Electrical Box Fill Calculator (NEC 314.16)

What electrical box fill means

Box fill is the free space an electrical box needs for everything inside it. NEC 314.16 requires boxes to be large enough to give free space for all the enclosed conductors, and the volume of the box, calculated under 314.16(A), can never be less than the fill calculated under 314.16(B). In practice you add up a volume allowance for each wire, ground, clamp, fitting and device, then compare the total with the box's volume in cubic inches.

How NEC 314.16 works

There are two halves to the calculation. 314.16(A) gives the volume of the box: the table value for standard metal boxes in Table 314.16(A), or the volume marked on the box, plus the marked volume of any plaster ring, extension ring or domed cover. 314.16(B) gives the volume you need, by adding the allowances from (B)(1) through (B)(6), each taken from the conductor table below. Locknuts, splicing connectors and bushings need no allowance. If the required volume is no more than the box volume, the box passes; an exact fit passes.

Conductor volume allowances

The same table sets the allowance for conductors, grounds, clamps, fittings, devices and terminal blocks. The size used for each item is stated in its rule below.

Conductor size	Allowance (in³)	Allowance (cm³)
18 AWG	1.50	24.6
16 AWG	1.75	28.7
14 AWG	2.00	32.8
12 AWG	2.25	36.9
10 AWG	2.50	41.0
8 AWG	3.00	49.2
6 AWG	5.00	81.9

How to count a cable

Each conductor that comes from outside the box and ends or is spliced inside it counts once, at its own size. A conductor that passes through the box without a splice or termination also counts once. An unbroken conductor that is looped or coiled in the box, with a length of at least twice the free-conductor length required by 300.14 (6 in, so 12 in), counts twice. A conductor with no part leaving the box, such as a pigtail with both ends inside, is not counted, whatever its length.

Counting multipliers for conductors under 314.16(B)(1). Wire connectors add nothing.

Equipment grounding conductors

Up to four equipment grounding conductors (or equipment bonding jumpers) that enter the box share one allowance, based on the largest grounding conductor. Each additional one adds ¼ of that allowance (314.16(B)(5)). For example, nine 12 AWG grounds cost 2.25 + 5 × 0.5625 = 5.0625 in³. A proposal to charge a full allowance for each additional group of four was submitted for the 2026 NEC and was rejected by the code-making panel, so this calculator keeps the quarter-allowance rule.

Internal clamps and support fittings

If the box has one or more internal cable clamps, whether factory or field supplied, make a single allowance based on the largest conductor present in the box (314.16(B)(2)). Luminaire studs and hickeys get one allowance for each type of fitting, also based on the largest conductor in the box (314.16(B)(3)); a stud and a hickey together are two allowances, and two studs are still one.

Devices, yokes and wide devices

Each yoke or strap that carries one or more devices gets a double allowance, based on the largest conductor connected to a device on that yoke (314.16(B)(4)). It is the yoke that is counted, not each device on it. A device or equipment wider than a single 2 in device box gets a double allowance for each gang needed to mount it. GFCI and other large devices are not treated differently by the code; box depth for them is a separate 314.24 question.

Terminal blocks

Where a terminal block is present, make one allowance for each terminal block assembly, based on the largest conductor terminated on it (314.16(B)(6)). The wires that land on it still count under (B)(1). This applies to all terminal blocks, including grounding terminal blocks.

Standard box volumes and how many wires fit

Table 314.16(A) gives the volume of standard metal boxes. The last seven columns are the most conductors of each size the box holds when nothing else is counted: no devices, clamps, fittings or grounds. Any of those reduce the count.

Box	in³	cm³	18	16	14	12	10	8	6
3 × 2 × 1½ in device box	7.50	123	5	4	3	3	3	2	1
3 × 2 × 2 in device box	10.00	164	6	5	5	4	4	3	2
3 × 2 × 2¼ in device box	10.50	172	7	6	5	4	4	3	2
3 × 2 × 2½ in device box	12.50	205	8	7	6	5	5	4	2
3 × 2 × 2¾ in device box	14.00	230	9	8	7	6	5	4	2
3 × 2 × 3½ in device box	18.00	295	12	10	9	8	7	6	3
4 × 2⅛ × 1½ in device box	10.30	169	6	5	5	4	4	3	2
4 × 2⅛ × 1⅞ in device box	13.00	213	8	7	6	5	5	4	2
4 × 2⅛ × 2⅛ in device box	14.50	238	9	8	7	6	5	4	2
4 × 1¼ in round/octagonal	12.50	205	8	7	6	5	5	4	2
4 × 1½ in round/octagonal	15.50	254	10	8	7	6	6	5	3
4 × 2⅛ in round/octagonal	21.50	353	14	12	10	9	8	7	4
4 × 1¼ in square	18.00	295	12	10	9	8	7	6	3
4 × 1½ in square	21.00	344	14	12	10	9	8	7	4
4 × 2⅛ in square	30.30	497	20	17	15	13	12	10	6
4-11/16 × 1¼ in square	25.50	418	17	14	12	11	10	8	5
4-11/16 × 1½ in square	29.50	484	19	16	14	13	11	9	5
4-11/16 × 2⅛ in square	42.00	689	28	24	21	18	16	14	8
3¾ × 2 × 2½ in masonry box/gang	14.00	230	9	8	7	6	5	4	2
3¾ × 2 × 3½ in masonry box/gang	21.00	344	14	12	10	9	8	7	4
FS single cover/gang (min. depth 1¾ in)	13.50	221	9	7	6	6	5	4	2
FD single cover/gang (min. depth 2⅜ in)	18.00	295	12	10	9	8	7	6	3
FS multiple cover/gang (min. depth 1¾ in)	18.00	295	12	10	9	8	7	6	3
FD multiple cover/gang (min. depth 2⅜ in)	24.00	395	16	13	12	10	9	8	4

Worked examples

Example 1 — receptacle fed by two 12/2 cables

Two 12/2 cables end in the box (4 insulated conductors and 2 grounds), with one single-gang receptacle. Conductors: 4 × 2.25 = 9.00. Grounds: one allowance for both = 2.25. Receptacle yoke: 2 × 2.25 = 4.50. Total required = 15.75 in³. A 4 × 2⅛ × 2⅛ device box (14.5 in³) is 1.25 in³ short; a 3 × 2 × 3½ device box (18.0 in³) passes with 2.25 in³ to spare.

Example 2 — mixed 12 and 14 AWG with a clamp

One 12/3 cable and one 14/2 cable end in the box (3 × #12 and 2 × #14 conductors, two grounds, the larger being #12), with internal clamps and one #12 receptacle. Conductors: 3 × 2.25 + 2 × 2.00 = 10.75. Grounds: 2.25. Clamps (largest conductor #12): 2.25. Receptacle: 4.50. Total required = 19.75 in³. A 4 in square box 1½ in deep (21.0 in³) passes with 1.25 in³ to spare.

Example 3 — junction box with three 12/3 cables

Three 12/3 cables are spliced in the box: 9 × 2.25 = 20.25, plus one allowance for the three grounds = 2.25, for 22.50 in³ with no devices. The common 4 × 2⅛ in square box (30.3 in³) passes at 74.3 % full; the smallest standard volume that fits is 24.0 in³ (an FD multiple-cover box), and the smallest standard square box that fits is the 4-11/16 × 1¼ (25.5 in³).

Minimum box sizing

The calculated required volume is a property of your contents. The box capacity is a property of the box. The calculator sorts all 24 standard boxes by volume and reports the smallest volume that is at least what you need, lists the smaller boxes with how far each falls short, and gives the smallest fit in each box family because families are not interchangeable (a device box cannot replace a square box). Boxes marked with a larger volume than the table can be used at their marked volume. The result is about volume only; it does not say a particular manufacturer or product is required.

Common mistakes

Counting every wire nut. Splicing connectors, locknuts and bushings add nothing.
Forgetting the grounds. They are one allowance for up to four, and a quarter allowance each beyond that.
Counting a device as one wire. A yoke is two allowances, and a wide device is two for each gang.
Counting a pigtail. A conductor that never leaves the box is not counted. Its length does not matter.
Sizing a clamp or fitting by its own conductor. They use the largest conductor present in the box.
Using the nominal box size. Use the Table 314.16(A) volume or the volume marked on the box.
Forgetting a ring or cover. Its marked volume adds to the box volume.
Using this for 4 AWG and larger. Those boxes must also meet 314.28, which is a different calculation.

Assumptions and limitations

Rules follow NEC 314.16 as shown in NFPA's code-development record for the 2026 edition; the clamp, support-fitting and conductor-counting wording is supported by earlier-edition code text with no 2026 revision found.
Only conductors from 18 AWG through 6 AWG are supported. Boxes with 4 AWG or larger conductors must also comply with 314.28, which is not calculated.
"Largest conductor present" means the largest size among all counted conductors, grounds, yoke-connected conductors and terminal-block conductors.
A cable that passes through counts its conductors once, and its ground counts as one entering ground.
Loops apply to insulated conductors. Looped grounds, the domed-luminaire exception and conduit bodies are not calculated.
A box with a barrier is treated one space at a time: enter that space's volume as a marked volume. Each space must pass on its own.
This is a volume check only. Box depth (314.24), listing, derating, heat and manufacturer limits are separate.
Volumes are computed with exact integer arithmetic, so a requirement exactly equal to the box volume passes.

NEC edition and scope

The current edition is NFPA 70-2026. For every rule used here, the 2020, 2023 and 2026 editions give the same numbers. Older editions differ: the 2017 NEC counted all grounds as one allowance, and terminal blocks were added in the 2023 NEC. Jurisdictions adopt new editions on their own schedule, so check which edition your inspector enforces.

References

Frequently asked questions

What is electrical box fill?

Box fill is the free space an electrical box needs for everything inside it. NEC 314.16 requires the box volume to be at least the total of the volume allowances for its conductors, grounds, clamps, fittings and devices.

How do you calculate electrical box fill?

Add one Table 314.16(B)(1) allowance for each conductor that enters and ends or is spliced in the box, and one for each that passes through. Add one allowance for all grounding conductors up to four, plus a quarter allowance for each beyond four, one for internal clamps, one for each type of luminaire stud or hickey, two for each device yoke, and one for each terminal block. The total must not exceed the box volume.

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

A 4 × 2⅛ in square box is 30.3 in³, so it holds 13 conductors at 2.25 in³ each when nothing else is counted. Device yokes, clamps, grounds and fittings each use part of that space, so the practical number is lower.

Do ground wires count in box fill?

Yes. Up to four equipment grounding conductors share one allowance, based on the largest one, and each additional grounding conductor adds a quarter allowance. A proposal to change this for the 2026 NEC was rejected by the code-making panel.

Do wire nuts count in box fill?

No. Small fittings such as locknuts, splicing connectors and bushings need no allowance, and the 2026 wording names splicing connectors explicitly.

Does a pigtail count in box fill?

A conductor with no part leaving the box, such as a pigtail with both ends inside, is not counted, whatever its length. The conductors of the cables entering the box still count normally.

Does a receptacle or switch count as two wires?

In volume terms, yes. Each yoke or strap that carries one or more devices gets a double allowance based on the largest conductor connected to it, and a device wider than a single 2 in device box gets a double allowance for each gang it needs.

What size electrical box do I need?

Calculate the required volume, then choose a box whose volume, from Table 314.16(A) or marked on the box, is at least that value. Box depth under 314.24 is a separate check that this calculator does not make.

When does NEC 314.28 apply instead of box fill?

Box fill under 314.16 covers conductors from 18 AWG through 6 AWG. A box with 4 AWG or larger conductors must also meet 314.28, which sets dimension rules for pull and junction boxes. This calculator does not calculate 314.28.

Related tools

Sizing the conductors for the same circuit? Check the run with the Voltage Drop Calculator. Wiring a security system into the box? Size its backup supply with the Battery Backup Calculator.

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