Box Fill Calculator: NEC 314.16 Made Simple

Box fill is the total volume, in cubic inches, that NEC 314.16 assigns to everything inside an outlet or junction box. Add up the allowances for conductors, clamps, support fittings, devices and grounds, then compare the total to the box's marked volume: the box passes when its volume is equal to or greater than the total.

Pick the box (or enter its marked volume), then add the conductors, devices, clamps and grounds inside it. The calculator counts each item the way NEC 314.16(B) requires and tells you whether the box passes.

Volume per conductor (Table 314.16(B))

Conductor size	Volume per conductor
18 AWG	1.50 in³
16 AWG	1.75 in³
14 AWG	2.00 in³
12 AWG	2.25 in³
10 AWG	2.50 in³
8 AWG	3.00 in³
6 AWG	5.00 in³

The volume of each item is based on conductor size. When conductors of different sizes are in the box, each counting rule tells you which size to use.

How to calculate box fill in four steps

List every item in the box: insulated conductors, grounds, internal clamps, fixture studs or hickeys, and each device yoke.
Convert each item to conductor allowances using the counting rules below.
Multiply each allowance by the volume for its size from Table 314.16(B), then add them up.
Compare to the box volume. Use the volume marked on the box, plus any marked volume of a plaster ring, extension ring or raised cover attached to it.

What counts, and how much (314.16(B))

Conductors. Each conductor that originates outside the box and terminates or is spliced inside counts once. A conductor that passes through without a splice counts once. An unbroken loop or coil of 12 inches or more counts twice. Conductors that never leave the box (such as pigtails) do not count.
Internal cable clamps. One or more internal clamps together count as one conductor of the largest size in the box. External clamps and connectors don't count.
Support fittings. Each type of fixture stud or hickey counts as one conductor of the largest size in the box.
Devices. Each yoke or strap holding a switch, receptacle or other device counts as two conductors of the largest size connected to that device. A device wider than a single 2-inch device box counts double for each gang it needs.
Equipment grounding conductors. All of them together count as one conductor of the largest EGC in the box. An additional set of isolated-ground EGCs counts one more.

Up to four fixture wires smaller than 14 AWG, plus one EGC, coming from a luminaire canopy into the box do not count.

Worked example: a two-gang box

Two 12/2 NM cables come in (power in, and power out to the next outlet) and one 12/2 NM cable leaves for a light. The box holds a 20 A receptacle and a switch on separate yokes, and it has internal cable clamps. Because the switch and light are on the same 20 A circuit, the switch leg must be 12 AWG too; 14 AWG anywhere on a 20 A circuit violates 240.4(D).

Item	Counts as	Volume
4 insulated 12 AWG conductors (two cables)	4 × 12 AWG	9.00 in³
2 insulated 12 AWG conductors (light cable)	2 × 12 AWG	4.50 in³
All grounds together	1 × 12 AWG (largest EGC)	2.25 in³
Internal clamps	1 × 12 AWG (largest conductor)	2.25 in³
Receptacle yoke (12 AWG connected)	2 × 12 AWG	4.50 in³
Switch yoke (12 AWG connected)	2 × 12 AWG	4.50 in³
Total required		27.00 in³

A 2-gang box needs at least 27 in³. Many 2-gang nonmetallic boxes are marked in the 30–35 in³ range, which would pass; a shallow or old-work box may not. For nonmetallic boxes, the volume marked by the manufacturer inside the box is the value to use. Standard metal boxes have their volumes listed in Table 314.16(A).

Worked example: GFCI in a single-gang box

One 12/2 NM cable feeds a GFCI receptacle and a second 12/2 leaves from its load terminals to protect the next outlet. The box has internal clamps.

Item	Counts as	Volume
4 insulated 12 AWG conductors	4 × 12 AWG	9.00 in³
All grounds together	1 × 12 AWG	2.25 in³
Internal clamps	1 × 12 AWG	2.25 in³
GFCI yoke	2 × 12 AWG	4.50 in³
Total required		18.00 in³

A box marked 18 in³ passes with zero margin, which is legal but leaves a deep GFCI and four conductors fighting for space. Most installers go to a deeper box or a 2-gang box with a single-gang mud ring here.

Worked example: ceiling box for a luminaire

One 14/2 cable feeds the box and a second 14/2 continues to the next light. The box has internal clamps and a fixture stud. The luminaire's own leads are 16 AWG fixture wires plus a ground.

Item	Counts as	Volume
4 insulated 14 AWG conductors	4 × 14 AWG	8.00 in³
All grounds together	1 × 14 AWG	2.00 in³
Internal clamps	1 × 14 AWG	2.00 in³
Fixture stud	1 × 14 AWG	2.00 in³
Luminaire fixture wires (four or fewer, smaller than 14 AWG, plus EGC)	Not counted	0
Total required		14.00 in³

Allowances by box volume

How many conductor allowances fit in a box of a given marked volume, when every item is the same size:

Marked box volume	14 AWG allowances (2.00 in³)	12 AWG allowances (2.25 in³)
14 in³	7	6
18 in³	9	8
20 in³	10	8
22.5 in³	11	10
25 in³	12	11
30 in³	15	13
35 in³	17	15

Remember that a device uses two allowances, all grounds together use one and clamps use one. In an 18 in³ box with clamps and one device on 12 AWG, that leaves 8 − 1 − 1 − 2 = 4 allowances for insulated conductors.

Tips that avoid failed inspections

GFCI and AFCI devices are deep. The code counts them like any other yoke, but they take far more physical space. Upsizing the box saves time at trim-out.
Count every ground as one allowance, not one per wire. This is the most commonly over-counted item.
Don't forget the clamps. Plastic boxes with built-in cable clamps count one allowance for the clamps.
Use extension rings or mud rings with marked volume. Their marked volume adds to the box volume.
Watch mixed sizes. A device yoke counts at the largest conductor connected to that device, not the largest in the box, while clamps and fixture studs count at the largest conductor in the box and grounds at the largest equipment grounding conductor. Mixed sizes are common where a 15 A lighting circuit and a 20 A receptacle circuit share a box.
Count the 12-inch loop twice. A conductor looped through unbroken for a future device, 12 inches or longer, counts as two.

Estimating note

Box fill is decided at takeoff, not at rough-in. Deep boxes, 4-inch square boxes with rings and extension rings cost more and take longer to set than shallow single-gang boxes, so flag GFCI, dimmer, smart-switch and multi-cable locations on the plans and price the right box from the start.

Frequently asked questions

How many wires can go in a single-gang box?

It depends on the box volume. An 18 in³ single-gang box holds nine 14 AWG allowances (9 × 2.00 = 18.00 in³) or eight 12 AWG allowances (8 × 2.25 = 18.00 in³). Remember that a device counts as two and all grounds together count as one.

Do wire nuts count in box fill?

No. Wire connectors, pigtails that stay inside the box, and grounding clips are not counted.

Does a GFCI count differently from a regular receptacle?

No. Any single device on one yoke counts as two conductors of the largest size connected to it.

Does a wire that passes straight through count?

Yes, once. A conductor that runs through the box without a splice or termination counts as one allowance, unless it is an unbroken loop of 12 inches or more, which counts as two.

How do I find the volume of a plastic box?

It is stamped or molded inside the box by the manufacturer. Use that marked value.

Estimating rough-in for a whole house? SparkQuote counts devices and boxes from the plans and applies your labor units to every one. See the electrical estimator.

Last updated: October 6, 2026. Based on the 2023 NEC 314.16. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.

Box Fill Calculator