How to Calculate Box Fill
Every box has a volume in cubic inches, and every item you put in it consumes a volume allowance based on the largest conductor present. The sum must not exceed the box volume.
What counts, and how much
- Each conductor counts as one allowance, valued by the largest conductor size in the box.
- All equipment grounding conductors together count as a single allowance.
- Each switch or receptacle counts as two allowances.
- Each cable clamp counts as one allowance.
- Support fittings for a luminaire count as one allowance.
Worked method
Find the volume allowance for the largest conductor size, count the items above, multiply and total, then compare with the box volume stamped on the box or listed in the tables.
The catch that trips people up is the largest conductor: a single 12 AWG conductor among 14 AWG conductors raises every allowance to the 12 AWG value.
Frequently asked
- Does a grounding conductor count in box fill?
- Yes, but all of them together count only once, at the value of the largest conductor.
- Do wire nuts count?
- Connectors themselves are not counted; what is counted is the conductors entering the box.
- How do I find the box volume?
- It is usually stamped inside the box, and for common box types it is listed in the reference tables. The calculator includes a preset list of common boxes.
Related calculators
Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.