Box Fill Calculator (NEC 314.16)
Make sure a junction or device box is large enough. Codewire totals the required volume from conductors, devices, clamps and equipment grounds and compares it to your box volume, per NEC 314.16.
How to use it
- Enter the box volume (or pick a common box) in cubic inches.
- Add conductors by size and count, the number of device yokes, clamps and the largest equipment ground.
- Read the required volume versus the box volume with an instant fits / too-small result.
Each conductor uses the volume allowance from Table 314.16(B) (e.g. 2.25 cu in for 12 AWG). Each device yoke counts as two of the largest conductor, all clamps count as one, and all equipment grounds count as one of the largest ground.
Frequently asked questions
How do you calculate box fill?
Add the Table 314.16(B) volume allowance for each conductor, plus two allowances per device yoke (based on the largest conductor), one for all internal clamps, and one for all equipment grounding conductors. Compare the total to the box's cubic-inch volume.
How many cubic inches does a 12 AWG wire need in a box?
2.25 cubic inches per 12 AWG conductor, per NEC Table 314.16(B). 14 AWG needs 2.0, 10 AWG needs 2.5.
All Codewire calculators · Free for electricians. Verify against the NEC edition adopted by your local AHJ.