Estimate box volume from conductor count, the allowance per conductor, and a two-volume device allowance.
| Volume (cm³) | – |
Use this calculator for a first check of a plastic box. Enter how many conductors count toward fill, the cm³ each size needs, and how many devices (switches or receptacles).
Volume (cm³) = (conductors × cm³ each) + (devices × 2 × cm³ each). The double device count is a common NEC-style allowance. There is no cm³ unit picker; values stay in cubic centimetres.
Confirm with the code you work under. Earths, clamps, and internal fittings may count differently.
Conductors: Number of conductors counted toward fill.
cm³ each: Volume allowance per conductor. 2.5 mm² is often around 8 cm³ in metric tables.
Devices: Number of devices. Each is counted as two conductor volumes here.
Volume (cm³): Estimated box fill in cubic centimetres.
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