Box Fill Calculator
Check whether the conductors, devices, clamps and grounds going into an outlet or junction box fit its volume under NEC 314.16 — for electricians and inspectors sizing a box before it is closed up.
Last checked against the code
Box Fill Calculator
NEC (US)How this was derived
- Conductors314.16(B)(1)
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- Internal clamps314.16(B)(2)
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- Studs and hickeys314.16(B)(3)
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- Devices314.16(B)(4) — double per yoke
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- Equipment grounds314.16(B)(5) — all of them, once
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- Total required
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- Box volume314.16(A)
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How this is calculated
Box fill is an accounting problem, not a percentage one. Every box has a free space in cubic inches, and NEC 314.16 assigns each thing inside it a share of that space — a volume allowance. Add the allowances up, compare against the box, done. There is no 40% limit here as there is in conduit: a box may be filled to 100% of its listed volume, because the allowances already build in the room a conductor needs to be folded back.
The allowance for one conductor depends only on its size, and Table 314.16(B) has seven rows: 1.50 in³ for 18 AWG up to 5.00 in³ for 6 AWG. Every term in the calculation is some whole-number multiple of one of those seven numbers.
Conductors count individually under (B)(1). Anything entering, leaving or passing through gets one allowance. A conductor that originates and terminates inside the box — a pigtail — gets none, because it never crosses the boundary.
Then two rules that pull in opposite directions, and getting them the wrong way round is the classic box fill mistake.
Under (B)(5), all the equipment grounding conductors together count once, based on the largest one present. Four grounds is one allowance, not four. They are bare or thinly insulated and dress tightly into the back of the box, so they genuinely displace far less than their number suggests.
Under (B)(4), each device yoke counts double. One duplex receptacle is two allowances. The second one stands in for the device body itself, which occupies real depth that no conductor count would capture.
Two smaller terms finish it. Internal cable clamps get one allowance between them however many there are, under (B)(2) — clamps on the outside of the box do not count at all. Fixture studs and hickeys get one allowance each, counted individually, under (B)(3).
All four of those non-conductor terms are keyed to the largest conductorinvolved, not to the number of things. That is why adding a single 10 AWG conductor to a box full of 14 AWG raises the cost of the device, the clamps and the grounds at once, and can fail a box that a straight conductor count says is fine.
Worked example
One full calculation with real numbers, so you can follow along and check the tool by hand.
A single-gang device box — 3 × 2 × 3½, which Table 314.16(A) lists at18 in³. Into it come two 12/2-with-ground cables, and on the front goes one duplex receptacle. No internal clamps, no fixture stud.
First, count what actually crosses the box wall. Two 12/2 cables carry two hots and two neutrals: four 12 AWG conductors. The two grounds are conductors as well, but they are handled by their own rule and must not be added here — this is the step where a calculation most often goes wrong before any arithmetic has happened.
Conductors. 12 AWG is 2.25 in³ each, so 4 × 2.25 =9.00 in³.
Grounds. Both grounds together are a single allowance at the largest ground size, which is 12 AWG: 1 × 2.25 = 2.25 in³. Had there been four cables and four grounds, this line would still read 2.25.
Device. One yoke, doubled: 1 × 2 × 2.25 =4.50 in³. The receptacle has two outlets and one strap, so it is one device, and the doubling is what accounts for its body.
The total. 9.00 + 2.25 + 4.50 = 15.75 in³ required against 18 in³ available. It fits, with2.25 in³ to spare — exactly one more 12 AWG conductor's worth, and 87.5% of the box used.
That spare inch is worth reading carefully, because it is the whole margin. Add internal clamps and the box takes another 2.25 in³ and lands on exactly 18.00 — legal, and with nothing left. Add a third cable instead and it is over. This is why a 3 × 2 × 2¼ box at 10.5 in³, which looks like a perfectly ordinary single-gang box on the shelf, cannot legally take this very common arrangement at all.
Visual comparison
| Category | Bar | Volume (in³) | Basis |
|---|---|---|---|
| Conductors | 9 | 4 × 2.25 | |
| Device | 4.5 | 1 × 2 × 2.25 — double per yoke | |
| Grounds | 2.25 | 1 × 2.25 — all grounds together, counted once | |
| Clamps and studs | 0 | None in this box | |
| Total required | 15.75 | 87.5% of the box | |
| Box capacity | 18 | 3 × 2 × 3½ device box |
Source: NEC 2023 314.16(B)
| Box | Bar | 12 AWG conductors | Volume |
|---|---|---|---|
| 3 × 2 × 1½ | 3 | 7.5 in³ | |
| 3 × 2 × 2½ | 5 | 12.5 in³ | |
| 4″ | 6 | 15.5 in³ | |
| 3 × 2 × 3½ | 8 | 18 in³ | |
| 4″ square × 1½ | 9 | 21 in³ | |
| 4″ square × 2⅛ | 13 | 30.3 in³ | |
| 4 11/16″ square × 2⅛ | 18 | 42 in³ |
Source: Derived from NEC 2023 Table 314.16(A) volumes ÷ the 314.16(B) allowance
Reference tables
Two tables. The first is Table 314.16(B) in full — it is only seven rows, it is the piece worth committing to memory, and the third column does the doubling for you because the device term is where mental arithmetic slips.
The second lists the standard metal boxes with their volumes. Use it instead of the calculator when you are working backwards: you know roughly what the fill will be and want to know which box to put on the van. The two conductor columns are deliberately labelled bare — they are the volume divided by the allowance, with nothing else in the box, and a real box with a device and grounds in it will take three or four fewer. Note also what is not in this table: every nonmetallic box on the market, none of which Table 314.16(A) covers.
| Conductor | Per conductorin³ | Per device yokein³ |
|---|---|---|
| 18 AWG | 1.5 | 3 |
| 16 AWG | 1.75 | 3.5 |
| 14 AWG | 2 | 4 |
| 12 AWG | 2.25 | 4.5 |
| 10 AWG | 2.5 | 5 |
| 8 AWG | 3 | 6 |
| 6 AWG | 5 | 10 |
| Box | Volumein³ | Max 14 AWGbare | Max 12 AWGbare | In practice |
|---|---|---|---|---|
| 3 × 2 × 1½ device box | 7.5 | 3 | 3 | Shallow. Rarely enough for anything but a single switch loop. |
| 3 × 2 × 2 device box | 10 | 5 | 4 | |
| 3 × 2 × 2¼ device box | 10.5 | 5 | 4 | |
| 3 × 2 × 2½ device box | 12.5 | 6 | 5 | |
| 3 × 2 × 2¾ device box | 14 | 7 | 6 | |
| 3 × 2 × 3½ device box | 18 | 9 | 8 | The deep single-gang box. The usual answer when a shallower one fails. |
| 4 × 2⅛ × 1½ device box | 10.3 | 5 | 4 | |
| 4 × 2⅛ × 1⅞ device box | 13 | 6 | 5 | |
| 4 × 2⅛ × 2⅛ device box | 14.5 | 7 | 6 | |
| 4″ round/octagonal × 1¼ | 12.5 | 6 | 5 | |
| 4″ round/octagonal × 1½ | 15.5 | 7 | 6 | The standard ceiling box for a luminaire outlet. |
| 4″ round/octagonal × 2⅛ | 21.5 | 10 | 9 | |
| 4″ square × 1¼ | 18 | 9 | 8 | |
| 4″ square × 1½ | 21 | 10 | 9 | |
| 4″ square × 2⅛ | 30.3 | 15 | 13 | The workhorse junction box. Takes a plaster ring to become a device box. |
| 4 11/16″ square × 1¼ | 25.5 | 12 | 11 | |
| 4 11/16″ square × 1½ | 29.5 | 14 | 13 | |
| 4 11/16″ square × 2⅛ | 42 | 21 | 18 | The largest box in the table. Past this, 314.28 takes over. |
| 3¾ × 2 × 2½ masonry box | 14 | 7 | 6 | |
| 3¾ × 2 × 3½ masonry box | 21 | 10 | 9 |
Notes and exceptions
A device wider than one gang counts double for every gang. The ordinary case is a single yoke in a single 2 inch gang, which is what this calculator models. A device whose strap physically spans two gangs — some large dimmers, timers and smart controls — takes two allowances per gang occupied, so four in total. Add the extra by hand, or enter an extra yoke to stand in for it.
A looped conductor can count twice. Where a conductor runs through a box unbroken but is left long enough to be twice the minimum free length — that is, a loop of at least 12 inches beyond what 300.14 requires — 314.16(B)(1) counts it as two conductors rather than one. A deliberately generous service loop in a junction box is the case to watch.
Isolated grounds get a second allowance. Where an additional set of equipment grounding conductors is present under 250.146(D) — the isolated-ground receptacle arrangement used to keep noise off sensitive equipment — that second set gets its own single allowance on top of the first. Two allowances total, not one and not one per wire.
Fill is not the only rule about what goes in a box. 300.14 requires at least 150 mm (6 in.) of free conductor at each outlet, with at least 75 mm (3 in.) outside the opening. A box can pass 314.16 comfortably and still be unworkable because the conductors were cut short, and no volume calculation will tell you that.
Common mistakes
Counting the equipment grounds one by one
All of them together are a single allowance, however many there are. A box with four 12 AWG grounds under one wirenut gets 2.25 in³, not 9.00. Counting them individually is the most common error in box fill and it fails boxes that pass comfortably.
NEC 2023 314.16(B)(5)
Counting a device once instead of twice
Each yoke is double the allowance for the largest conductor connected to it. One duplex receptacle on 12 AWG is 4.50 in³, not 2.25. The confusion runs the other way too — a duplex is one yoke, not two devices, so it is 2 × 2.25 rather than 4 × 2.25.
NEC 2023 314.16(B)(4)
Mixing new-work and old-work box volumes
Nominally identical plastic boxes differ by several cubic inches. A new-work single-gang is often 18 to 22.5 in³; the old-work box that replaces it in the same wall is frequently 14 to 16, because the wings and clamp mechanism eat the depth. Read the marking on the box in front of you.
NEC 2023 314.16(A)(2)
Entering the grounds in the conductor count as well
The two rules are exclusive. If the grounds are included in the conductor rows and the grounds allowance is also applied, every ground is paid for twice — a four-cable box comes out around 9 in³ heavier than it is, which will condemn a perfectly legal box.
NEC 2023 314.16(B)(1), (B)(5)
Using the table volume for a box with a plaster ring
A plaster ring, extension ring or raised cover adds its own marked volume to the box. That is often 2 to 6 in³ of genuine extra capacity being left on the table — and it is exactly the margin that decides whether a crowded 4 inch square passes.
NEC 2023 314.16(A)(1)
This tool provides planning estimates. Always verify final values against your local code and a licensed electrician.
Frequently asked questions
How many 12 AWG wires fit in an 18 cubic inch box?
Eight, if nothing else is in the box: 18 ÷ 2.25 = 8 exactly. In practice almost nothing else is true, because a real box also holds a device and some grounds. Add one duplex receptacle and the grounds and only four 12 AWG conductors will fit — the device takes 4.50 in³ and the grounds 2.25 in³, leaving 11.25 in³, which is five conductors’ worth with the fifth not quite paid for.
Do ground wires count toward box fill?
Yes, but all of them together count as one conductor, based on the largest ground present. Two grounds, four grounds and eight grounds all add the same single allowance — 2.25 in³ for 12 AWG. This is the opposite of conduit fill, where every conductor including the equipment grounding conductor takes its own space, and the difference between the two rules catches people who move between the two calculations.
Does a duplex receptacle count as one device or two?
One yoke, counted at double the volume allowance. A duplex receptacle has two outlets but a single strap, so it is one device under 314.16(B)(4) and contributes two allowances — 4.50 in³ on 12 AWG. A device physically wider than one 2 inch gang is the exception: it takes double for each gang it occupies, so a large dimmer spanning two gangs is four allowances.
What is the volume of a standard plastic old-work box?
There is no standard, which is the point of the marking requirement. Common single-gang old-work boxes run from about 14 to 18 in³ depending on manufacturer, against 18 to 22.5 for a new-work box of the same nominal size. Table 314.16(A) does not cover nonmetallic boxes at all, so the marked figure moulded into the back of the box is the only correct number to use.
Do pigtails and wirenuts count toward box fill?
Neither does. A conductor that both originates and terminates inside the box — a pigtail from a wirenut to a device — is not counted at all, because it never enters or leaves. Wirenuts, tape and other splicing materials have no allowance either. Only conductors crossing the boundary of the box count, plus the fittings and devices that 314.16(B) lists explicitly.
Does this apply to conductors larger than 6 AWG?
No. Table 314.16(B) stops at 6 AWG, and a box containing 4 AWG or larger is sized under 314.28 instead. That section works from raceway entry geometry rather than volume: a straight pull needs eight times the trade diameter of the largest raceway, an angle pull six times, and the calculation is about being able to physically pull the conductors without damaging them rather than about free space.