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Box Fill Calculator
NEC 314.16. Calculates the minimum cubic-inch volume required for a box based on conductors, devices, and hardware.
Box Fill (NEC 314.16)
How NEC 314.16 box fill works
Each item inside a box is counted as a number of "conductor equivalents," and each conductor has a minimum volume in cubic inches based on the wire gauge. Add up the equivalents, multiply by the volume per conductor, and that's your minimum box size.
Conductor counting rules (NEC 314.16(B))
- Each conductor entering the box = 1
- All grounding conductors combined = 1
- Cable clamps inside the box = 1 total regardless of quantity
- Each device (switch, outlet) = 2
- Conductors passing through without splice = 1 each
Volume per conductor by gauge
- 14 AWG2.00 in³
- 12 AWG2.25 in³
- 10 AWG2.50 in³
- 8 AWG3.00 in³
- 6 AWG5.00 in³
Tip: box volume is stamped inside
Most metal boxes have the cubic inch volume stamped on the inside. Plastic boxes typically list it on the packaging or manufacturer spec sheet.
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