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Box Fill Calculator

The math inspectors check before they approve your rough-in.

Every wire, device, clamp, and ground crammed into an electrical box consumes a code-defined volume, and overstuffed boxes are one of the most common inspection failures — overheated splices and damaged insulation start fires. This calculator applies NEC Table 314.16(B) conductor volumes to your wire count, adds the allowances for devices, clamps, and grounds, then checks the total against Table 314.16(A) box volumes to tell you pass or fail — and the smallest standard box that passes.

Why people use this

Electricians use it before rough-in inspection because box fill is the math inspectors actually check — and it fails more jobs than any other count. DIYers use it before buying boxes: the smallest box that passes is rarely the one on the shelf at eye level.

How it works

1. Count conductors of each gauge entering the box (each wire counts once; pigtails are free).
2. Add the number of devices (switches/receptacles), whether internal clamps are present, and how many ground wires terminate in the box.
3. The calculator sums required cubic inches: conductors × volume each, devices × 2, clamps × 1, grounds × 1 (all at the volume of the largest conductor present).
4. Pick a box — see PASS/FAIL with margin, plus the smallest standard box that fits your count.

Common questions

Do pigtails count toward box fill?

No. Under NEC 314.16(B)(1), conductors originating inside the box and terminating inside it — like pigtails — do not count. Only conductors that pass through or terminate from outside the box count, plus one allowance for all grounds combined, one for internal clamps, and two per yoke-mounted device.

Does a GFCI or dimmer count differently than a regular switch?

Same count — two volume allowances per yoke — but deep devices like GFCIs and dimmers physically crowd the box, so electricians often upsize beyond the minimum. A box that barely passes the fill math can still be miserable to close up; going one size deeper costs pennies.

What is the most common box fill violation?

Forgetting that grounds count. Four 12 AWG conductors in a standard 3×2×2½ box seem fine until the ground allowance, clamp allowance, and device allowances push the requirement past 12.5 cubic inches. This calculator counts every allowance the inspector counts.

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