🔌 Electrical Box Fill Calculator
Work out the minimum box volume required for your wiring, following the NEC 314.16 box fill rules.
What is Box Fill?
Box fill calculations determine the minimum interior volume an electrical junction box must have to safely hold all the conductors, devices, and fittings inside it, following National Electrical Code (NEC) Article 314.16.
How It’s Calculated
Each item inside the box counts as one or more “fill units” based on the volume allowance of the largest conductor present: Required Volume = Total Fill Units × Conductor Allowance. Under NEC rules, every current-carrying conductor counts as 1 unit; all grounding conductors together count as just 1 unit; internal cable clamps count as 1 unit; and each device (like a switch or receptacle) counts as 2 units.
- 14 AWG: 2.00 cu in per conductor
- 12 AWG: 2.25 cu in per conductor
- 10 AWG: 2.50 cu in per conductor
- 8 AWG: 3.00 cu in per conductor
- 6 AWG: 5.00 cu in per conductor
If you enter your box’s rated volume, this calculator shows the remaining capacity. This tool is for general planning and educational purposes — always verify against the full NEC 314.16 rules and your local electrical code, and have work inspected by a licensed electrician.