Box Fill Calculator

By Giorgi Gaprindashvili · Updated Jul 2026 · Based on NEC 314.16(B)

Size an electrical box to NEC 314.16(B). Add conductors by gauge, devices, clamps, and grounds for a required volume and a recommended box.

How to Use

Pick the NEC edition - choose the code edition your jurisdiction enforces. It changes how grounding conductors are counted.
Add conductors by size - add a row for each wire gauge entering the box and set the count. Each conductor is counted at its own size per 314.16(B)(1).
Add devices and fittings - enter switches/receptacles (each counts double), internal clamps, and fixture studs or hickeys.
Enter grounds - set the number and largest size of equipment grounding conductors. Wire nuts and pigtails are not counted.
Read the analysis - the panel shows required volume, the minimum box that fits, and a pass/fail if you select your box size.

Quick Answer

Box fill is the total volume your conductors, devices, clamps, and grounds require inside an electrical box under NEC 314.16(B). Count each conductor at its own size from Table 314.16(B), add a double allowance for each device, a single allowance for clamps and for each fixture stud or hickey, and a grounding allowance, then choose a box from Table 314.16(A) with at least that volume. A common single-gang device box holds 18 cubic inches; this tool computes your exact requirement and recommends the smallest box that passes.

What Counts Toward Box Fill

Every allowance uses the largest conductor in the box, except conductors themselves, which are each counted at their own size. Internal cable clamps add one allowance total; wire connectors and pigtails that stay in the box add none.

NEC Table 314.16(B) Volume Allowances

Conductor Size (AWG)	Volume Allowance (in³)	Volume (cm³)
18 AWG	1.50	24.6
16 AWG	1.75	28.7
14 AWG	2.00	32.8
12 AWG	2.25	36.9
10 AWG	2.50	41.0
8 AWG	3.00	49.2
6 AWG	5.00	81.9

Allowances apply to 18 AWG through 6 AWG. Conductors 4 AWG and larger that enter a box must be sized by NEC 314.28, not 314.16.

NEC Table 314.16(A) Common Box Capacities

Box (trade size)	Volume (in³)	Common Name	Max 12 AWG
3 × 2 × 2 device	10.0	single-gang	4
3 × 2 × 2½ device	12.5	single-gang deep	5
3 × 2 × 3½ device	18.0	single-gang deepest	8
4 × 4 × 1¼ square	18.0	4-square	8
4 × 4 × 1½ square	21.0	1900 box	9
4 × 4 × 2⅛ square	30.3	4-square deep	13
4-11/16 × 1½ square	29.5	4-11/16	13
4-11/16 × 2⅛ square	42.0	4-11/16 deep	18

A box marked with a volume larger than the table value may be used at its marked capacity. The "Max 12 AWG" column counts conductors only, before clamps, devices, or grounds.

Worked Examples

Each example is computed with the same per-gauge method this calculator uses. A 14-2 cable means two insulated conductors plus a bare ground; a 12-2 means two 12 AWG conductors plus a ground.

Scenario	What Is Counted	Required Volume	Minimum Box
Single switch, one 14-2 cable, internal clamp	2 × #14 + clamp + 1 device + 1 ground	12.0 in³	3 × 2 × 2½ (12.5 in³)
Receptacle, two 12-2 cables, internal clamps	4 × #12 + clamp + 1 device + 2 grounds	18.0 in³	4-square or 1900 box
Junction box, three 12-3 cables spliced	9 × #12 + clamp + 3 grounds	24.75 in³	4-11/16 × 1½ (29.5 in³)

The receptacle case is the textbook NEC example: four conductors, a clamp, a device, and two grounds all reduce to eight 12 AWG allowances, which is 18 cubic inches.

How the Calculation Works

All allowances come from NEC Table 314.16(B). Conductors use their own size; every other item uses the largest conductor in the box, except grounds, which use the largest grounding conductor.

Conductor fill = sum of (count × allowance) for each gauge, per 314.16(B)(1)
Clamp fill = 1 × largest-conductor allowance if any internal clamp is present
Support fitting = 1 × largest-conductor allowance for each fixture stud or hickey
Device fill = 2 × largest-conductor allowance for each device yoke
Ground fill 2020 = 1 allowance for up to 4 grounds, then +¼ allowance per ground over 4
Ground fill 2017 = 1 allowance for all grounds, plus 1 more for an isolated set
Required volume = conductor + clamp + fitting + device + ground fill

The minimum box is the smallest entry in Table 314.16(A) whose volume is at least the required volume. All volumes are in cubic inches.

Code References

Conductor fill counts each conductor that enters and terminates, is spliced, or passes through the box once, at its own size from Table 314.16(B). A conductor coiled at least twice the free-conductor length counts twice.NEC (NFPA 70) §314.16(B)(1)
One or more internal cable clamps count as a single allowance based on the largest conductor in the box. Connectors whose clamping mechanism is outside the box are not counted.NEC (NFPA 70) §314.16(B)(2)
Each fixture stud or hickey counts as one allowance based on the largest conductor in the box.NEC (NFPA 70) §314.16(B)(3)
Each device yoke or strap counts as a double allowance based on the largest conductor connected to that device.NEC (NFPA 70) §314.16(B)(4)
In the 2020 and later editions, up to four grounding conductors count as one allowance and each additional ground adds one-quarter allowance, based on the largest grounding conductor. In the 2017 edition, all grounds count as one allowance, with an additional allowance for an isolated set per 250.146(D).NEC (NFPA 70) §314.16(B)(5)
Box volumes are taken from Table 314.16(A) or the volume marked on the box. Wire connectors and conductors that do not leave the box are not assigned a volume allowance.NEC (NFPA 70) §314.16(A)

Next Steps

Box fill is one check in a rough-in. These tools cover what usually comes next:

Check conduit fillSize the raceway feeding the box for the same conductors.
Size the conductorsConfirm the wire gauge by load and ampacity before counting fill.

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Box fill is governed by NEC Article 314. Browse the full electrical calculator collection.

Frequently Asked Questions

How do you calculate box fill?

Add up the volume allowances for everything in the box using NEC Table 314.16(B). Count each conductor at its own size, add two allowances for each device yoke, one allowance for internal clamps, one for each fixture stud or hickey, and a grounding allowance. Multiply each allowance by its cubic-inch value, total them, and pick a box from Table 314.16(A) with at least that volume. The calculator above does all of this and recommends the smallest box that passes.

Do neutrals count for box fill?

Yes. A neutral is a current-carrying conductor, so it is counted once at its own size like any hot conductor. In a 12-2 cable, both the black and white conductors count as 12 AWG allowances.

Do grounds count in box fill?

Yes, but how they count depends on the NEC edition. Under the 2020 and later codes, up to four grounding conductors count as a single allowance and each ground beyond the fourth adds a quarter allowance. Under the 2017 code, all grounds together count as one allowance. Both use the largest grounding conductor in the box. Select your edition in the calculator to apply the correct rule.

Do wire nuts count in box fill?

No. Standard wire connectors and push-in connectors have no box fill allowance under NEC 314.16. Pigtails and other short conductors that do not leave the box are also not counted.

How many wires fit in a 1900 box?

A 1900 box, the trade name for a 4 × 4 × 1½ inch square box, has a volume of 21 cubic inches. That holds up to nine 12 AWG conductors or ten 14 AWG conductors counting conductors only. Once you add clamps, a device, and grounds, the usable conductor count drops, which is why the calculator totals every item.

How many wires can I put in a 4-square box?

A standard 4-square box, 4 × 4 × 1¼ inches, holds 18 cubic inches, which is eight 12 AWG or nine 14 AWG conductors before any clamps or devices. The deeper 4 × 4 × 2⅛ version holds 30.3 cubic inches. Use the calculator to subtract clamp, device, and ground allowances from that capacity.

What is the difference between box fill and box volume?

Box volume is the physical capacity of the box, either marked on the box or read from Table 314.16(A). Box fill is the total volume your conductors, devices, clamps, and grounds require per Table 314.16(B). The installation is compliant when the required box fill is less than or equal to the box volume.

Updated Jul 2026 · See our Methodology

This calculator follows NEC (NFPA 70) Article 314.16(B) volume allowances and Table 314.16(A) box capacities for 18 AWG through 6 AWG conductors. Conductors 4 AWG and larger use 314.28. Local codes, amendments, and the adopted NEC edition vary by jurisdiction. Use for planning estimates and verify with your AHJ or a licensed electrician. See our Data Sources and Methodology.