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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

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.

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