What box fill means
Box fill is the count of wires, devices and clamps inside an electrical box, converted to cubic inches and checked against the box's volume.
Where it matters on a electrical job
Stuffing a smart switch or a GFCI into an old shallow box is where box fill bites. Do the count before you promise a quick device swap, and carry a deeper box or a box extender on the truck.
What the model code says
- Each 14 AWG conductor counts as 2.00 cubic inches and each 12 AWG conductor as 2.25 cubic inches. (IRC 2021, Chapter 39)
- Boxes of 100 cubic inches or less must be marked with their cubic-inch capacity, other than the standard boxes in the code's table. (IRC 2021, Chapter 39)
Check which code edition your building department enforces for box fill before you quote.
What it costs
Jobs that involve box fill are priced in the electrical outlet installation cost guide: about $190 typical, usually $130 to $260. The guide shows its sources and what moves the price. The box fill calculator prices a job line by line.
Work it out
- Free calculator: Box fill calculator
- Cost guide: Electrical outlet installation cost
- Licensing: Electrical license rules by state
- Tools, templates and guides: Free electrical resources
Sources: 2021 International Residential Code, Chapter 39, Power and Lighting Distribution (ICC). Checked October 9, 2026.