothing. That is what a pigtail is, and it is the single most searched question about this article.
Grounds pool. Every equipment grounding conductor entering the box shares one allowance between them, sized on the largest one. From the 2020 edition each one beyond the fourth adds a quarter.
Each yoke of devices takes a double allowance. One receptacle is two allowances, not one, and this is the deduction people forget. A device too wide to mount in a single two inch device box takes a double allowance for every gang position it needs.
All the internal clamps together take one allowance. A connector clamping outside the box takes none.
The four different "largest conductor" rules
This is the part no other calculator gets right, and it is worth a cubic inch or more in a mixed box. Four separate allowances are sized on a "largest conductor" and each one draws from a different set:
- Clamps and support fittings
- The largest conductor present anywhere in the box.
- A device yoke
- The largest conductor connected to a device on that yoke. Not the largest in the box. A box fed by one 10/2 and two 14/2 with the receptacle wired on the 14 AWG needs 19.50 cubic inches; size the yoke on the 10 AWG instead and you get 20.50, a whole cubic inch of headroom that is not there.
- The grounding pool
- The largest grounding conductor entering the box.
- A terminal block
- The largest conductor landed on that assembly.
A worked example
Three 12/2 cables into a single gang plastic box, with one receptacle, and no clamps in the box:
- Six conductors, 12 AWG
- Two per cable, each counted on its own size. Six times 2.25 is 13.50.
- One yoke, receptacle
- A double allowance on the 12 AWG landing on it. 4.50.
- Three grounds
- Sharing one allowance, sized on the largest of them. Not three allowances. 2.25.
- Total
- 20.25 cubic inches. A box marked 20.0, which is a real product sold for this job, is 0.25 short. A $21.0 cubic inch box takes it.
Why the number on the box is not the answer
The conductor count stamped inside a box, and the same column in the published table, is the volume divided by one conductor's allowance and nothing else. It assumes no device, no ground and no clamp. It is the figure for a box used as a pure splice point with nothing landed in it, which almost never happens.
The published table calls a 12.5 cubic inch device box a 5 conductor box for 12 AWG. Put one 12/2 cable and one receptacle in it and you have used 11.25 cubic inches. It holds one cable. The table is not wrong; it is answering a different question, and the gap between the two questions is this whole page.
Manufacturers' markings sometimes round the wrong way too. A box stamped "24.5 cubic inches, 11 #12" has divided 24.5 by 2.25 to get 10.9 and rounded up. The code rounds down.
The pancake box, which is the tightest case there is
A metal pancake box is 6.0 cubic inches, and it is what gets fitted where a ceiling has no depth to give. One 14/2 cable into it needs 6.0 cubic inches: two conductors at 2.00 and the ground taking one allowance. It fits exactly, with nothing to spare.
One 12/2 cable into the same box needs 6.75, which is 0.75 over. A pancake box cannot legally take a single 12/2 cable. Nothing about the installation looks different, and the difference is entirely in the ground: the cable's own grounding conductor still counts even though the light fixture's does not.
That last part is the exception worth knowing. Where a luminaire with a domed canopy sits under the box, its grounding conductor and up to four of its fixture wires smaller than 14 AWG drop out of the calculation entirely. The feed cable's ground is not covered by that, which is why a pancake box with one 14/2 in it is at exactly its limit rather than comfortably inside it.
The edition matters, and most calculators do not ask
Six 12/2 cables spliced in a 4 inch square 2-1/8 inch deep box, 30.3 cubic inches, with no device: twelve conductors at 2.25 is 27.00, and the six grounds are one allowance under the 2017 code and 1.5 allowances from 2020. So the box needs 29.25 cubic inches under 2017 and 30.375 from 2020. It passes under one edition and fails under the other by 0.075 of a cubic inch.
Adoption is by state, and sometimes by city. As of September 2026, roughly half the country is on the 2023 edition, most of the rest on 2020, and a handful on 2017 or older. This calculator defaults to 2023 and shows all four side by side whenever they disagree, which is the only honest way to answer a question whose answer depends on your inspector.
Where this tool will mislead you
It does not tell you the box will physically close. Volume compliance and fitting are different things. A GFCI, a dimmer and a smart switch each count as exactly two allowances, the same as a plain receptacle, and all three are visibly bigger. If your margin is small, use a deeper box regardless of what the arithmetic says.
It does not size a box for 4 AWG or larger conductors. Those have no volume allowance in this article. NEC 314.28 sizes those boxes by the geometry of the pull, which is a different calculation, and this tool refuses rather than approximating it.
It does not know whether your box is fan rated. A box supporting a ceiling-suspended fan has to be listed and marked for it, which is a mechanical question about the box, not a volume question about the wires.
It does not apply the Canadian Electrical Code. The CEC handles box fill differently and this tool is US only. If you searched for this from Canada, the numbers here do not apply to you.
It is not an inspection. Your jurisdiction may amend the code or may be on an older cycle, and either can change the answer. The edition selector is there for exactly that reason, but a local amendment is beyond anything this page can know.
Terminology
- Box fill
- The total volume everything in the box is deemed to occupy. Not the same as how full it looks.
- Volume allowance
- The cubic inches the code assigns to one conductor of a given size. Everything else in the calculation is a multiple of one of these.
- Yoke or strap
- The metal bracket a receptacle or switch is built on, and what the code counts rather than counting devices. Two switches on one strap is one yoke.
- Equipment grounding conductor
- The bare or green wire. Called an EGC in the code, and pooled into a single allowance rather than counted individually.
- Pigtail
- A short jumper made up inside the box that does not leave it. Counts as nothing.
- Device box
- The rectangular box a receptacle or switch mounts in, as opposed to a round or square junction box.
- Conduit body
- An LB, LL, T or similar fitting. One may only contain splices if it is marked with its own volume, and then these same rules apply to it.
Questions about box fill
Does the ground wire count in box fill?
Yes, but not one for one. Every equipment grounding conductor entering the box goes into a single pool, and the whole pool takes one allowance, sized on the largest ground in it. Four 12/2 cables bring four grounds and those four together count as one 12 AWG conductor, 2.25 cubic inches.
From the 2020 edition each ground beyond the fourth adds a quarter of an allowance. Six grounds is 1.5 allowances, eight is two, and seventeen is 4.25. Before 2020 any number of them shared one allowance, however many there were.
A grounding pigtail that starts and ends inside the box is not counted at all, because it never enters the box from outside.
Do pigtails count toward box fill?
No. A conductor that starts and ends inside the box, which is what a pigtail is, counts as nothing. The rule counts conductors that come in from outside and land or splice inside, plus conductors that pass through without being cut. A pigtail does neither.
This is why this calculator asks for the cables entering the box rather than for a wire count. Cutting a six inch jumper to feed a second receptacle changes nothing about the box fill, and a tool that asks "how many wires" invites you to count it.
Do wire nuts count toward box fill?
No, and neither do push-in or lever connectors. They are small fittings, in the same category as locknuts and bushings, and take no allowance of their own.
The 2026 edition names splicing connectors in that list explicitly, which settles a question people have been asking for years. It changes no arithmetic: they never counted. If your jurisdiction is on an earlier edition the answer is the same, it is just not written down as plainly.
How many 12/2 cables fit in a single gang box?
With a receptacle or a switch in the box, two in a typical 18.0 cubic inch box, and three once the box is 20.25 cubic inches or larger.
With no device in the box at all, three still fit in 18.0 cubic inches with 2.25 to spare, and a fourth pushes it to 20.25. The four grounds still take an allowance between them, which is the line the old charts leave out: 18.0 cubic inches is what eight bare 12 AWG conductors need, and four cables are not eight bare conductors.
Three 12/2 cables and one receptacle need 20.25 cubic inches: six conductors at 2.25 each, a double allowance for the yoke, and one allowance shared by the three grounds. That is why a box marked 20.0 cubic inches, which is a real box sold for exactly this job, misses it by 0.25 of a cubic inch.
If the box has internal clamps, add another allowance: the same three cables then need 22.5 cubic inches, and a 22 cubic inch box no longer takes them either.
What size box do I need for three 12/2 and a receptacle?
20.25 cubic inches, so in practice a 21.0 cubic inch box or larger. A 4 inch square 1-1/2 inch deep steel box at 21.0 works with 0.75 to spare, and a 22 cubic inch plastic box works.
Four 12/2 and a receptacle need 24.75, which is why an inspector will fail a 22.5 cubic inch box holding four cables even though it has been done that way for decades. It is over by 2.25 of a cubic inch.
Does a GFCI count as more than a regular receptacle?
Not under the code. A GFCI receptacle, an AFCI receptacle, a dimmer and a smart switch all take exactly the same allowance as a plain receptacle: two, sized on the largest conductor landing on that yoke.
Physically they are all bigger, and that is a real problem the code does not measure. The stated reason for the 2020 grounding change was crowding caused by exactly these devices. So a box that passes by a tenth of a cubic inch with a smart switch in it passes on volume and may still not close. This calculator says so when you pick one of them.
Do cable clamps count toward box fill?
Internal clamps do, and they count once in total however many are in the box, sized on the largest conductor present anywhere in it. One clamp and six clamps are the same one allowance.
A connector whose clamping mechanism is outside the box takes nothing at all, which covers most cable connectors on a metal box. So the question is not whether the cable is clamped, it is whether the thing doing the clamping is inside the box taking up room.
Most single gang plastic new work boxes have no clamps, because the cable may instead be secured to the framing within eight inches of the box. Many old work and ceiling boxes do have them, and this calculator flags the ones that do.
Why does my box say 11 #12 when the calculation gives 10?
Because the marking rounded up and the code rounds down. A box marked 24.5 cubic inches divided by 2.25 for 12 AWG is 10.9, and the code allows 10.
The deeper issue is that both figures answer a question you probably are not asking. The conductor count on a box, and the same column in the published table, is the volume divided by one conductor's allowance and nothing else: no device, no ground, no clamp. It is the count for a box used as a pure splice point with no ground in it, which almost never happens.
A 12.5 cubic inch device box is listed as holding 5 conductors of 12 AWG. Put one 12/2 cable and one receptacle in it and you have used 11.25 of its 12.5 cubic inches. It holds one cable.
What changed for box fill in the 2020 NEC?
The grounding conductors. Before 2020, any number of grounds in a box shared a single volume allowance, so a junction box with a dozen of them counted them as one. From 2020, up to four share one allowance and each one beyond the fourth adds a quarter.
It matters at five cables and up. Six 12/2 cables with no device at all need 29.25 cubic inches under the 2017 code and 30.375 from 2020, so a 4 inch square 2-1/8 inch deep box at 30.3 cubic inches passes under one edition and fails under the other by 0.075 of a cubic inch.
For a two or three cable residential box the change does nothing, which is why most people have never noticed it.
Does a plaster ring or extension add to the box volume?
Only if it is marked with its own volume, or is itself one of the standard sizes. A ring marked 3.6 cubic inches adds 3.6. An unmarked mud ring adds nothing you are allowed to count, which is the opposite of what most people assume, and it is worth checking before you rely on the extra room.
A barrier goes the other way. Where a box is divided by a securely installed barrier, the volume is apportioned between the resulting spaces and only the conductors on your side of it belong in your calculation. An unmarked barrier is treated as taking half a cubic inch if it is metal and one cubic inch if it is not.
Does this work for a junction box with 4 AWG or larger conductors?
No, and it declines rather than guessing. Conductors of 4 AWG and larger have no volume allowance in this article at all. Those boxes are sized by NEC 314.28, which is a completely different rule: it measures the geometry of the pull, so a straight pull needs eight times the trade size of the largest raceway and an angle pull needs six times plus the sum of the others.
That is a dimensional calculation about bending radius, not a volume calculation about free space, and substituting the 6 AWG allowance would give you a confident answer to a question the rule does not ask.
Where these numbers come from, and what is not reproduced here
The rules are rules. They are short factual statements of what the code requires, they are stated in dozens of places, and they are restated on this page in this project's own words. No table text, note text or commentary from NFPA 70 is reproduced anywhere here. The National Electrical Code is copyrighted by the NFPA, who make it free to read online. Free to read is not a licence to republish, and this page does not.
The volume allowances are the seven figures the article rests on and they are transcribed, because they cannot be derived from anything. The wording of every rule was checked against the International Residential Code's own reproduction of this article, which cites the NEC subsection beside each paragraph, and against published code-change summaries that quote the superseded and current wording side by side for each edition. Read 2026-09-08.
The box volumes are dimensional facts about manufactured products, stamped inside the box and printed in every manufacturer's catalogue. The conductor counts in the table on this page are not transcribed at all: each one is computed from the volume and the allowance when the page is built, which is what the published table does. A test asserts that computation reproduces every published cell, so a typo on either side fails the build rather than shipping.
The marked box examples are examples. They are dated, they name the catalogue number they were read from, and they are not a substitute for reading your own box: the same nominal size differs between manufacturers and between the new work and old work versions of one manufacturer's own line.
What is deliberately absent. Conductors of 4 AWG and larger, which NEC 314.28 sizes by the geometry of the pull. Fan-rated boxes under 314.27. The listed clamp assembly provision added in 2020, because its wording could not be read from a source that resolved and approximating it would be worse than omitting it. Conduit bodies, beyond pointing out that one may only hold splices if it is marked with a volume, in which case these rules apply to it. And the Canadian Electrical Code, which is a different standard.
This is a calculator, not an inspection and not legal advice. If a figure here disagrees with your code book or with something you have measured, tell me and it gets fixed.