Electrical Box Fill Calculator (NEC 314.16)

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Use the largest wire gauge entering the box. Devices, clamps, and fittings are counted at the volume of the largest conductor in the box per NEC 314.16(B).
Count each unbroken wire that passes through or terminates in the box. Do not count pigtails (short wires that start and end inside the box). Count looped conductors with 6-inch or longer free length as two conductors.
wires
Count each device yoke (switch, receptacle, GFCI, etc.) mounted in the box. Each single-gang yoke counts as 2 conductor volumes using the largest wire that terminates on it.
yokes
If the box has built-in cable clamps inside the box body, count ONE volume allowance total (regardless of how many clamps there are), sized to the largest conductor.
Count each luminaire stud or hickey inside the box. Each one adds one conductor volume based on the largest conductor in the box.
fittings
Enter how many grounding (bare or green) conductors enter the box. All of them together count as ONE volume allowance sized to the largest grounding conductor in the box.
wires
Minimum box volume requiredFits in most single-gang boxes
16

Total NEC 314.16(B) fill volume - the box must meet or exceed this

Conductor volume8
Device volume4
Clamp volume2
Support fitting volume0
Grounding conductor volume2
Unitin³
Volume per conductor2
Conductors8
Devices4
Clamps2
Fittings0
Grounds2

Minimum box volume: 16.00 in3

  • Total fill required: 16.00 in3.
  • The smallest standard metal box that fits is the 4" × 2-1/8" round/octagonal (21.5 in3).
  • Boxes with factory-installed internal clamps use one combined clamp allowance regardless of how many clamps are present.

Next stepVerify that the box you select also has its volume marked in cubic inches (required for non-standard boxes) or appears in NEC Table 314.16(A). Conduit bodies are governed by a separate section (314.16(C)).

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