NEC calculators
Box fill calculator (NEC 314.16)
Add up the volume allowances for every conductor, device, clamp, fitting and grounding conductor, compare them with a standard or marked box plus rings, and see how many more wires fit, under the edition you work to.
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How to use the box fill calculator
Choose the NEC edition, then the box. The list holds every standard metal box in Table 314.16(A) with its minimum volume; for a nonmetallic box or any box not in the table, choose “Other box” and enter the volume marked inside it. Add the marked volume of a plaster ring, extension ring or domed cover if there is one. Then count what is in the box: insulated conductors by size, unbroken loops, internal clamps, luminaire studs or hickeys, device yokes, equipment grounding conductors and, from the 2023 edition, terminal blocks.
The result shows the volume the contents need, the volume the box gives, the margin, and how many more conductors of a size you choose would still fit. It also names the smallest box of the same kind that works, which is the quick answer when a box fails. Every line of the calculation cites the subsection it comes from, so you can show an inspector the work.
How 314.16 counts what is in the box
Box fill is a volume budget. Each conductor size has an allowance in cubic inches, from 1.50 in³ for 18 AWG and 1.75 in³ for 16 AWG through 2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG and 2.50 in³ for 10 AWG, up to 3.00 in³ for 8 AWG and 5.00 in³ for 6 AWG (Table 314.16(B), renumbered Table 314.16(B)(1) in 2023). Everything in the box is charged a number of those allowances, and the total has to fit in the box volume:
- Conductors (314.16(B)(1)): each conductor that comes in from outside and is spliced or terminated counts once; so does each one that passes through without a splice. A conductor that never leaves the box, such as a pigtail, counts zero.
- Loops: an unbroken conductor looped or coiled to at least twice the 6 in. free length of 300.14, that is 12 in. or more, counts twice.
- Internal cable clamps (314.16(B)(2)): one allowance for all of them, at the largest conductor in the box. Connectors whose clamps sit outside the box count nothing.
- Support fittings (314.16(B)(3)): one allowance for each type of luminaire stud or hickey, at the largest conductor.
- Devices (314.16(B)(4)): two allowances per yoke or strap, at the largest conductor connected to it. A device wider than a single 2 in. device box, such as a range or dryer receptacle, takes two allowances for each gang it needs.
- Equipment grounding conductors (314.16(B)(5)): the rule that changed most between editions, covered below.
- Terminal blocks (314.16(B)(6), from 2023): one allowance per block, at the largest conductor terminated on it.
- Nothing for locknuts, bushings and other small fittings. The 2026 edition adds splicing connectors to that list by name.
The box side of the budget is the Table 314.16(A) minimum volume for a standard metal box, or the marked volume for anything else, plus rings and covers that are marked with their volume. An extension ring made from a box size in the table counts at that box’s volume. Each space of a box divided by barriers is checked on its own, and an unmarked barrier takes 0.5 in³ (metal) or 1.0 in³ (nonmetallic).
Worked examples
Two receptacles in a 4 in square box with a plaster ring
1. Box volume (314.16(A), Table 314.16(A))
2. 5 × 12 AWG conductors, counted once each (314.16(B)(1))
3. 2 device yokes or gangs: two each, at the largest conductor connected (12 AWG) (314.16(B)(4))
4. Equipment grounding conductors (2): one allowance for up to four, at the largest (12 AWG) (314.16(B)(5))
5. Required volume (Table 314.16(B)(1) allowances)
Under the NEC 2023, the contents need 22.5 in³ (5 × 12 AWG plus the allowances for devices and grounding conductors); the 4 × 2-1/8 in square box with its ring gives 33.9 in³, so it fits with 11.4 in³ to spare.
A three-way switch on 14 AWG beside a receptacle on 12 AWG
1. Box volume (314.16(A))
2. 3 × 14 AWG conductors, counted once each (314.16(B)(1))
3. 2 × 12 AWG conductors, counted once each (314.16(B)(1))
4. Internal cable clamps: one allowance for all, at the largest conductor (12 AWG) (314.16(B)(2))
5. 1 device yoke or gang: two each, at the largest conductor connected (14 AWG) (314.16(B)(4))
6. 1 device yoke or gang: two each, at the largest conductor connected (12 AWG) (314.16(B)(4))
7. Equipment grounding conductors (2): one allowance for up to four, at the largest (12 AWG) (314.16(B)(5))
8. Required volume (Table 314.16(B)(1) allowances)
Under the NEC 2023, the contents need 23.5 in³ (3 × 14 AWG + 2 × 12 AWG plus the allowances for devices, clamps and grounding conductors); the box gives 25 in³, so it fits with 1.5 in³ to spare.
Eight 12 AWG grounds in a 4 × 1-1/2 in square box, NEC 2017
1. Box volume (314.16(A), Table 314.16(A))
2. 8 × 12 AWG conductors, counted once each (314.16(B)(1))
3. Equipment grounding conductors and bonding jumpers (8): one allowance for all, at the largest (12 AWG) (314.16(B)(5))
4. Required volume (Table 314.16(B) allowances)
Under the NEC 2017, the contents need 20.25 in³ (8 × 12 AWG plus the allowances for grounding conductors); the 4 × 1-1/2 in square box gives 21 in³, so it fits with 0.75 in³ to spare.
The same box under NEC 2023
1. Box volume (314.16(A), Table 314.16(A))
2. 8 × 12 AWG conductors, counted once each (314.16(B)(1))
3. Equipment grounding conductors (8): one allowance for up to four plus ¼ for each of the 4 more, at the largest (12 AWG) (314.16(B)(5))
4. Required volume (Table 314.16(B)(1) allowances)
Under the NEC 2023, the contents need 22.5 in³ (8 × 12 AWG plus the allowances for grounding conductors); the 4 × 1-1/2 in square box gives 21 in³, so it is 1.5 in³ too small.
Five 8 AWG in a 4 × 1-1/4 in round box, NEC 2020
1. Box volume (314.16(A), Table 314.16(A))
2. 5 × 8 AWG conductors, counted once each (314.16(B)(1))
3. Required volume (Table 314.16(B) allowances)
Under the NEC 2020, the contents need 15 in³ (5 × 8 AWG); the 4 × 1-1/4 in round or octagonal box gives 12.5 in³, so it is 2.5 in³ too small.
The second example is the one in Mike Holt’s classic EC&M box fill article, which counts five 14 AWG and six 12 AWG volumes for 23.5 in³. If you don’t know which conductor lands on which device, charging both yokes at the largest conductor in the box is the safe shortcut: 24 in³ here, half a cubic inch more.
Volume needed for common boxes
Most device boxes in a house hold one of a handful of arrangements. The table runs them through the same engine, assuming NM cable with one bare grounding conductor per cable. Ganged device boxes add their volumes, so a two-gang box of two 3 × 2 in sections gives twice the volume of one.
| Box contents | Volume needed | Smallest 3 × 2 in device box | Smallest square box |
|---|---|---|---|
| Single-pole switch, 14/2 cable in and out | 14 in³ | 3 × 2 × 2-3/4 (14 in³) | 4 × 1-1/4 (18 in³) |
| Receptacle at the end of a 12/2 run | 11.25 in³ | 3 × 2 × 2-1/2 (12.5 in³) | 4 × 1-1/4 (18 in³) |
| Receptacle or GFCI mid-run, 12/2 in and out | 15.75 in³ | 3 × 2 × 3-1/2 (18 in³) | 4 × 1-1/4 (18 in³) |
| Three-way switch, 14/2 in, 14/3 out | 16 in³ | 3 × 2 × 3-1/2 (18 in³) | 4 × 1-1/4 (18 in³) |
| Two switches, three 14/2 cables | 22 in³ | 2 ganged, 3 × 2 × 2-1/2 (25 in³) | 4-11/16 × 1-1/4 (25.5 in³) |
| Ceiling light: two 14/2 and a 14/3, clamps and a stud | 20 in³ | n/a | 4 × 1-1/2 (21 in³) |
| Junction box, four 12/2 cables spliced, clamps | 22.5 in³ | n/a | 4-11/16 × 1-1/4 (25.5 in³) |
| Junction box, six 12/2 cables spliced, clamps | 32.63 in³ | n/a | 4-11/16 × 2-1/8 (42 in³) |
A receptacle mid-run on 12 AWG is the case that catches people: it needs 15.75 in³, so the shallow device boxes are out and the smallest 3 × 2 in box that works is the 3 × 2 × 3-1/2 in (18 in³). A GFCI receptacle in that box leaves little room for its body. Deep boxes, or 4 in square boxes with a marked device ring, are cheap insurance.
Grounding conductors: how the rule changed
Through 2017, every equipment grounding conductor and bonding jumper in the box shared a single allowance at the largest of them, and an isolated-ground set for receptacles under 250.146(D) got one more. The 2020 edition replaced that with a sliding rule: up to four EGCs take one allowance, and each additional one adds a quarter. Isolated-ground conductors became ordinary EGCs in that count. In 2023 the count was limited to conductors that enter the box, so equipment bonding jumpers are no longer counted, and the quarter allowances are based on the largest EGC entering.
| EGCs entering | NEC 2017 | NEC 2020, 2023, 2026 |
|---|---|---|
| 1 | 2.25 in³ | 2.25 in³ |
| 2 | 2.25 in³ | 2.25 in³ |
| 4 | 2.25 in³ | 2.25 in³ |
| 5 | 2.25 in³ | 2.813 in³ |
| 6 | 2.25 in³ | 3.375 in³ |
| 8 | 2.25 in³ | 4.5 in³ |
| 10 | 2.25 in³ | 5.625 in³ |
| 4 + 2 isolated-ground | 4.5 in³ | 3.375 in³ |
On boxes with many cables the difference is real. Eight 12 AWG conductors spliced through a 4 × 1-1/2 in square box (21 in³) with their eight grounding conductors need 20.25 in³ under 2017 and fit; under 2020 and later they need 22.5 in³ and don’t. Check which edition your jurisdiction enforces on the NEC adoption map.
Two more edition notes. The 2023 edition corrected Table 314.16(A) for the 4 × 1-1/4 in round or octagonal box: the 2017 and 2020 tables printed five 8 AWG conductors, but 12.5 in³ ÷ 3.00 in³ = 4.17, so only 4 fit, and that is what the calculator uses under every edition. And the 2026 edition names splicing connectors among the fittings that need no allowance; nobody counted wire nuts before, but the text now says so.
Box volume: standard, marked and rings
Table 314.16(A) covers standard metal boxes only, and its volumes are minimums: the inside of a 4 × 2-1/8 in square box is 30.3 in³, not the 34 in³ its outside dimensions suggest. Nonmetallic boxes and metal boxes not in the table must be marked with their volume by the manufacturer, and that marking is the number to use. The same applies to plaster rings, domed covers and extension rings: only a marked volume counts, so an unmarked ring adds nothing. Conductors larger than 6 AWG are outside 314.16 altogether; boxes for 4 AWG and larger are sized by 314.28, from the raceway sizes instead of conductor volume.
Common mistakes
- Counting pigtails. A conductor that starts and ends in the box counts zero. Counting them makes boxes look fuller than they are.
- Forgetting the device. Each yoke takes two allowances; a GFCI or a smart switch takes no more volume in the calculation than a plain device, but much more in the box.
- Charging clamps at the wrong size. Clamps and studs go at the largest conductor in the box, not the smallest.
- Using the 2017 ground rule after 2020. More than four EGCs now add a quarter allowance each.
- Adding an unmarked ring. Only rings and covers marked with their volume count.
- Reading Table 314.16(A) with mixed sizes or devices. The table’s conductor counts apply only to boxes holding one size of conductor and nothing else; anything more needs the calculation.
Once the boxes work, check the raceways between them with the conduit fill calculator, and the conductor sizes with the wire size calculator.
Method and verification
Allowances and box volumes are the NEC values for each edition, checked against the 2017 text, an independent 2011 reprint and NFPA’s 2020, 2023 and 2026 revision records. The calculator adds an allowance for each item exactly as 314.16(B)(1) to (B)(6) describe and compares the sum with the box volume plus marked rings. Maximum counts are whole conductors only, which reproduces every Table 314.16(A) entry except the 8 AWG figure that 2023 corrected.
The tests include both of Mike Holt’s published examples (22.5 in³ with 5 more 14 AWG fitting, and 23.5 in³), the 2023 committee example of eight 12 AWG grounding conductors (4.5 in³ from 2020, 2.25 in³ in 2017), the isolated-ground, bonding-jumper and terminal-block rules in every edition, the round-box correction, and properties such as “more contents never need less volume”.
Covers outlet, device and junction boxes with conductors 18 to 6 AWG; conduit bodies (314.16(C)) and pull boxes (314.28) are not calculated. Updated 2026-10-14. Found a problem? Report it and we log every fix publicly.