Box Fill Calculator
Enter your conductor count and wire size to get the cubic inches required, based on NEC 314.16.
What your result means
This is the cubic inches your conductors need, at the NEC 314.16 per-wire volume (2.0 for #14, 2.25 for #12, 2.5 for #10). It's a simplified count: a full 314.16 calc also adds allowances for devices (twice the largest conductor), cable clamps (one), and grounds (one total). Compare the result to the volume stamped inside your box, upsize if it's close, and verify against code.
How to use this calculator
- Count conductors (hot, neutral, ground all count).
- Pick wire gauge.
- Read total cubic inches required.
- Compare to box volume (printed on box).
- Larger box if total exceeds capacity.
The formula
Each conductor needs a set volume by gauge: 2.0 cubic inches for #14, 2.25 for #12, 2.5 for #10. Multiply by the conductor count.
Worked example
Say you've got 6 conductors of #12 in a box. That's 6 times 2.25, or 13.5 cubic inches, which a standard 18 cubic inch box handles.
NEC 314.16 per-conductor volume
| Wire | Volume each |
|---|---|
| #14 AWG | 2.0 cu in |
| #12 AWG | 2.25 cu in |
| #10 AWG | 2.5 cu in |
Tips & gotchas
- NEC 314.16: each conductor adds 2.0 cu in (#14), 2.25 (#12), or 2.5 (#10).
- All conductors count, ground wire included.
- Each device (switch, outlet) adds twice the largest conductor's volume.
- Box volumes are printed inside the box.
- Always upsize if you're close to the max.
Frequently asked questions
How many wires fit in an electrical box?
Use the calculator. A standard 4x4 box is 18 to 21 cubic inches.
Does the ground wire count?
Yes. All conductors count.
What if I have too many wires?
Use a larger box or add an extension ring.
What's the NEC reference?
NEC 314.16, box fill calculations.
How do I find box volume?
It's stamped inside the box, or on the manufacturer's spec.
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