Preliminary numbers for estimating and for checking a plan. The NEC edition your local authority having jurisdiction adopted comes first. A licensed electrician sizes and installs the boxes.

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Calculator · Electrical

Box fill calculator

A 4 x 2-1/8 square box holds 30.30 in³. With a 14/3 cable on a switch, a 12/2 cable on a receptacle, clamps and a ground wire, the fill is 23.50 in³, so the box is 77.6% full and has 6.80 in³ left (NEC 314.16, Mike Holt's example). This page adds up the allowances of Table 314.16(B) for conductors, clamps, support fittings, devices and ground wires, compares them with the volume of the box and the rings on it, and names the smallest standard box that holds the fill. A single-gang box with a 12/2 cable to a switch is the usual surprise: 13.50 in³ in a 12.50 in³ box. For the size of the ground wire that goes in the box, use the equipment grounding conductor size calculator.

Choose a standard metal box from Table 314.16(A), or enter the volume marked on the box. Add the marked volume of any plaster ring, extension ring or domed cover.
Enter the conductors by size, loops of unbroken wire, internal clamps, support fittings, device gangs and ground wires. Leave out pigtails and jumpers that start and end in the box.
The result shows the fill required, the box volume, the room left or the overfill, how many more conductors fit, and the smallest standard box of the same type that holds the fill.

Outlet, device and junction boxes holding 6 AWG and smaller conductors, under 314.16(A) and (B)(1) to (5). Conduit bodies, terminal blocks and pull boxes are not worked. A licensed electrician installs the boxes.

Box

A standard metal box with no volume marked on it takes its volume from Table 314.16(A). A nonmetallic box, and any box of another kind, has its volume marked by the maker: choose the first entry and type it below.

Rings and covers added, in³ (optional)

Plaster rings, extension rings and domed covers that are marked with their volume. A ring with no marked volume adds nothing.

14 AWG conductors

Conductors that start outside the box and end or are spliced in it, plus conductors that pass through, each counted once. Hots, neutrals and switch legs. Leave out ground wires (below), and pigtails and jumpers that start and end in the box.

12 AWG conductors

10 AWG conductors

8 AWG conductors

6 AWG conductors

16 AWG conductors

Fixture wires and other small conductors. Up to four fixture wires smaller than 14 AWG that enter from a domed luminaire canopy and end in the box are left out.

18 AWG conductors

Loops or coils of unbroken conductor

A loop or coil at least twice the free conductor length of 300.14 (12 in. in the usual case) counts twice. Enter it here and not above.

Size of the looped conductor

Internal cable clamps

Clamps inside the box, factory or field fitted, take one allowance in all. A cable connector that clamps outside the box takes none.

Types of support fitting

Fixture studs and hickeys. One allowance for each type, however many of that type are in the box.

Device gangs, first group

A switch or receptacle on its own yoke is one gang. A device wider than a single 2 in. device box counts once for each gang it needs to mount. Two allowances for each.

Largest conductor on the first group

Device gangs, second group

For devices whose largest conductor is a different size, such as a 14 AWG switch next to a 12 AWG receptacle.

Largest conductor on the second group

Equipment grounding conductors entering the box

Bare or green ground wires, isolated ground wires included. Up to four share one allowance and each one above four adds a quarter. A bonding jumper that starts and ends in the box is not counted.

Largest grounding conductor

Example result for the default inputs

4 x 2-1/8 square: fits, 77.6% full. NEC 314.16.

Fill required

23.50 in³

Box volume

30.30 in³

Room left

6.80 in³

Calculation details
Box volume	4 x 2-1/8 square, 30.30 in³ from Table 314.16(A).
Conductors, 14 AWG	3 x 2.00 in³ = 6.00 in³. Each counted once.
Conductors, 12 AWG	2 x 2.25 in³ = 4.50 in³. Each counted once.
Internal cable clamps	1 x 2.25 in³ = 2.25 in³. One allowance for all clamps, at the largest conductor in the box, 12 AWG.
Device yokes, 14 AWG	2 x 2.00 in³ = 4.00 in³. Two allowances for each gang, at the largest conductor on the device, 14 AWG.
Device yokes, 12 AWG	2 x 2.25 in³ = 4.50 in³. Two allowances for each gang, at the largest conductor on the device, 12 AWG.
Equipment grounding conductors	1 x 2.25 in³ = 2.25 in³. One allowance for all of them, at the largest, 12 AWG.
Fill required	23.50 in³, the sum of the lines above (Table 314.16(B), 314.16(B)(1) to (5)).
Result	Fits. 23.50 of 30.30 in³ used, 77.6% full, 6.80 in³ left.
Room for more conductors	3 more 14 AWG, 3 more 12 AWG. Each new conductor of that size or smaller takes its own allowance. A new wire larger than the largest now in the box also raises the clamp, fitting and ground allowances, so run the numbers again.
Smallest box of this type that fits	4 x 2-1/8 square, 30.30 in³. The box you picked is the smallest 4 in. square box in Table 314.16(A) that holds this fill.

For estimation purposes only. Consult a licensed engineer for final designs.

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How box fill is worked

NEC 314.16 asks one question: is the box big enough for what is in it? The box has a volume in cubic inches, from Table 314.16(A) for a standard metal box or marked on the box by the maker. Each conductor, clamp, fitting, device and ground wire takes a volume allowance from Table 314.16(B), set by wire size. The allowances are added up, and the total must not be more than the box volume.

Allowance by wire size: Table 314.16(B), in³ per conductor18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00. Insulation type does not change it.
Box volume: box (table row or marked volume) + marked rings and coversA 4 x 1-1/2 square box is 21.00 in³. A plaster ring, extension ring or domed cover adds its marked volume only. Mike Holt's example adds a 3.60 in³ plaster ring to a 30.30 in³ box for 33.90 in³.
Conductors, 314.16(B)(1): one allowance for each conductor that comes from outside the box or runs through itA conductor that starts outside and ends or is spliced in the box counts once. One that passes through unbroken counts once. A loop or coil at least twice the free length of 300.14 counts twice: with two 12 AWG conductors, one loop makes 4 allowances in all, 9.00 in³. A conductor that never leaves the box, a pigtail or a jumper, counts for nothing.
Clamps and fittings, 314.16(B)(2) and (3): one allowance for all internal clamps, one for each type of support fitting, at the largest conductor in the boxWith 14 AWG conductors, a 12 AWG ground wire and a clamp, the clamp counts at 12 AWG: 2.25 in³, not 2.00. Locknuts and bushings take nothing.
Devices, 314.16(B)(4): two allowances for each yoke or strap, at the largest conductor connected to itA 12 AWG receptacle takes 2 x 2.25 = 4.50 in³ on top of the conductors that end on it. A device wider than a single 2 in. device box takes two allowances for each gang needed to mount it.
Ground wires, 314.16(B)(5): one allowance for up to four, plus a quarter allowance for each one above four, at the largest ground wireTwo 12 AWG conductors with four ground wires take 6.75 in³. With five ground wires it is 7.31 in³. From the 2020 edition an isolated ground wire is counted like any other ground wire.
Test: fill required <= box volume23.50 in³ required in a 30.30 in³ box leaves 6.80 in³, 77.6% full. Exactly full passes: the NEC says the box volume may not be less than the fill. Room for more is the space left divided by the allowance of the wire.

Eight boxes through the same formula

The first four rows are worked examples from EC&M, and the page gives the same totals. Four more are common jobs: a switch in a single-gang box in 12 AWG and in 14 AWG, a ring that makes a 4 x 1-1/2 square box big enough, and a nonmetallic box that is exactly full.

Fill required, box volume and result for eight boxes
Box and wiring	Fill required, in³	Box, in³	Result
4 x 2-1/8 square, 14/3 and 12/2 NM, switch and receptacle (Mike Holt, EC&M 2019)	23.50	30.30	Fits, 77.6% full
4 x 2-1/8 square with a 3.60 in³ plaster ring, two receptacles, five 12 AWG (Mike Holt, EC&M 2016)	22.50	33.90	Fits, 66.4% full
4 x 1-1/2 square, eleven 14 AWG allowances (EC&M 2005)	22.00	21.00	Over by 1.00
4 x 1-1/2 square, nine 12 AWG and nothing else (Mike Holt, EC&M 2019)	20.25	21.00	Fits, 96.4% full
3 x 2 x 2-1/2 device box, 12/2 NM to a switch	13.50	12.50	Over by 1.00
3 x 2 x 2-1/2 device box, 14/2 NM to a switch	12.00	12.50	Fits, 96.0% full
4 x 1-1/2 square with an 8.5 in³ ring, three 12 AWG cables, switch and receptacle	29.25	29.50	Fits, 99.2% full
Nonmetallic box marked 18 in³, six 14 AWG, one device, one ground	18.00	18.00	Fits, 100.0% full

The 4 x 1-1/2 square box lists 9 conductors of 12 AWG in Table 314.16(A), but with a clamp, a switch and a ground wire it holds 5. Table 314.16(A) below is the NEC table of standard metal boxes. Its counts hold only where no clamps, fittings, devices or ground wires are in the box, which is why the calculator works from the volume.

Table 314.16(A), metal boxes

Standard metal boxes, minimum volume and maximum number of conductors by wire size
Box, in.	Volume, in³	18 AWG	16 AWG	14 AWG	12 AWG	10 AWG	8 AWG	6 AWG
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 per gang	14.0	9	8	7	6	5	4	2
3-3/4 x 2 x 3-1/2 masonry box per gang	21.0	14	12	10	9	8	7	4
FS box, single cover per gang, 1-3/4 deep	13.5	9	7	6	6	5	4	2
FD box, single cover per gang, 2-3/8 deep	18.0	12	10	9	8	7	6	3
FS box, multiple cover per gang, 1-3/4 deep	18.0	12	10	9	8	7	6	3
FD box, multiple cover per gang, 2-3/8 deep	24.0	16	13	12	10	9	8	4

Standards and data sources

NEC 2023, 314.16, Table 314.16(A) and Table 314.16(B) - the box volume rules, the allowances for conductors, clamps, support fittings, devices and ground wires, the table of metal box volumes and the allowance per conductor. The NEC text was read as others print it, not in NFPA's viewer.
2024 International Residential Code, Chapter 39, E3905.12 (extracts NEC 2023 314.16) - the text of 314.16(A) and 314.16(B)(1) to (5) in its 2023 form, with every paragraph tagged to its NEC source, the 4 in. round box count of 4 at 8 AWG, and the one-quarter allowance for each ground wire above four.
NEC 2011 and 2008 reprints in the Thomas & Betts Steel City and Appleton catalogs - the full Table 314.16(A) with the FS and FD box rows and its footnote, the text of 314.16 in those editions, and the older 8 AWG count of 5 for the 4 in. round box.
Mike Holt and L. Keith Lofland in EC&M, Box Fill Calculations (2016, 2019, 2020) - the worked examples this page opens on and checks itself against: 23.50 in³ in a 4 x 2-1/8 square box, and a box with a plaster ring that has room for five more 14 AWG conductors. Lofland adds that Table 314.16(A) counts hold only for same-size conductors with nothing else in the box.
Electrical License Renewal and Leviton on the 2020 change to 314.16(B)(5) - the 2017 and 2020 wording of the ground wire rule, and the statement that the 2020 text no longer needs a separate rule for isolated ground wires.

Limitations of this calculator

This page gives preliminary numbers for estimating and for checking a plan. A licensed electrician sizes and installs the boxes under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
The NEC text was read as the 2024 International Residential Code extracts it, as the 2011 and 2008 NEC reprints in manufacturer catalogs print it, and as EC&M, Electrical License Renewal and Leviton restate it, not in NFPA's own viewer. The rules follow the 2023 wording as the IRC prints it. Check the edition your AHJ adopted.
Ground wires follow the 2020 and 2023 rule: up to four share one allowance and each one above four adds a quarter. In the 2017 edition and earlier all ground wires took one allowance, and an isolated ground set under 250.146(D) took a second allowance of its own. Older articles and some chart sites still say so.
One count in Table 314.16(A) differs between prints: the 4 x 1-1/4 round or octagonal box holds 4 conductors of 8 AWG in the 2024 IRC and 5 in the 2008 and 2011 reprints. The page uses 4, which is what the volume gives (12.50 in³ / 3.00 in³ = 4.17). Box fill on this page always comes from the volume, so the count does not enter the result.
This page does not cover conduit bodies (314.16(C)), the terminal block allowance that the 2023 edition added to 314.16(B) as EC&M reports it, boxes with barriers (an unmarked barrier takes 0.5 in³ in a metal box and 1.0 in³ in a nonmetallic one), and conductors larger than 6 AWG. Boxes with 4 AWG or larger conductors also fall under 314.28, and the pull box rules are not worked here.
Leave out what the code leaves out: pigtails and jumpers that start and end in the box, locknuts and bushings, and an equipment grounding conductor or up to four fixture wires smaller than 14 AWG that enter from a domed luminaire canopy and end in the box. Count the ground wires of the cables, one allowance for up to four.
Rings and covers add volume only when marked with it, or when made from a listed box in Table 314.16(A). The page takes the volume you type. It does not check that a ring fits a box, that a nonmetallic box is marked, or the depth rules for boxes and devices.
The masonry, FS and FD rows of Table 314.16(A) are printed per gang, and the page multiplies them by the gangs you enter. The 3 x 2 and 4 x 2-1/8 device box rows carry no such label in the prints read, so they are one box each. For device boxes ganged into a row, enter the marked volume as the sum of the sections, as 314.16(A) adds the volume of assembled sections.
The device groups are two, each with the size of its largest conductor. Counts run up to 200 conductors of one size, and a marked box volume runs from 1 to 500 in³. The marking rule of 314.16(A)(2) is written for boxes of 100 in³ or less.

Common mistakes with box fill

Using the table count when the box has a device or a clamp. A 4 x 1-1/2 square box lists 9 conductors of 12 AWG in Table 314.16(A), but the table applies only when the box has no clamps, fittings, devices or ground wires. With one clamp, one switch and a ground wire, 4 allowances are used before the first conductor, so the box holds 5 conductors of 12 AWG, not 9.
Counting a switch or receptacle once. A device yoke takes two allowances, at the largest conductor connected to it. A 12 AWG switch takes 4.50 in³ plus the conductors that end on it. A 12/2 NM cable to a switch in a 3 x 2 x 2-1/2 device box comes to 13.50 in³ against 12.50 in³, so the box is over by 1.00 in³. The same job in 14 AWG is 12.00 in³ and fits.
Counting every ground wire, or none. All the ground wires in a box take one allowance, up to four of them, at the largest ground wire. Two 12 AWG conductors in a 4 x 2-1/8 square box with four ground wires come to 6.75 in³. With five ground wires the fifth adds a quarter allowance and the total is 7.31 in³. Leaving the ground wires out of the count understates the fill.
Counting pigtails, or missing the pass-through wire. A pigtail that starts and ends in the box is not counted. A wire that runs through the box without a splice is counted once, and a loop of unbroken wire that is at least twice the free length of 300.14 is counted twice. Nine 12 AWG conductors take 20.25 in³ of a 21.00 in³ box, and a tenth would not fit.
Pricing the clamp and the fittings at the wrong wire size. The clamp allowance, the support fitting allowance and the ground wire allowance are taken at the largest conductor in the box, not at the wire most of the box holds. Four 14 AWG conductors, a clamp and a 12 AWG ground wire come to 12.50 in³ here, with the clamp and the ground at 2.25 each, not 2.00.
Adding a ring or cover that is not marked. Only a volume marked on the ring or cover, or one made from a listed box, is added. Mike Holt's plaster ring is marked 3.60 in³ and takes his box from 30.30 to 33.90 in³, which leaves room for five more 14 AWG. A ring with no marking adds nothing, and a box that fits only with it needs a larger box.

Common questions about box fill

How many 12 AWG wires fit in a 4 inch square box?

A 4 x 1-1/2 square box is 21.00 in³ and Table 314.16(A) lists 9 conductors of 12 AWG, a 4 x 2-1/8 square box lists 13. Those counts hold only when the box has no clamp, device, support fitting or ground wire. With one clamp, one switch and a ground wire, the 4 x 1-1/2 box holds 5 conductors of 12 AWG. Use the calculator for your own mix.

How is a switch or receptacle counted in box fill?

Two conductor allowances for each yoke, at the largest conductor connected to the device. For 12 AWG that is 4.50 in³ for each receptacle or switch, on top of the conductors that end on it. A device wider than a single 2 in. device box takes two allowances for each gang it needs to mount.

Do ground wires count in box fill?

Yes, as one allowance for up to four of them, at the size of the largest ground wire. Each ground wire above four adds a quarter allowance. A bonding jumper that starts and ends in the box is not counted. Two 12 AWG conductors with one ground wire in a box are 6.75 in³, and with five ground wires 7.31 in³.

Does an isolated ground receptacle need extra box volume?

Not under the 2020 and 2023 text. The 2017 edition and earlier gave an additional set of ground wires under 250.146(D) an allowance of its own. The 2020 edition changed 314.16(B)(5) so that up to four ground wires share one allowance and each one above four adds a quarter, which removed the separate isolated ground rule. Check the edition your AHJ adopted.

Do pigtails and wire nuts count?

A pigtail or jumper that starts and ends in the box is not counted, because no part of it leaves the box. Wire connectors, locknuts and bushings take no allowance either. The wires that enter the box and are spliced to the pigtail are counted, each once.

Does a plaster ring or extension ring add volume?

Yes, if it is marked with its volume in cubic inches or is made from a box listed in Table 314.16(A). In Mike Holt's example a 3.60 in³ plaster ring takes a 30.30 in³ box to 33.90 in³ and the fill of 22.50 in³ leaves room for five more 14 AWG conductors. A cover with no marked volume adds nothing.

What if the box is overfilled?

Use a larger box, or add a marked extension ring. A 12/2 NM cable to a switch overfills a 3 x 2 x 2-1/2 device box, 13.50 in³ against 12.50 in³, and the next size up, the 3 x 2 x 2-3/4 device box at 14.00 in³, holds it. Do not remove ground wires or clamps from the count, and do not fit the wires in by compressing them.

How is a loop of wire counted?

A loop or coil of unbroken conductor at least twice the free conductor length of 300.14 counts twice. The free length is 6 in. in the usual case, so the loop must be 12 in. or more. Two 12 AWG conductors and one such loop in a 4 x 1-1/2 square box come to 4 allowances, 9.00 in³.

Is a box exactly full allowed?

Yes. 314.16 says the volume of the box may not be less than the fill, so equal is allowed. A nonmetallic box marked 18.00 in³ with six 14 AWG conductors, one device and a ground wire is 18.00 in³ and passes with no room left. A later change would overfill it.

Does this replace an electrician?

No. This page is for estimating and for checking numbers on a plan. A licensed electrician sizes and installs the boxes under a permit, and the local authority having jurisdiction inspects them.