Box Fill Calculator (NEC 314.16): Rules & Box Volumes

Updated 2026-09-29

Short answer: add up an allowance for every conductor entering the box (#14 = 2.00 in³, #12 = 2.25 in³, #10 = 2.50 in³), two allowances per device yoke, one for internal clamps, one per fixture stud, and one for all the grounds (plus a quarter for each ground over four). The total must not exceed the box volume.

What counts, and how much (314.16(B))

Item	Counts as
Each conductor that enters and ends or is spliced	1 allowance of its size
Conductor passing through unbroken	1 allowance
Pigtails and jumpers that do not leave the box	0
Internal cable clamps (any number)	1 allowance, largest conductor
Each fixture stud or hickey	1 allowance, largest conductor
Each device yoke	2 allowances, largest conductor on that device
Equipment grounding conductors, up to 4	1 allowance, largest ground
Each ground beyond 4 (2020 NEC and later)	1/4 allowance

Volume per conductor (Table 314.16(B))

Size	in³ each
#6 AWG	5.00
#8 AWG	3.00
#10 AWG	2.50
#12 AWG	2.25
#14 AWG	2.00
#16 AWG	1.75
#18 AWG	1.50

Metal box volumes (Table 314.16(A))

Box	Volume in³	Max #14	Max #12
4 × 1-1/4 round / octagon	12.5	6	5
4 × 1-1/2 round / octagon	15.5	7	6
4 × 2-1/8 round / octagon	21.5	10	9
4 × 1-1/4 square	18.0	9	8
4 × 1-1/2 square	21.0	10	9
4 × 2-1/8 square	30.3	15	13
4-11/16 × 1-1/4 square	25.5	12	11
4-11/16 × 1-1/2 square	29.5	14	13
4-11/16 × 2-1/8 square	42.0	21	18
3 × 2 × 1-1/2 device	7.5	3	3
3 × 2 × 2 device	10.0	5	4
3 × 2 × 2-1/4 device	10.5	5	4
3 × 2 × 2-1/2 device	12.5	6	5
3 × 2 × 2-3/4 device	14.0	7	6
3 × 2 × 3-1/2 device	18.0	9	8
4 × 2-1/8 × 1-1/2 device	10.3	5	4
4 × 2-1/8 × 1-7/8 device	13.0	6	5
4 × 2-1/8 × 2-1/8 device	14.5	7	6

"Max" is the number of allowances the box holds, before counting devices, clamps and grounds as allowances. Plastic and nonmetallic boxes use the volume marked on the box.

Worked example: receptacle in a 4" square box

A 4" square × 1-1/2" box (21.0 in³) with four #12 conductors, one receptacle, internal clamps and two grounds:

4 × #12 = 4 × 2.25 = 9.00 in³
Receptacle yoke = 2 × 2.25 = 4.50 in³
Clamps = 2.25 in³
Grounds (two) = 2.25 in³
Total 18.00 in³ ≤ 21.0 in³: the box is big enough.

The grounding rule since 2020

Before the 2020 NEC, all equipment grounding conductors in a box counted as a single allowance no matter how many there were. Since 2020, up to four count as one allowance and each one beyond four adds a quarter allowance. Boxes with many grounds that passed under older editions can now fail. Check which edition your jurisdiction enforces, and see the conduit fill calculator for the raceway side.

Questions electricians ask

How many 12 AWG wires can go in a 4x4 box?

A 4" square × 1-1/2" deep box (21 in³) holds nine #12 conductor allowances (9 × 2.25 = 20.25 in³). Remember that a device counts as two allowances, clamps as one and the grounds as one.

How do devices count in box fill?

Each yoke or strap holding a device (switch, receptacle) counts as two conductors of the largest size connected to it (314.16(B)(4)). A device wider than 2" counts two allowances for each gang it needs.

Do ground wires count in box fill?

Up to four equipment grounding conductors together count as one allowance of the largest ground. Since the 2020 NEC, each additional ground beyond four adds a quarter allowance.

Do pigtails count in box fill?

No. A conductor that starts and ends inside the box, such as a pigtail or a jumper, is not counted (314.16(B)(1) exception). Fixture wires coming from a canopy also do not count in some cases.