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Box Fill Calculator

Use this box fill calculator to find the correct size of electrical utility box to fit the conducting wires, grounding wires, and devices or equipment you would need to install and have it pass the National Electrical Code®.

Hots and neutrals. Count each one.
All of them together count as one.
Each yoke counts as two conductors.
All clamps together count as one.
Marked on the box. A single gang plastic box is usually 18–22 in³.
Clear
Volume required18 in³8 conductor equivalents at 2.25 in³ each
Conductor equivalents8
Current-carrying conductors4one each
Grounding conductors2 counting as 1all grounds together count once, at the largest size
Devices1 counting as 2each yoke counts as two conductors
Cable clamps1 counting as 1all internal clamps together count once
Box volume18 in³
Resultcompliant0 in³ spare
Fill100%
Spare capacity0 more conductors
Smallest standard box that fits18 in³rounded to the next half cubic inch

The formula

NEC 314.16: each conductor counts as its own volume allowance

Not everything counts once

NEC 314.16 has three rules that trip people up. All the equipment grounding conductors together count as a single conductor, at the volume of the largest. All internal cable clamps together also count as one. And every device yoke — a switch or a receptacle — counts as two conductors of the largest size connected to it.

Wires that pass through the box without splicing still count, and a wire entering and leaving counts once. Pigtails entirely inside the box do not count at all.

Overfilling a box is not a paperwork problem. Crammed conductors are hard to fold back without nicking insulation, and the heat has nowhere to go.