ParrotNotes

Box Fill Calculator

Work out the box fill for an electrical box from its conductors, devices, clamps and grounds under NEC 314.16, and the smallest box that fits. Count the wires, grounds and devices, and check a box from Table 314.16(A) or a plastic box by its marked volume.

Your numbers

Count each hot, neutral or traveler that enters and is spliced or ends here, or passes through. A loop of 12 in or more counts two. Pigtails that stay in the box count zero.

Up to four count as one; each one beyond four adds a quarter.

Each switch, receptacle or GFCI on its own strap counts two, at the largest wire on it.

A device wider than a 2 in device box counts two for each gang it takes: a 2-gang range receptacle adds 1 here.

One allowance for all of them. Connectors that clamp outside the box count zero.

One allowance.

One allowance.

Box fill (volume required)

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Smallest standard metal box that fits
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Smallest device box
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Smallest square box
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Smallest round or octagonal box
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How box fill works

NEC 314.16 says a box must hold at least the volume its contents need. 314.16(A) gives the box's volume: from Table 314.16(A) for standard metal boxes, or as marked on plastic boxes, plus any marked ring or domed cover. 314.16(B) gives what the contents need: a number of volume allowances, each worth the cubic inches in Table 314.16(B) for its wire size. If the box volume is at least the total, the box is big enough.

Worked example: a 4 in square box feeding a duplex receptacle, with three 12/2 cables coming in. Six 12 AWG conductors are 6 × 2.25 = 13.50 in³; the three grounds count once, 2.25; the receptacle counts double, 4.50; internal clamps one more, 2.25. That is 22.50 in³. A 4 × 1-1/2 in square box (21.0 in³) is too small on its own; with a plaster ring marked 4.5 in³ it has 25.5 in³ and passes, and a 4 × 2-1/8 in box (30.3 in³) passes outright.

What counts and what does not, 314.16(B)

Conductors: one allowance for each wire that originates outside the box and is spliced or terminated inside, and one for each that passes through without a splice. An unbroken loop at least twice the 6 in free length of 300.14, so 12 in or more, counts twice. A wire no part of which leaves the box (a pigtail or jumper) counts zero. Exception: a ground and up to four fixture wires smaller than 14 AWG coming in from a domed luminaire canopy may be left out.

Internal cable clamps: one allowance for all of them, at the largest conductor in the box; connectors that clamp outside the box count zero. Fixture studs and hickeys: one allowance for each type, at the largest conductor. Devices: two allowances for each yoke or strap, at the largest wire connected to it, and a device wider than a 2 in device box counts two for every gang it takes. Grounds and bonding jumpers: one allowance for up to four, plus a quarter for each beyond four, at the largest ground.

Box volumes, Table 314.16(A)

Standard metal boxes and the most conductors of one size each holds when there is nothing else in it (no clamps, devices, fittings or grounds): the volume divided by the Table 314.16(B) allowance, rounded down. The code table prints the same numbers, except 5 rather than 4 for 8 AWG in the 4 × 1-1/4 in round box (12.5 ÷ 3.00 = 4.17).

Volume of standard metal boxes and the maximum number of conductors by size, NEC Table 314.16(A)
Box (in)in³181614121086
4 × 1-1/4Round or octagonal box12.58765542
4 × 1-1/2Round or octagonal box15.510876653
4 × 2-1/8Round or octagonal box21.51412109874
4 × 1-1/4Square box18121098763
4 × 1-1/2Square box211412109874
4 × 2-1/8Square box30.32017151312106
4-11/16 × 1-1/4Square box25.5171412111085
4-11/16 × 1-1/2Square box29.5191614131195
4-11/16 × 2-1/8Square box422824211816148
3 × 2 × 1-1/2Device box7.55433321
3 × 2 × 2Device box106554432
3 × 2 × 2-1/4Device box10.57654432
3 × 2 × 2-1/2Device box12.58765542
3 × 2 × 2-3/4Device box149876542
3 × 2 × 3-1/2Device box18121098763
4 × 2-1/8 × 1-1/2Device box10.36554432
4 × 2-1/8 × 1-7/8Device box138765542
4 × 2-1/8 × 2-1/8Device box14.59876542
3-3/4 × 2 × 2-1/2Masonry box149876542
3-3/4 × 2 × 3-1/2Masonry box211412109874
FS, 1-3/4 in deep, single cover/gangFS or FD box13.59766542
FD, 2-3/8 in deep, single cover/gangFS or FD box18121098763
FS, 1-3/4 in deep, multiple cover/gangFS or FD box18121098763
FD, 2-3/8 in deep, multiple cover/gangFS or FD box2416131210984

Allowance per conductor, Table 314.16(B): 18 AWG 1.5 in³, 16 AWG 1.75 in³, 14 AWG 2 in³, 12 AWG 2.25 in³, 10 AWG 2.5 in³, 8 AWG 3 in³, 6 AWG 5 in³.

For electricians and electrical distributor reps

Box fill is where a rough-in quote meets the inspection. On the walk-through, note each box location with the circuits and cables landing in it, the devices, any fixture with a stud or hickey, and whether the customer wants deep boxes or rings. Read the list back before you leave so the box order and the bid match the job.

Sources, each read on 2026-10-30. The edition of the NEC your jurisdiction has adopted and your inspector have the final say:

Running the cable in conduit? The conduit fill calculator checks the raceway, and the wire size calculator and voltage drop calculator pick the conductor for the load and the run.

More for electrical sales: Electrical distributor sales, Distributor sales, Industrial distributor sales.

The box count starts on the site walk

How many circuits land in each box, which devices go where and what the customer changed their mind about all come up on the walk-through. Record it in ParrotNotes and you get a transcript, a summary and the action items, so the material list and the box sizes match what was agreed.

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Box fill calculator FAQ