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Electrical box fill calculator
Cubic inches in the box, and the box that holds them.
Open the Box Fill toolFree. Runs in your browser, no account, and keeps working offline.
What it does
Enter the wires in the box by size, the devices, whether it has internal cable clamps, any fixture studs, and the equipment grounds. The calculator adds up the cubic inches the way NEC 2023 §314.16(B) counts them and names the smallest standard metal box that holds it.
Devices count twice, clamps once, and grounds as one allowance for the first four plus a quarter for each one after. Clamps, devices and grounds count at the largest wire in the box, which can only make the answer safer.
Pick a box to check it, or enter the volume stamped inside a plastic box. Box fill depends on the edition your jurisdiction has adopted: verify with your inspector (AHJ).
Worked example
A switch box with two 12/2 cables in and one out to a light (six 12 AWG insulated wires), internal clamps, one device and three grounds.
- Wires (6 × 2.25 in³)
- 13.50 in³
- Clamps
- 2.25 in³
- Device (counts twice)
- 4.50 in³
- Grounds
- 2.25 in³
- Required
- 22.50 in³
- Smallest standard metal box
- Square 4¹¹⁄₁₆×1½ (29.5 in³)
NEC 2023 §314.16 counting; verify with your inspector (AHJ).
Questions
How do you calculate box fill?
Count each insulated wire that enters and is spliced or landed, at its size's volume. Add one allowance for internal clamps, one per kind of fixture stud, two per device, and the ground allowance — all at the largest wire. The total must not exceed the box's volume.
How many cubic inches is a 12 AWG wire?
2.25 in³ under NEC 2023 §314.16(B). 14 AWG is 2.00 in³. See the box fill chart for every size.
Do grounds count in box fill?
Yes. Under NEC 2023 §314.16(B)(5), up to four grounds count together as one allowance at the largest conductor, plus a quarter allowance for each ground after four.
Do pigtails count?
A pigtail that starts and ends inside the box counts zero.