Junction Box Sizing Calculator

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NEC tables use cubic inches; cm³ is provided for reference.
The largest conductor entering the box. All conductors not counted individually are based on this size for clamps, fittings, and grounding.
Count every current-carrying conductor and neutral entering the box. Do not count equipment grounding conductors here (use the grounding field below).
Each switch, receptacle, GFCI, or AFCI yoke counts as two conductor allowances (NEC 314.16(B)(4)). Faceplates and cover plates do not count.
If the box has internal cable clamps, add one conductor allowance (based on largest conductor) regardless of how many clamps there are (NEC 314.16(B)(2)).
Each fixture stud or hickey inside the box counts as one conductor allowance based on the largest conductor (NEC 314.16(B)(3)).
All equipment grounding conductors together count as one allowance for the first four, plus 0.25 allowances for each conductor above four (NEC 314.16(B)(5)).
Total fill requiredNeeds deeper 4-inch square box
22.5in³

Minimum box volume needed per NEC 314.16

Conductor fill13.5in³
Device fill4.5in³
Clamp fill2.25in³
Fitting fill0in³
Grounding fill2.25in³
Recommended standard box4 x 2.125 in square (30.3 in³)
Conductors13.5
Devices4.5
Clamps2.25
Fittings0
Grounding2.25

22.50 in3 of fill required - 4 x 2.125 in square (30.3 in³)

  • Total box fill is 22.50 in3 (368.7 cm3). The box you select must equal or exceed this volume.
  • Device yokes account for 20% of the fill because each yoke counts as two conductor allowances (4.50 in3 each).
  • The 6 line/neutral conductors contribute 13.50 in3. Running fewer splices or using a larger box can resolve a tight fit.

Next stepChoose a listed metal box rated at or above the calculated volume. Check the manufacturer label or NEC Table 314.16(A) to confirm the box volume before installation.

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