Box Fill Calculator

Every outlet box, switch box, and junction box has a maximum cubic-inch volume that limits how many wires can go in it. If you go over that volume, you need a bigger box. The NEC assigns a volume allowance to each conductor, device, and clamp. This tool adds them up and tells you whether your box is big enough.

Tip: No matter how many ground wires you have, they all count as just one conductor of the largest size in the box.

Box Volume

Box Type

Box Volume (in³)

Conductors & Fittings (NEC 314.16(B))

Largest Conductor AWG Sets volume for devices, ground, clamps

14 AWG conductors 2.00 in³ each

12 AWG conductors 2.25 in³ each

10 AWG conductors 2.50 in³ each

8 AWG conductors 3.00 in³ each

6 AWG conductors 5.00 in³ each

Devices (switches/recep.) 2× largest conductor each

Equipment grounding conductors All grounds = 1× largest

Internal cable clamps

Box Fill Results

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Fill %

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Used (in³)

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Box Vol (in³)

Item	Count	Vol Each	Total	NEC Ref

Per NEC 314.16(B). Fill must not exceed 100% of listed box volume.