Box Fill Calculator
Estimate required box volume using conductor allowances, grounds, clamps, and device yokes. Always verify your box’s marked volume and local code requirements.
Developed by: Nohman Habib
1) Box volume
2) Conductors (entering the box)
Count conductors that enter the box and are spliced/terminated or pass through. Wire connectors don’t add volume.
| AWG | Count | Allowance (in³) |
|---|---|---|
| #18 | 1.50 | |
| #16 | 1.75 | |
| #14 | 2.00 | |
| #12 | 2.25 | |
| #10 | 2.50 | |
| #8 | 3.00 | |
| #6 | 5.00 |
3) Grounds, clamps, devices
4) Optional: pigtails that must be counted
Most pigtails do not count if they stay entirely within the box. If a local rule or special case requires counting, enter them here.
| AWG | Pigtails (count) |
|---|---|
| #18 | |
| #16 | |
| #14 | |
| #12 | |
| #10 | |
| #8 | |
| #6 |
Results
Breakdown
| Item | Allowances | Per allowance | Volume |
|---|---|---|---|
| Click Calculate to see details. | |||
Example data
| Scenario | Box volume | Conductors | Grounds | Clamp | Yokes | Required volume | Pass? |
|---|---|---|---|---|---|---|---|
| 1‑gang receptacle (typical) | 18 in³ | 4× #14 | 2× #14 (counts as 1) | Yes | 1× (2 allowances of #14) | 16 in³ | PASS |
| Overfilled #12 box | 20.3 in³ | 6× #12 | 1× #12 (counts as 1) | Yes | 1× (2 allowances of #12) | 22.5 in³ | FAIL |
| Deep box, two devices | 30 in³ | 8× #14 | 1× #14 (counts as 1) | Yes | 2× (4 allowances of #14) | 28 in³ | PASS |
Formulas used
Volume allowance per conductor is chosen by AWG size (e.g., #14 → 2.00 in³, #12 → 2.25 in³).
- Conductors: total conductor volume = Σ(countawg × allowanceawg)
- Grounds: if any grounds exist, they usually add 1 allowance of the largest ground size in the box
- Internal clamp: adds 1 allowance of the largest conductor present
- Support fittings: each adds 1 allowance of the largest conductor present
- Device yokes: each yoke adds 2 allowances based on the largest conductor connected to that yoke
Total required volume = conductors + grounds + clamp + support fittings + device yokes (+ optional counted pigtails).
How to use
- Enter your box volume (best: the volume printed/marked on the box).
- Count and enter conductors entering the box by AWG size.
- Enter equipment grounds and choose the ground size.
- Check internal clamp if the box has built-in cable clamps, and enter device yokes/support fittings if present.
- Click Calculate. If it fails, increase box volume (deeper/larger box, extension ring) or reduce conductors.
FAQs
1) Do wire nuts or connectors count toward box fill?
No. Small fittings like wire connectors do not add box fill volume in typical NEC-style calculations. The conductors and certain internal components are what consume the allowed space.
2) How are grounds counted?
Multiple equipment grounding conductors typically count as a single conductor allowance, based on the largest ground size present in the box.
3) Why does a device yoke add “two conductors”?
A switch or receptacle strap (yoke) usually requires a double allowance because it occupies space similar to two conductors, based on the largest conductor connected to that device.
4) Do pigtails count?
Most pigtails that stay entirely within the box do not count. If your local requirement or a special situation requires counting them, enter them in the “Optional pigtails” section.
5) What if the calculator shows FAIL?
Use a larger-volume box, a deeper box, or an extension ring. You can also reroute or reduce conductors/devices where allowed. The goal is to keep required volume at or below available volume.
6) Is this an official code compliance tool?
No. It’s an estimator based on common NEC-style rules. Always confirm with the box’s markings, your edition of the code, and any local amendments before installation.