Box Fill Calculator
Count your conductors, devices, and grounds — get the required cubic inches and whether your box is legal.
Insulated circuit conductors entering the box
Count each hot and neutral that enters or passes through. Pigtails and wires that never leave the box don't count.
Required volume
6.00 cu in
Box volume
21.0 cu in
✅ This box is big enough.
Conductors: 0.00
Devices: 4.00
Grounds: 2.00
Clamps: 0.00
Per NEC 314.16 and Tables 314.16(A)/(B), using the 2023 ground-wire counting rule (one allowance for up to four EGCs, ¼ allowance each beyond four). Luminaire studs, hickeys, and barriers add more — verify against the current code.
How box fill counting works
NEC 314.16(B) assigns a volume allowance to everything in the box: each insulated circuit conductor counts once at its own size; each device yoke counts twice at the largest conductor connected to it; internal cable clamps count once at the largest conductor in the box; and equipment grounding conductors get a single allowance at the largest EGC (plus a quarter-allowance each beyond four, under the 2023 code).
Wires that pass through unbroken count once. Pigtails that never leave the box count zero. Plastic boxes have their volume stamped inside — use the "custom volume" option for those.