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Box Fill Calculator

Box fill = conductors + clamps + fittings + 2 per device + grounds, each at its wire's volume; four 12 AWG wires, two grounds, a switch and clamps need 18 cubic inches.

Insulated conductors that end or splice in the box
Fixture wires, 18 and 16 AWG

Count each wire that comes in from outside once, black, white and any other colors. Bare and green grounds go below.

All grounds together count once, up to four.

One per device strap. A two-gang strap is two.

Fixture stud or hickey

Box volume required

18cubic inches

Smallest standard box that fits: Device box, 3 x 2 x 3-1/2 in, 18 cu in.
In cubic centimeters
295cm³
Largest wire in the box
12 AWG
Allowance per wire
2.25cu in
The math

  1. 12 AWG conductors

    4 × 2.25 cu in = 9 cu in

  2. 2 equipment grounding conductors, 12 AWG

    1 × 2.25 cu in = 2.25 cu in

  3. Internal cable clamps

    1 × 2.25 cu in = 2.25 cu in

  4. Device yokes (2 each)

    2 × 2.25 cu in = 4.5 cu in

Total 18 cu in. Clamps, fittings and devices are counted at the largest wire in the box.

NEC 2023 (NFPA 70): Table 314.16(B) volume allowances, counting rules of 314.16(B)(1) to (5); box volumes from Table 314.16(A). Your state or city may adopt an older edition or add local amendments.

Two rules catch people out: a box marked with a volume overrides the table, and fill is a minimum, not a target. Go one size up when it is close. Most places require a permit and inspection for new circuits, and many only let a licensed electrician or the homeowner of a single-family house do the work. Turn the breaker off and test the wires dead before you touch them, and have a licensed electrician check anything you are unsure about.

Electrical box fill is the total of a volume allowance for every wire, clamp, fitting, device and ground in the box, each valued at the wire's size from NEC Table 314.16(B). Four 12 AWG wires, two grounds, one switch and internal clamps need 18 cubic inches, which a 3 × 2 × 3½ in device box holds exactly. This box fill calculator covers NEC 314.16 and gives the box size in cubic inches and the smallest standard box that fits.

How box fill is calculated

The 2023 NEC 314.16(B) and its identical residential text (IRC E3905.12.2) count each item once, twice or not at all:

  • Each insulated conductor that starts outside the box and ends or is spliced inside counts one allowance. A wire that runs through unbroken counts one; a loop 12 in or longer counts two; a pigtail that starts and ends inside counts none.
  • Internal cable clamps: one allowance in total, at the largest wire in the box, however many clamps.
  • Luminaire studs and hickeys: one allowance for each type present, at the largest wire.
  • Device yokes: two allowances for each yoke (a switch or duplex receptacle is one yoke, a two-gang strap is two), at the largest wire connected to it.
  • Equipment grounding conductors: one allowance for all of them, up to four, at the largest ground. Each ground beyond four adds one quarter of an allowance.
  • Small fittings such as locknuts and bushings count nothing.

Fill (cu in) = sum of allowances × volume per wire size

Volume allowance per conductor, Table 314.16(B)

Wire sizeVolume allowance
18 AWG1.50 cu in
16 AWG1.75 cu in
14 AWG2.00 cu in
12 AWG2.25 cu in
10 AWG2.50 cu in
8 AWG3.00 cu in
6 AWG5.00 cu in

Worked example: a switch box with 12 AWG cable

Two 12-2 cables enter the box (one in, one out), which gives four insulated conductors and two grounds, with one switch and clamps:

  • Conductors: 4 × 2.25 = 9.00 cu in
  • Grounds: 1 allowance × 2.25 = 2.25 cu in
  • Clamps: 1 × 2.25 = 2.25 cu in
  • Switch: 2 × 2.25 = 4.50 cu in
  • Total: 18.00 cu in

The calculator suggests the 3 × 2 × 3½ in device box, marked 18.0 cu in, so it is exactly full: fine by code, tight to work in. Swap in 14 AWG and the same box needs 16 cu in, and it holds nine 14 AWG conductors. Mixed sizes count at their own volumes, but clamps and devices always count at the largest wire: two 14 AWG and two 12 AWG wires, one 14 AWG ground, a luminaire stud and one switch total 17.25 cu in.

Electrical box size chart

These standard metal box volumes come from Table 314.16(A). Boxes not in the table, including all plastic ones, carry their volume stamped inside, and that marking is what counts.

BoxVolume
Handy box, 4 × 2⅛ × 1⅞ in13.0 cu in
4 in round or octagon, 1½ in deep15.5 cu in
Device box, 3 × 2 × 3½ in18.0 cu in
4 in square, 1½ in deep21.0 cu in
4 in square, 2⅛ in deep30.3 cu in
4-11/16 in square, 2⅛ in deep42.0 cu in

A junction box with six 12 AWG wires and three grounds and no devices needs 6 × 2.25 + 2.25 = 15.75 cu in, so the 4 in round box at 15.5 cu in is just too small and the calculator moves up to the 18.0 cu in box.

When the box is too small

Go up a size, add a box extension, or split the splices into two boxes. A deeper box costs a few dollars and saves hot, jammed wires. Pick your own box in the "Box you plan to use" list to see a pass or fail.

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