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Box fill calculator

Volume required · 314.16(B)18.00in³MINIMUM VOLUMENEC 2023 Table 314.16(B)(1) · smallest listed metal box: 4 x 1-1/4 square, 18.0 in³

2 in³ each

2.25 in³ each · a 12-2 cable is 2

2.5 in³ each

3 in³ each

5 in³ each

Internal cable clamps

Luminaire studs or hickeys, per type (0–2)

Receptacles, switches

EGCs entering the box

From the box marking or Table 314.16(A); leave blank to list boxes that fit

4 × 12 AWG conductors314.16(B)(1), NEC 2023 (allowances identical in 2011 and 2017)
9.00 in³
Internal cable clamps314.16(B)(2), NEC 2023 (allowances identical in 2011 and 2017)
2.25 in³
Device yoke 1 (12 AWG)314.16(B)(4), NEC 2023 (allowances identical in 2011 and 2017)
4.50 in³
2 equipment grounding conductors (12 AWG)314.16(B)(5), NEC 2023 (allowances identical in 2011 and 2017)
2.25 in³
18 in³4 × 12 AWG · 9Internal cable clamps · 2.25Device yoke 1 · 4.52 equipment grounding · 2.2518 in³ needed
Table 314.16(A) boxes that fit
  • 4 x 1-1/4 square · 18.0 in³
  • 3 x 2 x 3-1/2 device · 18.0 in³
  • FD — single cover (2-3/8) FD · 18.0 in³
  • FS — multiple cover (1-3/4) FS · 18.0 in³
  • 4 x 1-1/2 square · 21.0 in³
  • 3-3/4 x 2 x 3-1/2 masonry box/gang · 21.0 in³
  • 4 x 2-1/8 round/octagonal · 21.5 in³
  • FD — multiple cover (2-3/8) FD · 24.0 in³

Allowances from Table 314.16(B)(1): 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 in³. A conductor that starts outside and ends or splices in the box counts once; one passing through unbroken counts once; a loop at least twice the 300.14 length counts twice; pigtails that never leave the box count nothing. Clamps, each support-fitting type and each device yoke take one or two allowances at the largest conductor present; up to four grounds take one allowance, then a quarter each (2020/2023 wording). Nonmetallic boxes are marked with their volume; use that figure.

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Count the conductors entering the box by size, say whether it has internal clamps or a stud or hickey, add the devices and the grounds, and read the cubic inches the NEC requires. Each part of the count is struck with its rule: 314.16(B)(1) conductors, (B)(2) clamps, (B)(3) support fittings, (B)(4) devices at two allowances per yoke, (B)(5) grounds at one allowance for up to four. Leave the box volume blank to see which Table 314.16(A) metal boxes are big enough; type it to check the box in your hand.

Worked examples

Receptacle fed through: two 12-2 NM cables
4 × 12 AWG at 2.25 = 9.00; internal clamps 2.25; one yoke 2 × 2.25 = 4.50; two grounds count once, 2.25. Total 18.0 in³. A 4 × 1½ square (21.0 in³) fits; a 3 × 2 × 3½ device box (18.0) fits exactly. Open this run
Two switches on three 14-2 cables
6 × 14 AWG at 2.00 = 12.0; clamps 2.0; two yokes 2 × 2 × 2.0 = 8.0; three grounds 2.0. Total 24.0 in³: a 4-11/16 × 1¼ square (25.5) or a 4 × 2⅛ square (30.3). Open this run
Splice box, six 12 AWG, no devices, 18 in³ box
6 × 2.25 = 13.5 plus 2.25 for the grounds = 15.75 in³. The 4 × 1¼ square at 18.0 in³ has 2.25 to spare. Open this run
Ceiling box with a fixture stud, 15.5 in³ octagon
4 × 14 AWG = 8.0; clamps 2.0; one support fitting type 2.0; grounds 2.0. 14.0 in³ in a 4 × 1½ octagonal box (15.5): fits. Open this run

How it is worked out

Volume required = Σ conductors × Table 314.16(B)(1) allowance (18 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 in³) + one allowance for internal clamps + one allowance per support-fitting type + two allowances per device yoke (per gang for a device wider than 2 in) + one allowance for up to four equipment grounding conductors plus a quarter allowance for each beyond four, each at the largest conductor the rule names.

Conductors that originate and end inside the box (pigtails) and up to four fixture wires from a canopy are not counted; a loop at least twice the 300.14 length counts twice.

The box must have at least that volume: its marking, or Table 314.16(A) for standard metal boxes.

The figures behind it

NEC Table 314.16(B)(1) and 314.16(B) rules
Allowance per conductor, verified identical in NFPA 2011, NFPA 2017 and the IRC 2024 reprint of the 2023 table. Rule text 2017 verbatim; (B)(5) uses the 2020/2023 wording (one allowance for up to four EGCs, then one quarter each), and (B)(6) terminal blocks, new in 2023, is supported in the engine. Source
NEC Table 314.16(A), Metal Boxes
Standard trade sizes with minimum volumes, 2017 text cross-checked with 2011 and IRC 2024. One printed cell disagrees between editions (the 8 AWG count for the 4 × 1¼ round box: 5 in 2011/2017, 4 in IRC 2024); the calculator computes counts from volume rather than reading that cell. Source

Questions people ask

How many 12 AWG wires fit in a 4 × 1½ square box?
21.0 ÷ 2.25 = 9 conductors if nothing else is in the box. Each device takes two allowances, clamps one, and the grounds together one, so a receptacle on two cables (4 conductors + clamps + yoke + grounds = 18.0) already uses most of it.
Do the grounds count?
All the equipment grounding conductors together count as one conductor of the largest size present, for up to four of them; each one beyond four adds a quarter allowance (2020 and 2023 wording). In the 2017 edition they counted once regardless of number.
What about a box with no volume printed on it?
Standard metal boxes are listed in Table 314.16(A); the calculator lists those that fit. Nonmetallic boxes must be marked with their volume (314.16(A)(2)); use the marking.
Do wire nuts or pigtails take space?
Not in the calculation. A conductor no part of which leaves the box is not counted, and connectors are not counted. The allowances already include working room.

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