NEC Box Fill Calculator
Calculate the minimum required box volume per NEC Article 314.16 based on conductors, devices, and fittings inside the box.
Formula — NEC Article 314.16(B)
Total Fill (in³) = Conductor Fill + Device Fill + Ground Fill + Clamp Fill + Stud Fill
- Conductor Fill: (Hot count + Neutral count) × Volume per conductor
- Ground Fill: If any grounds present → 1× Volume per conductor (largest ground)
- Device Fill: Device count × 2 × Volume per conductor (largest conductor)
- Clamp Fill: If internal clamps present → 1× Volume per conductor
- Stud Fill: If fixture studs/hickeys present → 1× Volume per conductor
Volume allowances per NEC Table 314.16(B):
18 AWG=2.0 in³ | 16 AWG=2.25 in³ | 14 AWG=2.5 in³ | 12 AWG=2.75 in³ |
10 AWG=3.0 in³ | 8 AWG=3.5 in³ | 6 AWG=5.0 in³
Assumptions & References
- Based on NEC 2020 Article 314.16 — Conductors in Outlet, Device, and Junction Boxes.
- All equipment grounding conductors (regardless of count) are counted as a single conductor using the volume of the largest grounding conductor present.
- Each single yoke/strap device counts as two conductors at the volume of the largest conductor connected to that device.
- Conductors that originate and terminate within the box (e.g., pigtails ≤ 6 in) are not counted.
- Conductors passing through the box without splice or termination each count as one conductor.
- This calculator assumes all conductors are the same AWG for simplicity. For mixed sizes, calculate each group separately per NEC 314.16(B)(1).
- Always verify with your local AHJ (Authority Having Jurisdiction) and the adopted NEC edition.