Box Fill Calculator

How to use the box fill calculator

Count the conductors by size that enter the box (hots, neutrals, travelers). A wire that passes through unbroken counts once.
Tick internal cable clamps if the box has them.
Add device yokes (each switch or receptacle strap) and the largest wire connected to them.
Enter the number of equipment grounding conductors.
Pick a standard box or type the volume marked inside the box.

Box fill formula (NEC 314.16(B))

Required volume = Σ(conductors × allowance) + clamps (1×) + support fittings (1× each) + device yokes (2× each) + grounds (1×, +¼ each beyond 4)

Conductor size	Volume allowance
6 AWG	5.00 in³
8 AWG	3.00 in³
10 AWG	2.50 in³
12 AWG	2.25 in³
14 AWG	2.00 in³
16 AWG	1.75 in³
18 AWG	1.50 in³

Worked example

A receptacle fed through with two 12/2 NM cables: four 12 AWG conductors (4 × 2.25 = 9.00 in³), internal clamps (2.25), one device yoke (2 × 2.25 = 4.50) and two grounds counted once (2.25). Total: 18.0 in³. A 3 × 2 × 3-1/2 device box (18.0 in³) just makes it; a 4 × 1-1/2 square box (21.0 in³) gives some breathing room.

Standard metal box volumes (Table 314.16(A))

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

Max conductor counts assume no clamps, devices or grounds.

Box fill vs conduit fill

Boxes are sized by volume under 314.16; raceways are sized by cross-sectional area under Chapter 9. If you're planning the conduit between boxes, use the conduit fill calculator.

Frequently asked questions

How do you calculate box fill?

Add up the volume allowances for everything in the box using NEC 314.16(B): each conductor, one allowance for internal clamps, one per type of support fitting, two per device yoke and one for the grounding conductors. Compare the total with the box volume.

How many 12 AWG wires fit in a 4" square box?

A 4 × 1-1/2 square box has 21.0 in³. At 2.25 in³ per 12 AWG conductor, that is 9 conductors if nothing else is in the box. Each device, clamp set and the grounds take their share too.

Do ground wires count in box fill?

Yes. Up to four equipment grounding conductors together count as one conductor of the largest ground size. Under the 2020 and 2023 NEC, each ground beyond four adds a quarter allowance.

Do wire nuts and pigtails count?

No. Conductors that start and end inside the box, such as pigtails, are not counted, and connectors like wire nuts are not counted. Up to four fixture wires from a domed luminaire canopy are also exempt.

How many cubic inches does a switch take?

Each yoke or strap counts as two conductors of the largest wire connected to it: 4.0 in³ with 14 AWG or 4.5 in³ with 12 AWG.

What about plastic boxes?

Nonmetallic boxes are not listed in Table 314.16(A); they must be marked with their volume in cubic inches. Enter that number in the custom volume field.

Last reviewed 2026-11-15. Values from NFPA 70 (NEC) 2017–2023. For estimating only — confirm with the code edition adopted by your authority having jurisdiction.