Electrical Box Fill Calculator

Count the wires, devices and fittings in a box and check it against the box’s cubic-inch rating.

Count each hot, neutral and traveler that enters; pigtails that stay in the box don’t count.
Stamped inside the box; a common single-gang plastic box is 18 to 22.5.
Fractions and decimals both work.
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Required volume
Volume allowances
Per-conductor volume
Box volume
Room left
Work

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NEC Box Fill Rules (314.16(B))

Wire sizeVolume per conductor
18 AWG1.50 in³
16 AWG1.75 in³
14 AWG2.00 in³
12 AWG2.25 in³
10 AWG2.50 in³
8 AWG3.00 in³
6 AWG5.00 in³

Each insulated conductor entering the box counts once. Each device (switch or receptacle) counts twice, based on the largest wire connected to it. All internal cable clamps together count once; all equipment grounding conductors together count once (plus one more for an isolated ground set); each fixture stud or hickey counts once.

Worked Example

Tips

This page uses one wire size for everything (the largest), which is conservative when sizes are mixed. Check the local code in force (NEC 2023 in many areas) and have work inspected.