Box fill (314.16)

A planning aid. It uses the NEC 2023 rules; check the edition your jurisdiction has adopted and its amendments, and the inspector has the last word.

How to use it

Choose the box: one of the metal boxes of Table 314.16(A) (round and octagonal, square, device, masonry, FS/FD), or Plastic or other box and type the volume marked inside it. Add a marked extension or plaster ring if there is one.
Count the conductors by size with the +/− buttons: each one that comes in and is spliced or lands on a device. Pass-through conductors and unbroken loops of 12 in or more are under Unbroken conductors.
Tick internal cable clamps, and count fixture studs or hickeys, devices (switches and receptacles, two allowances per yoke) and wide devices (two per gang), the equipment grounding conductors (one allowance for all of them) and any isolated-ground set.
Pigtails and up to four small fixture wires under a canopy are listed but not counted.

The bar at the top shows the cubic inches needed against the box, coloured by what uses them, and stays in view as you scroll. Below: the count line by line with the section for each, and the metal boxes that would fit (tap one to switch to it).

How it is worked out

Volumes per conductor are from Table 314.16(B)(1): 18 AWG 1.5 in³, 16 AWG 1.75, 14 AWG 2.0, 12 AWG 2.25, 10 AWG 2.5, 8 AWG 3.0 and 6 AWG 5.0. Clamps, support fittings and grounds use the largest conductor in the box; a device uses the largest conductor connected to it (you can choose). Box volumes are Table 314.16(A). Conductors larger than 6 AWG fall under 314.28 (pull and junction boxes) instead.

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