Box Fill Calculator
By Andrew ·
Quick answer
Box fill adds up volume allowances under NEC 314.16(B): one for each insulated conductor that enters the box, two for each device yoke, one for all internal clamps, one for each type of fixture stud or hickey, and one for up to four grounds. Each allowance is the Table 314.16(B) volume for the wire, 2.25 in³ for 12 AWG, so a receptacle fed through with 12/2 cable needs 18 in³.
Calculator
- Results
- Volume allowances
- 8
- Box volume needed
- 16 in³
- Big enough?
- Yes
From NEC 314.16(B), with every conductor the same size. Use the volume stamped in the box, plus any plaster ring or extension marked with its own.
What counts, item by item
NEC 314.16(B) turns everything in a box into volume allowances. Count the allowances, multiply by the volume for the wire size, and compare the total with the volume of the box.
- Conductors, (B)(1): each conductor that comes in from outside the box and ends, is spliced or passes through counts once. An unbroken loop or coil 12" or longer counts twice.
- Internal cable clamps, (B)(2): one allowance for all of them, sized by the largest conductor in the box. A connector that clamps the cable outside the box adds nothing.
- Fixture studs and hickeys, (B)(3): one allowance for each type, sized by the largest conductor in the box.
- Devices, (B)(4): two allowances for each yoke or strap holding a switch, receptacle or other device, sized by the largest conductor connected to it. A device wider than a single 2" gang gets two for each gang it needs.
- Equipment grounding conductors, (B)(5): one allowance for up to four, plus a quarter allowance for each one after the fourth, sized by the largest ground.
Some things take no allowance: a conductor that never leaves the box, such as a pigtail; wire connectors; and up to four fixture wires smaller than 14 AWG, with the equipment grounding conductor, that come in from a domed luminaire or similar canopy and end in the box.
Volume for each conductor
| Wire size | Volume each |
|---|---|
| 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 |
With mixed sizes, each conductor counts at its own size. The clamp and fixture stud allowances use the largest conductor in the box, a device uses the largest conductor connected to it, and the ground allowance uses the largest ground. The calculator takes one wire size, so for a mixed box enter the largest: the answer comes out on the safe side.
The table stops at 6 AWG. Boxes for 4 AWG and larger conductors are sized by pull distances instead of volume.
Worked examples
- Cables
- two 12/2 with ground, one in and one out
- Device
- one duplex receptacle
- Box
- 3" × 2" × 3-1/2" metal device box, 18.0 in³, internal clamps
- Insulated conductors
- 4 allowances
- Receptacle yoke
- 2 allowances
- Internal clamps
- 1 allowance
- Two grounds
- 1 allowance
- Volume needed
- 8 × 2.25 in³ = 18 in³
- Big enough?
- Yes, with nothing to spare
Add a third 12/2 cable and the count goes to 10, or 22.5 in³, too much for this box. A 4" square box 2-1/8" deep (30.3 in³) with a device ring has room.
- Insulated conductors
- 12
- Grounds
- 6
- Internal clamps
- yes
- Box volume
- 30.3 in³
- Grounds
- 1 + 2 × 1/4 = 1.5 allowances
- Total
- 12 + 1 + 1.5 = 14.5 allowances
- Volume needed
- 14.5 × 2.25 in³ = 32.625 in³
- Big enough?
- No, 2.325 in³ short
A 4-11/16" square box 2-1/8" deep (42.0 in³) fixes it, or an extension ring or raised cover marked with enough volume to make up the difference.
Box volumes
Use the volume marked on the box. Table 314.16(A) lists the standard metal boxes; nonmetallic boxes, and boxes not in the table, are marked by the maker. A plaster ring, extension ring or domed cover adds the volume marked on it.
| Box | Volume | 14 AWG | 12 AWG |
|---|---|---|---|
| 4" round or octagonal, 1-1/2" deep | 15.5 in³ | 7 | 6 |
| 4" round or octagonal, 2-1/8" deep | 21.5 in³ | 10 | 9 |
| 4" square, 1-1/2" deep | 21.0 in³ | 10 | 9 |
| 4" square, 2-1/8" deep | 30.3 in³ | 15 | 13 |
| 4-11/16" square, 1-1/2" deep | 29.5 in³ | 14 | 13 |
| 4-11/16" square, 2-1/8" deep | 42.0 in³ | 21 | 18 |
| 3" × 2" device box, 2-1/2" deep | 12.5 in³ | 6 | 5 |
| 3" × 2" device box, 3-1/2" deep | 18.0 in³ | 9 | 8 |
Where box fill goes wrong
- Counting a device once instead of twice, or by the smallest wire on it instead of the largest.
- Counting every ground separately, or leaving the grounds out. The first four share one allowance.
- Counting pigtails and wire connectors, which take no allowance.
- Counting a loop of 12" or more as one conductor instead of two.
- Reading the volume of a different box. Device boxes that look alike come in several depths.
The quarter allowance for each ground after the fourth came in with the 2020 NEC, and the calculator uses it. Under the 2017 NEC all the equipment grounding conductors in a box counted as one allowance together, with one more for a separate isolated-ground set. Use the edition your jurisdiction enforces; the AHJ has the final say.
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Common questions
How many 12 AWG wires can go in a 4" square box?
A 4" square box 1-1/2" deep holds 21.0 in³, which is 9 allowances of 2.25 in³; at 2-1/8" deep (30.3 in³) it is 13. Devices, clamps and grounds come out of those numbers too.
Do ground wires count in box fill?
Yes. Up to four equipment grounding conductors count as one allowance together, sized by the largest, and each one after the fourth adds a quarter allowance (2020 and 2023 NEC).
How much box fill does a switch or receptacle take?
Two allowances for each yoke, sized by the largest conductor connected to it: 4 in³ for a device on 14 AWG and 4.5 in³ on 12 AWG.
Do wire nuts and pigtails count in box fill?
No. Wire connectors take no allowance, and neither does a conductor that never leaves the box, such as a pigtail. Internal cable clamps do take one allowance for all of them.