Wiring & Boxes · CEC 12-3034 / NEC 314.16
Box Fill Calculator
Count conductors, clamps, and device yokes against a box's marked volume so junction boxes and outlet boxes aren't overcrowded.
1. The Box
2. What's in the Box
3. Fill Verdict
Volume allowance methodTOTAL VOLUME REQUIRED
18.5 in³
BOX CAPACITY — USED
132%
Overfilled. This box needs more volume than it has. Upsize to the next box size or remove a clamp/splice.
How each item is counted
- Each conductor that enters the box counts as one allowance of its size — even if it just passes through.
- One or more internal cable clamps count as a single conductor allowance of the largest wire present.
- Each switch or receptacle (yoke) counts as two allowances of the largest conductor connected to it.
- All grounding conductors together count as one allowance of the largest ground wire.
- Wires that originate and end entirely outside the box (like a loop for a future fixture) don't count.
Box fill logic
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