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Box Fill Calculator

Count conductor volume allowances to confirm a device box is legally large enough.

Conductors in the box

Count each conductor that enters the box and terminates, and each conductor that passes through unspliced. A conductor that both enters and leaves without splicing counts once. Pigtails entirely inside the box are not counted at all.

2.00 in³ each

2.25 in³ each

2.50 in³ each

3.00 in³ each

5.00 in³ each

Devices, clamps and fittings

314.16(B)(4) — each yoke counts as two conductors of the largest size connected to it.

314.16(B)(2) — all clamps together count as one conductor, no matter how many.

314.16(B)(3) — fixture studs and hickeys, one conductor each.

314.16(B)(5) — all grounds together count as one conductor.

The single ground allowance is taken at the largest ground present.

in³

Marked on plastic boxes. Metal box volumes are in NEC Table 314.16(A).

Required box volume

18.00 in³8 conductor allowances

A 18.0 in³ box is large enough, with 0.00 in³ to spare.
Conductor allowances4 conductors counted individually
9.00 in³
Device, clamp and fitting allowancesTaken at 12 AWG, the largest conductor present
6.75 in³
Grounding allowanceAll grounds as one 12 AWG conductor
2.25 in³
Smallest standard box that works3" × 2" × 3-1/2" device box
18.0 in³

Legal, but tight

You are using 100% of the box. It passes, but a box filled to the legal limit is hard to make up neatly and harder still to service later. Step up one size if you have the framing for it.
How the required volume is built up
ItemCounted asAllowance sizeVolume
12 AWG conductors4 conductors2.25 in³9.00 in³
Device yokes (1)2 conductors2.25 in³4.50 in³
Internal cable clamps (2)1 conductor total2.25 in³2.25 in³
Grounding conductors (2)1 conductor total2.25 in³2.25 in³
Required18.00 in³
Standard boxes checked against this makeup
BoxVolumeSpare after makeupStatus
3" × 2" × 2" device box10.0 in³-8.00 in³Too small
3" × 2" × 2-1/4" device box10.5 in³-7.50 in³Too small
3" × 2" × 2-1/2" device box12.5 in³-5.50 in³Too small
3" × 2" × 2-3/4" device box14.0 in³-4.00 in³Too small
3" × 2" × 3-1/2" device box18.0 in³0.00 in³Compliant
4" square × 1-1/4" deep18.0 in³0.00 in³Compliant
4" square × 1-1/2" deep21.0 in³3.00 in³Compliant
4" square × 2-1/8" deep30.3 in³12.30 in³Compliant
4-11/16" square × 1-1/4" deep25.5 in³7.50 in³Compliant
4-11/16" square × 1-1/2" deep29.5 in³11.50 in³Compliant
4-11/16" square × 2-1/8" deep42.0 in³24.00 in³Compliant
18 cu in. plastic device box18.0 in³0.00 in³Compliant
20 cu in. plastic device box20.0 in³2.00 in³Compliant
22.5 cu in. plastic device box22.5 in³4.50 in³Compliant
32 cu in. plastic 2-gang box32.0 in³14.00 in³Compliant
42 cu in. plastic 2-gang box42.0 in³24.00 in³Compliant

Quick answer

NEC box fill limits how many conductors, devices, clamps, and equipment grounds can share a box. Each item counts as a volume allowance based on the largest wire in the box (for example, 14 AWG counts 2.00 in³ each in a standard calculation). If fill exceeds the box volume, you need a larger box or fewer devices — overcrowded boxes are a common inspection failure. Use the calculator with your box size, wire sizes, and device count; always follow the box manufacturer's listed volume and local amendments.

What this assumes

  • Volume allowances per conductor come from NEC Table 314.16(B).
  • All equipment grounding conductors together count as a single conductor of the largest size present.
  • Each yoke or strap counts as two conductors of the largest size connected to that device.

Important

Box fill is a code minimum, not a workmanship target. Boxes filled to the legal limit are difficult to make up neatly and harder to service later.

Understanding the result

How the count works

NEC 314.16(B) assigns a cubic-inch allowance to each conductor size, then counts: each conductor entering and terminating counts as one; all grounds together count as one; all internal cable clamps together count as one; and each device yoke counts as two. Conductors that pass through without splice count as one each.

Why overfilled boxes fail

An overfilled box forces conductors against device terminals and makes it nearly impossible to fold the makeup back cleanly. That damages insulation, loosens terminations over time, and is a common inspection failure on residential rough-in.

Pass-through conductors add up fast

A three-gang switch box with power in, power out, and three switch legs can accumulate volume allowances quickly. Each 12 AWG conductor counts as 2.25 cubic inches. Six conductors at 12 AWG is 13.5 cubic inches before you count the device yokes — which is why a shallow 22 cubic inch box fails where a 30 cubic inch box passes.

Box volume is stamped on the box

Listed boxes have their internal volume marked on the back or inside. If the stamp is missing or illegible, use NEC Table 314.16(A) for standard metal boxes. Plastic boxes without a volume stamp are not code-compliant for use as device boxes.

Extensions are a legitimate fix

When a rough-in box is borderline, a box extension ring adds volume without replacing the entire box. For a finished wall, a mud ring and deeper extension is cheaper than opening drywall to swap boxes. Plan box depth at rough-in to avoid this entirely.

Common questions

What size box do I need for 4 wires and a switch?
A single-gang box with four 14 AWG conductors, one ground, and one switch needs at least 12.25 cubic inches: four conductors at 2.00 cubic inches each (8.00), one ground (2.00), and one device yoke counting as two conductors (4.00). A standard 18 cubic inch single-gang box covers this with room to work.
How many wires can fit in a 4x4 box?
A 4x4x1-1/2 inch box provides 21 cubic inches. With 12 AWG conductors at 2.25 cubic inches each, you can fit roughly eight conductors plus one ground before device allowances. With no devices installed, nine 12 AWG conductors pass. Add a switch or receptacle and the count drops by 4.5 cubic inches per yoke.
Do ground wires count in box fill?
Yes, but all equipment grounding conductors in the box count as a single conductor of the largest ground wire size present. Four 12 AWG grounds and one 10 AWG ground together count as one 10 AWG conductor — 2.50 cubic inches, not 10.00.
What is the box fill for 12 AWG wire?
Each 12 AWG conductor is allotted 2.25 cubic inches under NEC Table 314.16(B). A device yoke counts as two conductors of the largest size connected to that device, so a switch wired with 12 AWG consumes 4.50 cubic inches for the yoke alone.
How big of a junction box do I need for 6 wires?
Six 12 AWG conductors need 13.5 cubic inches (6 × 2.25), plus 2.25 for the ground counted as one conductor, totaling 15.75 cubic inches with no devices. A 4x4x1-1/2 inch box at 21 cubic inches handles this. Six 10 AWG conductors need 18.0 cubic inches before grounds and devices.
Does a dimmer switch count as two wires in box fill?
Yes. NEC 314.16(B)(4) counts each yoke-mounted device as two conductors of the largest wire connected to it. A dimmer on 14 AWG adds 4.00 cubic inches (2 × 2.00). Large smart dimmers also take physical space, so a box that passes on paper can still be unworkable.
What size box for a GFCI outlet?
A GFCI on a 12/2 feed (one hot, one neutral, one ground in, no pass-through) needs 11.25 cubic inches: three conductors at 2.25 (6.75), one ground (2.25), and one device yoke as two conductors (4.50). Use at least a 20 cubic inch box — GFCIs are physically large and need room beyond the minimum.
How many 14 AWG wires in a single gang box?
A standard 18 cubic inch single-gang box with no device holds seven 14 AWG conductors plus one ground counted as one: eight allowances at 2.00 cubic inches equals 16.00 cubic inches. Add a receptacle and you lose 4.00 cubic inches for the yoke, leaving room for only five conductors plus ground.
Do wire nuts count toward box fill?
No. NEC 314.16 counts conductors, clamps, and device yokes — not wire nuts, connectors, or cable connectors. However, a box packed to the legal limit with wire nuts is still nearly impossible to service, which is why experienced electricians size up before they hit the code minimum.
What happens if a box is overfilled?
An overfilled box fails inspection under NEC 314.16. Beyond the code issue, forced conductors against device terminals cause insulation damage and loose connections that arc over time. The fix is a box extension or a larger box — both require opening the wall.

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