Switch & Outlet Device Box Fill Calculator
Free online tool • 2026 NEC Compliant
Residential device boxes — the boxes that hold switches, receptacles, and GFCI outlets — are the most commonly overfilled boxes in electrical work. The NEC requires that the combined volume of all conductors, devices, and clamps stays within the box's rated cubic-inch capacity. Use this calculator to check any device box fill before you close the wall.
Conductors (entering or leaving box)
All EGCs count as 1 × the largest
Each device = 2× largest wire on that yoke
Results
PASSHow to Use This Calculator
Select box type
Choose from standard device box sizes (3×2 series) from NEC Table 314.16(A).
Add conductors
Enter 12 or 14 AWG wire gauge and the count of conductors entering the box.
Enter device count
Enter the number of switches or receptacles on single-gang plates.
Add ground wires
Enter the number of equipment grounding conductors — all count as one.
Frequently Asked Questions
How many 12 AWG wires can fit in a standard switch box?
A common 3×2×2.5" box has a volume of 12.5 cubic inches. With 12 AWG wire (2.25 in³ each), the 40% limit allows up to 5 conductors — but you must also account for the switch device itself (2× the wire volume = 4.5 in³) and ground wires (1× volume = 2.25 in³). A 12.5 in³ box typically holds 2 conductors + 1 device + 1 ground in 12 AWG.
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