Box Fill Calculator (NEC 314.16)
Enter what's going into an electrical box and see the required cubic inches, line by line, against the box's volume. This box fill calculator covers the stated 2017, 2020 and 2023 NEC profiles for 18–6 AWG wire—not 2026, conduit bodies, or barrier apportionment.
Calculate your box fill
Example result — 2023: 16.00 in³ required · 18.00 in³ available · Within the box's volume, 2.00 in³ to spare.
| Item | Rule | in³ |
|---|---|---|
| 4 × 14 AWG wires | 314.16(B)(1) | 8.00 |
| Internal clamps: 1 allowance at 14 AWG | 314.16(B)(2) | 2.00 |
| Switch: 2 allowances at 14 AWG | 314.16(B)(4) | 4.00 |
| 2 entering grounds: 1 allowance at 14 AWG | 314.16(B)(5) | 2.00 |
| Required | 16.00 |
Smallest whole-box reference volume listed here: 18.0 in³ (3 × 2 × 3½ in. device or 4 × 1¼ in. square). This is a volume comparison, not confirmation that either box suits the device or installation.
Room for 1 more independent 14 AWG wire, or 0 more 12 AWG wires, or 0 more 10 AWG wires. These are alternatives, not a combined allowance; a new cable, ground, device or changed connection needs a new calculation.
This compares volume allowances only. It doesn't confirm box depth, device fit, support or approval of the installation.
Do it by hand
Every allowance in this worksheet comes from one number: the allowance for a wire size. Multiply counts by allowances, add the lines, and compare the total with the box.
| Line | Count | × in³ each | = in³ |
|---|---|---|---|
| Wires, one line per size | ___ | ___ | ___ |
| Unbroken loops of 12 in. or more (count each twice) | ___ | ___ | ___ |
| Internal clamps (1 total, at largest wire in the box) | ___ | ___ | ___ |
| Fixture stud (1, at largest wire in the box) | ___ | ___ | ___ |
| Hickey (1, at largest wire in the box) | ___ | ___ | ___ |
| Each device (2, at largest wire connected to it; 2 per gang if it needs more than one) | ___ | ___ | ___ |
| Grounds (2020/2023: 1 for up to four entering, + ¼ for each extra, at the largest entering ground; 2017: 1 at the largest ground present, plus an allowance for an additional isolated set) | ___ | ___ | ___ |
| Terminal block assemblies (1 each, at largest wire on it; 2023) | ___ | ___ | ___ |
| Required volume | ___ | ||
| Box volume + marked added volume | ___ |
If required is less than or equal to available, the box has enough volume for these allowances. An exact tie is enough. NEC 314.16, volume comparison.
Volume allowance per wire size
| Wire size (AWG) | in³ per allowance |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
These are volumes, not wire areas or ampacities, so don't mix them up with conduit fill. Sources: 2008 NEC text of 314.16, reprinted with NFPA permission (14–10 AWG values confirmed for the 2026 NEC by Minnesota DLI's inspection checklist, p. 6, item 046). The table stops at 6 AWG. Boxes with 4 AWG or larger wire also have to meet the pull- and junction-box sizing rules in 314.28, which this calculator doesn't cover.
What counts toward box fill
| In the box | Counts as | Sized by |
|---|---|---|
| Each insulated wire that enters and is spliced, terminated or passes through | 1 each | Its own size |
| An unbroken loop or coil at least twice the 6 in. free-length minimum (so 12 in. or more) | 2 | Its own size |
| All internal cable clamps together | 1 | Largest wire in the box |
| Fixture studs | 1 for the type | Largest wire in the box |
| Hickeys | 1 for the type | Largest wire in the box |
| Each device mounting strap (yoke) | 2 | Largest wire connected to that device |
| A device wider than a single 2 in. device box | 2 for each gang it needs to mount | Largest wire connected to it |
| Grounds entering, one to four (2020/2023) | 1 total | Largest entering ground or bonding jumper |
| Each entering ground beyond four (2020/2023) | + ¼ each | Largest entering ground or bonding jumper |
| Entering grounds (2017) | 1 total | Largest ground or bonding jumper present, including wholly internal ones |
| An additional isolated-ground set (2017 NEC) | + 1 | Largest ground in that set |
| Each terminal block assembly (2023 NEC) | 1 | Largest wire on it |
No separate conductor/fitting allowance: wire nuts and other splicing connectors, locknuts, bushings, cable connectors whose clamping part is outside the box, and pigtails or bonding jumpers that never leave the box.
Exception: Grounds and up to four fixture wires smaller than 14 AWG that come from a domed fixture canopy and end in that box can be left out.
Sources: 2020 NEC 314.16(A) and (B), reproduced by BELL/Hubbell for wires, loops, clamps, supports, yokes, wide devices and the canopy exception; the 2017 and 2020 grounding text; the authorized Leviton 2020 grounding reprint; 2023 terminal-block text; and 2023/2026 small-fitting text.
The ordinary-wire rows and loop rows are mutually exclusive: an eligible unbroken loop gets two allowances total, not one ordinary allowance plus two more. “Largest” means physically largest wire, so 10 AWG is larger than 12 AWG. Wholly internal grounds are not added to the entering count, but their size matters under the 2017 wording.
The questions people argue about
Do grounds count? Yes. In the 2020/2023 profiles, one to four entering grounds share a single allowance based on the largest entering ground or bonding jumper. Under the 2020 and 2023 NEC, each entering ground past four adds a quarter allowance. So five 12 AWG grounds are 1¼ × 2.25 = 2.8125 in³, not 2.25 and not 4.50.
Do neutrals count? Yes. Every insulated neutral that enters counts once at its own size. Box fill measures space, not which wires carry current. Minnesota DLI's own example counts all eight insulated wires from four 14/2 cables (checklist, p. 6).
Do both sides of a splice count? Yes, if both pieces come in from outside the box. Two cable wires joined in the box are two conductors. What doesn't count is the pigtail that starts and ends inside.
Do pigtails ever matter? Their size can. If a 12 AWG pigtail is the biggest wire in a box of 14 AWG wires, the clamp allowance jumps to 2.25 in³. If that pigtail feeds a device, it can set the device's size too. That's what the internal-wire options are for. A larger wholly internal ground or bonding jumper also sets the 2017 ground allowance: one entering 14 AWG ground with a wholly internal 12 AWG bonding jumper needs a 2.25 in³ ground allowance in that profile, not 2.00 in³. 2017 grounding text.
Is a duplex receptacle two devices? No. It has one strap, so it takes two allowances, not four. The count follows straps, not outlets.
Do wire nuts count? No separate allowance is assigned to ordinary wire-splicing connectors in this calculation. The 2026 text of 314.16(B) explicitly names splicing connectors alongside locknuts and bushings. That wording change does not establish that every 2026 box-fill rule is identical to 2023; this calculator has no 2026 profile.
Worked examples
These nine worked examples are unofficial study material, not exam questions. EX02 is attributed to Minnesota DLI; the other eight are original examples. The calculations were independently recomputed and checked against the calculator. All unspecified added-volume, support-fitting, terminal-block and internal-larger-wire inputs are zero or absent; each cable in these examples has one equipment ground of the stated cable size.
EX01 — Switch box too small (2023)
Two 14/2 cables, internal clamps and one switch in a 3 × 2 × 2¾ in. device box (14.0 in³).
| Item | in³ |
|---|---|
| 4 × 14 AWG wires | 8.00 |
| Clamps, 1 × 2.00 | 2.00 |
| Switch, 2 × 2.00 | 4.00 |
| 2 grounds, 1 × 2.00 | 2.00 |
| Required | 16.00 |
Over the box's volume by 2.00 in³. The smallest whole-box reference volumes listed below that meet 16.00 in³ are 18.0 in³: a 3 × 2 × 3½ in. device box or a 4 × 1¼ in. square box. That does not establish physical fit or approval. The switch is the line people forget. It's two allowances, not one.
EX02 — Minnesota DLI's four-cable example (2026)
Four 14/2 cables, a switch, a receptacle and internal clamps. This repeats the worked example in Minnesota DLI's 2026 inspection checklist.
| Item | in³ |
|---|---|
| 8 × 14 AWG wires | 16.00 |
| 4 grounds, 1 × 2.00 | 2.00 |
| Switch, 2 × 2.00 | 4.00 |
| Receptacle, 2 × 2.00 | 4.00 |
| Clamps, 1 × 2.00 | 2.00 |
| Required | 28.00 |
The smallest whole-box reference volume listed below that meets 28.00 in³ is 29.5 in³, shown for a 4-11/16 × 1½ in. square box. A box marked 28.0 in³ or more would meet this volume total, but device fit and other requirements still apply. This is a verified official example, not general 2026 calculator coverage.
EX03 — Three wire sizes, one device (2023)
Two 14 AWG, six 12 AWG and two 10 AWG wires; five grounds, the largest 10 AWG; internal clamps; one receptacle whose largest connected wire is 12 AWG. The box is marked 34.0 in³.
| Item | in³ |
|---|---|
| 2 × 14 AWG | 4.00 |
| 6 × 12 AWG | 13.50 |
| 2 × 10 AWG | 5.00 |
| Clamps, 1 × 2.50 (largest wire in the box is 10 AWG) | 2.50 |
| Receptacle, 2 × 2.25 (its largest wire is 12 AWG) | 4.50 |
| 5 grounds, 1¼ × 2.50 | 3.125 |
| Required | 32.625 |
Within the box's volume, 1.375 in³ to spare. Each line uses its own size rule. The 10 AWG wire sets the clamp line but not the receptacle, because it isn't connected to the receptacle.
EX04 — Same box, different answer by edition
Six 12/2 cables spliced in a 4 × 2⅛ in. square box (30.3 in³). No clamps or devices.
| Item | 2017 | 2020 and 2023 |
|---|---|---|
| 12 × 12 AWG wires | 27.00 | 27.00 |
| 6 grounds | 1 × 2.25 = 2.25 | 1½ × 2.25 = 3.375 |
| Required | 29.25 | 30.375 |
| Result | Within, 1.05 in³ to spare | Over by 0.075 in³ |
Under the 2020/2023 rules, the fifth and sixth grounds add a quarter allowance each. That's enough to push the same inventory from within the old volume allowance to over the newer one. A 42.0 in³ reference box, or a suitable marked extension ring adding enough volume, clears this numerical shortfall; neither choice is an installation approval.
EX05 — Don't round before you compare (2023)
Ten 12 AWG wires and five 12 AWG grounds in a box marked 25.31 in³.
| Item | in³ |
|---|---|
| 10 × 12 AWG | 22.50 |
| 5 grounds, 1¼ × 2.25 | 2.8125 |
| Required | 25.3125 |
Over the box's volume by 0.0025 in³. Round 25.3125 to 25.31 and it looks like a tie. It isn't. Keep the exact total for the comparison and show the significant decimals in the result. The calculator retains 25.3125 and the 0.0025 in³ shortfall instead of displaying a false tie.
EX06 — Two-gang box: how many more wires fit? (2023)
A plastic two-gang box marked 32.0 in³. A 14/2 and a 14/3 feed a switch; a 12/2 in and a 12/2 out feed a receptacle. No clamps.
| Item | in³ |
|---|---|
| 5 × 14 AWG | 10.00 |
| 4 × 12 AWG | 9.00 |
| 4 grounds, 1 × 2.25 (largest is 12 AWG) | 2.25 |
| Switch, 2 × 2.00 (14 AWG connected) | 4.00 |
| Receptacle, 2 × 2.25 (12 AWG connected) | 4.50 |
| Required | 29.75 |
Within the box's volume, 2.25 in³ to spare. That's room for one more 12 AWG or one more 14 AWG wire (2.25 ÷ 2.25 = 1; 2.25 ÷ 2.00 = 1.125, round down to 1). The shortcut only works if nothing else changes. A fifth cable would also bring a fifth ground and add a quarter allowance.
EX07 — Ceiling light box with a fixture stud (2023)
A 4 × 1½ in. octagon box (15.5 in³) with a 14/2 in, a 14/2 out, internal clamps and a fixture stud. Two 18 AWG fixture wires and one 18 AWG equipment ground come from a domed fixture canopy and terminate in the box. No larger internal wire is present.
| Item | in³ |
|---|---|
| 4 × 14 AWG | 8.00 |
| 2 grounds, 1 × 2.00 | 2.00 |
| Fixture stud, 1 × 2.00 | 2.00 |
| Clamps, 1 × 2.00 | 2.00 |
| Eligible fixture wires and ground from the canopy | 0.00 |
| Required | 14.00 |
Within the box's volume, 1.50 in³ to spare. The fixture wires drop out under the canopy exception because there are no more than four of them and they're smaller than 14 AWG. The canopy ground is also omitted under that exception. The stud and the clamps still count.
EX08 — Terminal block assembly (2023)
Two 10 AWG and two 12 AWG wires; two 12 AWG grounds; one receptacle on 12 AWG; one terminal block assembly whose largest wire is 10 AWG. The box is marked 20.0 in³.
| Item | in³ |
|---|---|
| 2 × 10 AWG | 5.00 |
| 2 × 12 AWG | 4.50 |
| 2 grounds, 1 × 2.25 | 2.25 |
| Receptacle, 2 × 2.25 | 4.50 |
| Terminal block assembly, 1 × 2.50 | 2.50 |
| Required | 18.75 |
Within the box's volume, 1.25 in³ to spare. Count assemblies, not screw terminals. The dedicated terminal-block rule was added in 2023. An assembly entered under the 2017 or 2020 profile produces a scope error, not a zero allowance. Keep the actual inventory and applicable edition; resolve the treatment under that edition rather than switching editions or removing the entry just to get a result. 2023 NEC 314.16(B)(6).
EX09 — Junction box with a marked ring (2023)
Three 12/3 cables spliced, with internal clamps, in a 4 × 1½ in. square box (21.0 in³). A ring marked 3.5 in³ is added.
| Item | in³ |
|---|---|
| 9 × 12 AWG | 20.25 |
| 3 grounds, 1 × 2.25 | 2.25 |
| Clamps, 1 × 2.25 | 2.25 |
| Required | 24.75 |
Available: 21.0 + 3.5 = 24.5 in³. Over by 0.25 in³. The smallest whole-box reference volume listed below that meets the total on its own is 25.5 in³, shown for a 4-11/16 × 1¼ in. square box. This checks volume, not physical fit.
Box volume: standard boxes, plastic boxes and rings
Use the volume stamped on the box when there is one. A standard metal box that isn't marked uses the applicable NEC edition's table volume. Plastic boxes, and boxes of 100 in³ or less that aren't in the table, must be marked by the maker (2020 NEC 314.16(A)). Don't estimate from outside dimensions.
The reference volumes below are documented in the 2008 NEC table reproduced by Appleton and the City of Republic's 2012 IRC table, p. 7. They are a reference lookup, not a newly verified reproduction of every later edition's table. Confirm an unmarked box against the code edition used for the job or exam.
| Standard metal box (trade size, in.) | Volume (in³) |
|---|---|
| 4 × 1¼ round/octagonal | 12.5 |
| 4 × 1½ round/octagonal | 15.5 |
| 4 × 2⅛ round/octagonal | 21.5 |
| 4 × 1¼ square | 18.0 |
| 4 × 1½ square | 21.0 |
| 4 × 2⅛ square | 30.3 |
| 4-11/16 × 1¼ square | 25.5 |
| 4-11/16 × 1½ square | 29.5 |
| 4-11/16 × 2⅛ square | 42.0 |
| 3 × 2 × 1½ device | 7.5 |
| 3 × 2 × 2 device | 10.0 |
| 3 × 2 × 2¼ device | 10.5 |
| 3 × 2 × 2½ device | 12.5 |
| 3 × 2 × 2¾ device | 14.0 |
| 3 × 2 × 3½ device | 18.0 |
| 4 × 2⅛ × 1½ device | 10.3 |
| 4 × 2⅛ × 1⅞ device | 13.0 |
| 4 × 2⅛ × 2⅛ device | 14.5 |
| 3¾ × 2 × 2½ masonry box | 14.0 per gang |
| 3¾ × 2 × 3½ masonry box | 21.0 per gang |
| FS, single cover | 13.5 per gang |
| FD, single cover | 18.0 per gang |
| FS, multiple cover | 18.0 per gang |
| FD, multiple cover | 24.0 per gang |
A per-gang value is not the volume of an entire multi-gang box. Multiply it by the applicable gang count, or use the marked total; never do both. The calculator requires a gang count for these entries and excludes them from automatic whole-box suggestions.
Rings and covers. A mud ring, extension ring or domed cover adds volume only if it's marked with its volume or made from a box in the table. Add it once. Don't add it again if the box's marking already includes it.
Boxes with barriers. Figure each space separately. A barrier that isn't marked takes up 0.5 in³ if it's metal and 1.0 in³ if it's nonmetallic (314.16(A), added in 2017). The calculator does not subtract or apportion barrier volume. Its barrier option stops the result. Do not run each compartment against the full box volume. For a hand calculation, establish each compartment's net volume after the barrier deduction, then compare only the inventory in that space.
How many wires fit in a 4-square box? It depends on what else is in it. A 4 × 1½ in. square box holds 21.0 in³, which is nine 12 AWG wires (21.0 ÷ 2.25 = 9.33, round down) only if there are no grounds, clamps or devices. Add any of those and use the calculator.
Which code edition should you use?
Use the edition adopted by the authority having jurisdiction for the job, or the one your exam's candidate bulletin names. It may not be the newest one. The dropdown offers 2017, 2020 and 2023 only; do not choose 2023 as a substitute for a 2026 requirement.
| Edition | Ground and terminal-block treatment in this resource |
|---|---|
| 2017 | One allowance when grounds enter, based on the largest ground or bonding jumper present; an additional isolated-ground set adds one more allowance. |
| 2020 | Up to four entering grounds share one allowance; each entering ground past four adds ¼, based on the largest entering ground or bonding jumper. |
| 2023 | Uses the entering-ground approach and adds one allowance per terminal block assembly, based on its largest terminated conductor. |
| 2026 | Not a calculator profile. The cited 314.16(B) text explicitly names splicing connectors, and EX02 reproduces one official example; neither establishes full-edition coverage. |
Sources: 2017/2020 grounding text; 2020 authorized reprint; 2023 terminal-block text; 2026 small-fitting text. To read the code itself, use NFPA's free access page.
Mistakes that fail boxes
- Counting each ground as its own allowance. Up to four share one.
- Forgetting the fifth-ground quarter allowance under the 2020 and 2023 NEC.
- Leaving out the clamp line.
- Sizing the clamp line by the clamped cable instead of the largest wire in the box.
- Sizing a device by the largest wire in the box instead of the largest wire connected to it.
- Counting a switch or receptacle once instead of twice.
- Using a metal-box table volume for a plastic box instead of the volume stamped inside it.
- Rounding the required total before comparing it with the box.
- Treating an exact tie as a fail. Equal volume is enough.
A box that passes the math can still be a tight fit for a bulky GFCI or smart switch. Box fill sets the minimum; it doesn't guarantee the device will seat well. Outlet and device boxes also have to meet the applicable depth requirements in 314.24 (2020 NEC 314.16 scope).
Practice box fill for your exam
Choose one of EX01–EX09, cover its answer table, and calculate the required volume with the worksheet. Then compare your conductor, ground, clamp and device lines with the explanation. For EX04, solve both edition cases before revealing the totals. These are focused calculation exercises, not a full exam or a prediction of passing.
Sources and verification
By Castleport Test Prep Editorial Team
Source check: Sept. 30, 2026. We checked the cited rule excerpts, the historical reference tables and every worked-example total. The table sources retain their original editions; this is not a claim that a complete final 2026 code or every jurisdiction's amendments was reviewed. The calculator and examples were checked with exact arithmetic. AI tools helped prepare and check this page. This was a source and arithmetic check, not a review by a licensed electrician or inspector.
- BELL/Hubbell, 2020 NEC excerpts, catalog page D2: 314.16(A) and (B), including ordinary conductors, loops, clamps, supports, device yokes and the canopy exception.
- Leviton, authorized 2020 NEC grounding reprint: 314.16(B)(5).
- 2017 and 2020 grounding text, reproduced by ElectricalLicenseRenewal.com: largest ground present versus largest entering ground, and the additional isolated set.
- 2023 terminal-block text, reproduced by ElectricalLicenseRenewal.com: 314.16(B)(6).
- 2023 and 2026 small-fitting text, reproduced by ElectricalLicenseRenewal.com: the opening paragraph of 314.16(B), not the full section.
- Appleton, authorized 2008 NEC reprint, catalog p. 1479: per-size allowances and historical reference volumes.
- City of Republic, Electrical Box Fill Calculations, December 2016, pp. 7–8: 2012 IRC reference box dimensions, volumes and per-size allowances.
- 2017 barrier text, reproduced by ElectricalLicenseRenewal.com: 314.16(A), compartment apportionment and unmarked-barrier deductions.
- Minnesota DLI, 2026 inspection checklist, p. 6, item 046; City of Champlin mirror: the attributed 28 in³ example and 14–10 AWG values.
- NFPA, free access to codes and standards.
Read our editorial standards.
Castleport Test Prep is an independent exam prep publisher. We're not affiliated with, endorsed by or approved by the National Fire Protection Association, any licensing board or any testing company. NEC and National Electrical Code are used to identify their subjects, and trademarks belong to their respective owners. This calculator checks volume only and doesn't approve electrical work. The applicable authority having jurisdiction determines acceptance. This page covers the U.S. NEC, not Canadian electrical-code calculations.