Box fill calculator

Count what goes in the box. The total is the volume it needs under 314.16(B), and the list shows the standard metal boxes that take it.

Conductors that start or end in the box, or pass through it

14 AWG

12 AWG

10 AWG

8 AWG

6 AWG

A wire that passes through unbroken counts once. A pigtail that stays in the box doesn't count.

Devices (switches, receptacles)

Wire size on the devices

Fixture studs or hickeys

The box has internal cable clamps Equipment grounding conductors enter the box

Check against a box

18 cu in Fits

4 x 2-1/8 round/octagonal holds 21.5 cu in, 3.50 to spare.

4 × 12 AWG conductors9
Internal cable clamps (one or more)2.25
Device yoke (12 AWG)4.5
All equipment grounds (12 AWG)2.25

Standard boxes that take it

4 x 1-1/4 square18 cu in
3 x 2 x 3-1/2 device18 cu in
FD single cover/gang (2-3/8 min depth)18 cu in
FS multiple cover/gang (1-3/4 min depth)18 cu in
4 x 1-1/2 square21 cu in
3-3/4 x 2 x 3-1/2 masonry box/gang21 cu in

How box fill counts

Each conductor from outside the box: one allowance for its size (6 AWG 5 cu in, 8 AWG 3 cu in, 10 AWG 2.5 cu in, 12 AWG 2.25 cu in, 14 AWG 2 cu in, 16 AWG 1.75 cu in, 18 AWG 1.5 cu in).
All internal cable clamps together: one allowance for the largest conductor.
Each fixture stud or hickey: one allowance for the largest conductor.
Each device yoke: two allowances for the largest conductor on it.
All equipment grounds together: one allowance for the largest ground. Isolated grounds add one more.

Plaster rings, extension rings and domed covers add their marked volume. Nonmetallic boxes and boxes not in the table use the volume marked inside them.