How do you calculate box fill?

Add the volume allowance for every item in the box: one allowance per current-carrying conductor entering and terminating, one total for all internal clamps, two allowances per device yoke, and one total for all equipment grounding conductors. Sizes use the largest conductor present for clamps, devices, grounds and fittings. The total must not exceed the box's marked volume.

What is the volume allowance for 12 AWG?

A 12 AWG conductor is allowed 2.25 cubic inches. 14 AWG is 2.0, 10 AWG is 2.5, 8 AWG is 3.0 and 6 AWG is 5.0 cubic inches.

How many 12 AWG wires fit in an 18 cubic inch box?

18 / 2.25 gives 8 conductors if nothing else is in the box. Add a receptacle (two allowances, 4.5 in³) and a ground (one allowance, 2.25 in³) and only 4 current-carrying conductors remain.

Do ground wires count in box fill?

Yes, but all equipment grounding conductors together count as a single volume allowance based on the largest one. A separate isolated grounding system adds one more allowance.

Do wire nuts and pigtails count toward box fill?

No. Wire connectors are not counted, and a conductor that both begins and ends inside the box, such as a pigtail, is not counted either. Only conductors that enter or leave the box count.

How is a switch or receptacle counted?

Each yoke or strap holding one or more devices counts as two volume allowances based on the largest conductor connected to that device.

Box Fill Calculator

Count conductors, clamps, devices, grounds and support fittings, and see whether the required cubic inches fit the box you are installing.

Quick answer

Box fill is the sum of volume allowances: one per conductor entering the box, one total for all clamps, two per device yoke, one total for all grounds, each sized on the largest conductor present. 12 AWG is 2.25 in³, so six 12 AWG conductors, clamps, one receptacle and a ground need 13.5 + 2.25 + 4.5 + 2.25 = 22.5 in³ — too much for an 18 in³ box.

What is in the box

Conductor sizeConductors entering the boxLargest conductor in the boxDevices (yokes)Equipment grounding conductorsSupport fittings (studs, hickeys)Box volume (in³)Pick a common box

Internal cable clamps present (one allowance total)Separate isolated grounding system (one extra allowance)

required = (n × Vc) + clamps + (devices × 2 × Vmax) + grounds + fittings

Result

Required volume

22.50in³

Box volume

18.00in³

Fill

125%

Spare volume

-4.50in³

Over the box volume by 4.50 in³ — use a deeper box, an extension ring or a larger enclosure.

Item	Volume (in³)
6 × 12 AWG conductors	13.50
Internal clamps	2.25
Devices (1 × 2 allowances)	4.50
Grounding conductors	2.25
Support fittings	0.00
Total required	22.50

Breakdown of the volume allowances counted.

Volume allowance per conductor

Each conductor entering the box gets one allowance at its own size; clamps, devices, grounds and fittings use the largest conductor in the box.

Conductor size	Allowance (in³)	Device yoke (2×)	Max in 18 in³ box	Max in 30.3 in³ box
18 AWG	1.50	3.00	12	20
16 AWG	1.75	3.50	10	17
14 AWG	2.00	4.00	9	15
12 AWG	2.25	4.50	8	13
10 AWG	2.50	5.00	7	12
8 AWG	3.00	6.00	6	10
6 AWG	5.00	10.00	3	6

Source & method: Counts shown assume nothing else in the box. See the methodology page for how each figure is derived.

Common box volumes and realistic capacity

Capacity below assumes 12 AWG conductors, internal clamps, one device and one ground.

Box	Marked volume (in³)	Conductors only	With clamps, 1 device, ground
3" × 2" × 2" device box	10.0	4	0
3" × 2" × 2-1/4" device box	10.5	4	0
3" × 2" × 2-1/2" device box	12.5	5	1
3" × 2" × 2-3/4" device box	14.0	6	2
3" × 2" × 3-1/2" device box	18.0	8	4
4" square × 1-1/4" deep	18.0	8	4
4" square × 1-1/2" deep	21.0	9	5
4" square × 2-1/8" deep	30.3	13	9
4" octagon × 1-1/2" deep	15.5	6	2
4-11/16" square × 1-1/2" deep	29.5	13	9
4-11/16" square × 2-1/8" deep	42.0	18	14
Nonmetallic single-gang (18 in³)	18.0	8	4
Nonmetallic two-gang (32 in³)	32.0	14	10

Frequently asked

How do you calculate box fill?: Add the volume allowance for every item in the box: one allowance per current-carrying conductor entering and terminating, one total for all internal clamps, two allowances per device yoke, and one total for all equipment grounding conductors. Sizes use the largest conductor present for clamps, devices, grounds and fittings. The total must not exceed the box's marked volume.
What is the volume allowance for 12 AWG?: A 12 AWG conductor is allowed 2.25 cubic inches. 14 AWG is 2.0, 10 AWG is 2.5, 8 AWG is 3.0 and 6 AWG is 5.0 cubic inches.
How many 12 AWG wires fit in an 18 cubic inch box?: 18 / 2.25 gives 8 conductors if nothing else is in the box. Add a receptacle (two allowances, 4.5 in³) and a ground (one allowance, 2.25 in³) and only 4 current-carrying conductors remain.
Do ground wires count in box fill?: Yes, but all equipment grounding conductors together count as a single volume allowance based on the largest one. A separate isolated grounding system adds one more allowance.
Do wire nuts and pigtails count toward box fill?: No. Wire connectors are not counted, and a conductor that both begins and ends inside the box, such as a pigtail, is not counted either. Only conductors that enter or leave the box count.
How is a switch or receptacle counted?: Each yoke or strap holding one or more devices counts as two volume allowances based on the largest conductor connected to that device.

Gear for rough-in and device work

Tools for making up boxes and terminating devices.

Affiliate

Links go to Amazon search results, not a single listing — confirm ratings, insulation type and listing details before buying, and make sure the part is approved for your application.

What is in the box

Result

Volume allowance per conductor

Common box volumes and realistic capacity

Frequently asked

Gear for rough-in and device work

Gear for rough-in and device work

Related charts & calculators