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A standard metal box from NEC Table 314.16(A), or another box with its volume marked on it.
US boxes only. The listed metal boxes carry the table's volume, and keep it when a plaster ring, extension ring or raised cover is fitted: choose the box here and enter the ring's or cover's volume in its own field below. A plastic box, or any box not in the table, counts only the cubic inches durably marked on it — choose another box and enter that figure. Canadian, British and Australian boxes are not sized this way.
Extra cubic inches from a plaster ring, extension ring or raised cover: the volume marked on it, or a listed box's.
Section 314.16(A) counts the space of a ring or domed cover that is marked with its volume, or that is made from a box size listed in Table 314.16(A) — then that box's volume from the list above, so an unmarked extension ring made from a 4 × 1½ in square box adds 21.0 in³. Any other unmarked ring or cover adds nothing to the calculation, however much room it seems to give.
Insulated 14 AWG conductors entering the box, each counted once.
Count every insulated conductor that enters and is spliced or terminated, and every one that passes straight through. A 14/2 cable brings two insulated conductors (its bare ground is counted separately); 14/3 brings three. An unbroken loop of 12 in (305 mm) or more counts as two, and pigtails that begin and end in the box are not counted.
Insulated 12 AWG conductors entering the box, each counted once.
Count every insulated conductor that enters and is spliced or terminated, and every one that passes straight through. A 14/2 cable brings two insulated conductors (its bare ground is counted separately); 14/3 brings three. An unbroken loop of 12 in (305 mm) or more counts as two, and pigtails that begin and end in the box are not counted.
Insulated 10 AWG conductors entering the box, each counted once.
Count every insulated conductor that enters and is spliced or terminated, and every one that passes straight through. A 14/2 cable brings two insulated conductors (its bare ground is counted separately); 14/3 brings three. An unbroken loop of 12 in (305 mm) or more counts as two, and pigtails that begin and end in the box are not counted.
Insulated 8 AWG conductors entering the box, each counted once.
Count every insulated conductor that enters and is spliced or terminated, and every one that passes straight through. A 14/2 cable brings two insulated conductors (its bare ground is counted separately); 14/3 brings three. An unbroken loop of 12 in (305 mm) or more counts as two, and pigtails that begin and end in the box are not counted.
Insulated 6 AWG conductors entering the box, each counted once.
Count every insulated conductor that enters and is spliced or terminated, and every one that passes straight through. A 14/2 cable brings two insulated conductors (its bare ground is counted separately); 14/3 brings three. An unbroken loop of 12 in (305 mm) or more counts as two, and pigtails that begin and end in the box are not counted.
The bare or green grounds that come into the box — one per cable, usually.
Up to four count together as a single allowance at the size of the largest of them. From the 2020 NEC, each one beyond the fourth adds a quarter allowance; earlier editions count all of them once, apart from an additional set of isolated grounds for isolated-ground receptacles, which adds one more allowance.
The size of the largest equipment grounding conductor in the box.
In 14/2 and 12/2 cable the bare ground is the same size as the insulated conductors; in larger cables it can be smaller. Their allowance is taken at the largest.
The edition your state or city enforces; it changes only how the grounds are counted.
US jurisdictions adopt NEC editions on their own timetables, so the same box can be counted two ways in two towns. The difference appears when more than four equipment grounding conductors enter the box, and before 2020 in one more case: an additional set of isolated grounds, for isolated-ground receptacles under 250.146(D), adds a further allowance at the largest conductor in that set. The page has no field for that set — enter one more conductor of its size. The 2020 rule counts those grounds with the rest.
Whether the box has clamps inside it that grip the cables.
Any number of internal clamps count as one allowance, at the size of the largest conductor in the box. Connectors whose clamping part sits outside the box are not counted.
Fixture studs and hickeys count once for each type present, not once for each fitting.
Section 314.16(B)(3) makes "a single volume allowance … for each type of fitting", at the size of the largest conductor in the box: two luminaire studs together are one allowance, and a stud with a hickey two. Some handouts, the City of Republic's among them, count every stud and hickey separately, which overstates the fill.
Device yokes in the box — each switch, receptacle or combination device on one strap.
Each yoke counts as two allowances at the size of the largest conductor connected to it. A device too wide for a single-gang box, such as a 50 A range receptacle, counts two for each gang it takes — enter it as two. Devices on a different conductor size go in the second group below.
The size of the largest conductor connected to the devices above.
A switch on 14 AWG counts 4 in³; a receptacle on 12 AWG counts 4.5 in³.
Any devices whose largest conductor is a different size from the first group.
A box with a switch on 14 AWG and a receptacle on 12 AWG counts the two at their own sizes: put one in each group.
The size of the largest conductor connected to the second group of devices.
Leave it as it is when the second group is empty; it then counts nothing.
Volume the box's contents need (in³)
28 in³
The volume the box's contents need is below the box's volume shown with it — NEC Table 314.16(A), as the 2012 IRC reproduces it, gives it. The count is only as complete as what was entered: loops, pigtails and fixture wires follow the code's own rules for whether they count. Matching the figures quoted is not compliance. The rest of the requirement, and the installed work, are outside what this page can see.
- Box volume, with rings or covers (in³)
- 30.3
- Box volume less the contents' allowances (in³)
- 2.3
- Insulated conductors (in³)
- 16
- Equipment grounding conductors (in³)
- 2
- Devices, two allowances per yoke (in³)
- 8
- Internal cable clamps (in³)
- 2
- Luminaire studs and hickeys, one allowance per type (in³)
- 0
- Smallest volume in Table 314.16(A) that holds this (in³)
- 29.5
They open the calculator with your figures already in it
Electrical Box Fill Calculator (NEC 314.16, US): 28 in³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- City of Republic, Missouri, Community Development Department, Electrical Box Fill Calculations (revised December 2016): 2012 IRC Table E3905.12.2.1, the volume allowance per conductor — 18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 in³ — and Table E3905.12.1, the volumes of standard metal boxes, the residential code's copies of NEC Tables 314.16(B) and 314.16(A); and the counting rules of NEC 314.16(B)(1) to (5) with worked examples (eleven 14 AWG allowances, 22 in³; five 14 AWG and six 12 AWG, 23.5 in³ in a 4 × 2⅛ in square box). Its support-fitting rule, an allowance for each stud or hickey, is not the code's, which gives one for each type of fitting (Emerson's reprint, below); the page follows the code
- Minnesota Department of Labor and Industry, Electrical Inspection Checklist for One-Family Dwellings based on the 2023 NEC (revised 2023-05-06), item 046, NEC 314.16: each insulated wire one allowance, all grounding wires combined one, each switch or receptacle two, all internal cable clamps one; more than four equipment grounding conductors add a quarter allowance each; four 14/2 cables with a switch, a receptacle and clamps need 28 in³
- City of San Antonio Development Services, 2020 National Electrical Code training (November 2022): 314.16(B)(5), new in 2020 — an additional ¼ volume allowance for equipment grounding conductors exceeding four
- Emerson (Appleton and O-Z/Gedney), Capacity of Steel Boxes and Covers, commercial and industrial fittings catalogue page 444 (February 2017), which reprints NEC 314.16 from the 2008 edition with NFPA's permission: 314.16(A), a box's volume including plaster rings, domed covers and extension rings that are marked with their volume or made from a box size listed in Table 314.16(A); 314.16(B)(3), a single allowance for each type of support fitting, luminaire studs or hickeys, at the largest conductor in the box; 314.16(B)(5), one allowance for the equipment grounding conductors and, for an additional set of isolated ones under 250.146(D), one more; and Tables 314.16(A) and 314.16(B) in cm³ with in³ in brackets, a 4 × 2⅛ in square box at 497 cm³ (30.3 in³) and a 14 AWG conductor at 32.8 cm³ (2.00 in³)
- Leviton, Captain Code 2020 NEC change summary, 314.16(B)(5): from 2020 a quarter allowance for each equipment grounding conductor or bonding jumper beyond four, a change that "eliminates the need to specifically address" the isolated-ground conductors of 250.146(D)
Inputs used
- Box
- 4 × 2⅛ in square — 30.3 in³
- Volume marked on the box (in³)
- 22.5
- Volume of rings or raised covers (in³)
- 0
- 14 AWG conductors
- 8
- 12 AWG conductors
- 0
- 10 AWG conductors
- 0
- 8 AWG conductors
- 0
- 6 AWG conductors
- 0
- Equipment grounding conductors entering
- 4
- Largest grounding conductor
- 14 AWG
- NEC edition adopted where the box is
- 2020 NEC or later — grounds beyond four add a quarter each
- Internal cable clamps
- Yes — one or more clamps inside the box
- Luminaire studs or hickeys
- None
- Switches or receptacles
- 2
- Largest conductor on those devices
- 14 AWG
- Devices on a second conductor size
- 0
- Largest conductor on the second group
- 12 AWG
Intermediate steps
- Box volume, with rings or covers (in³)
- 30.3
- Box volume less the contents' allowances (in³)
- 2.3
- Insulated conductors (in³)
- 16
- Equipment grounding conductors (in³)
- 2
- Devices, two allowances per yoke (in³)
- 8
- Internal cable clamps (in³)
- 2
- Luminaire studs and hickeys, one allowance per type (in³)
- 0
- Smallest volume in Table 314.16(A) that holds this (in³)
- 29.5
Confidence note: The volume the box's contents need is below the box's volume shown with it — NEC Table 314.16(A), as the 2012 IRC reproduces it, gives it. The count is only as complete as what was entered: loops, pigtails and fixture wires follow the code's own rules for whether they count. Matching the figures quoted is not compliance. The rest of the requirement, and the installed work, are outside what this page can see.
What this calculation does not cover
- US only: the National Electrical Code's Section 314.16 as the 2012 IRC, Minnesota's 2023 checklist and San Antonio's 2020 training reproduce it. Canada's electrical code has its own box-fill rule and figures, and British and Australian accessory boxes are filled to the maker's instructions; none of them is this calculation.
- Conductors from 14 to 6 AWG only. Table 314.16(B) also gives 18 AWG at 1.50 in³ and 16 AWG at 1.75 in³ — fixture wires, which are often exempt when they come from a domed luminaire canopy — and nothing larger than 6 AWG belongs in this calculation; a box for 4 AWG and larger conductors is sized by its own rules in the same article.
- The count is yours, and the rules for it are the code's: a conductor passing straight through counts once, an unbroken loop of 12 in (305 mm) or more counts twice, a pigtail or bonding jumper that never leaves the box is not counted, and up to four fixture wires of 16 AWG or smaller and the grounds from a domed canopy may be left out. The page adds what it is given.
- The 2023 NEC also counts terminal blocks in a box, and some boxes carry other fittings the code assigns an allowance to; neither has a field here.
- Before the 2020 NEC, an additional set of isolated equipment grounding conductors, run to isolated-ground receptacles under 250.146(D), adds one more allowance at the largest conductor in that set. The 2017-or-earlier choice does not add it: enter one more conductor of that size. From 2020 those grounds are counted with the rest, a quarter each beyond the fourth.
- A box that is not in Table 314.16(A), and every nonmetallic box, is credited only with the volume durably marked on it. A ring or cover adds the volume marked on it, or, if it is made from a box size in Table 314.16(A), that box's table volume; otherwise nothing, however much room it seems to give.
- A box filled within its volume can still be the wrong box: whether it is rated for the location, supports the luminaire or fan, sits flush, and is accessible are separate requirements, and the work is the electrician's and the inspector's to judge.
Add the equipment this sizes
This result is a specification — 28 in³ — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations5
- City of Republic, Missouri, Community Development Department, Electrical Box Fill Calculations (revised December 2016): 2012 IRC Table E3905.12.2.1, the volume allowance per conductor — 18 AWG 1.50, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00 in³ — and Table E3905.12.1, the volumes of standard metal boxes, the residential code's copies of NEC Tables 314.16(B) and 314.16(A); and the counting rules of NEC 314.16(B)(1) to (5) with worked examples (eleven 14 AWG allowances, 22 in³; five 14 AWG and six 12 AWG, 23.5 in³ in a 4 × 2⅛ in square box). Its support-fitting rule, an allowance for each stud or hickey, is not the code's, which gives one for each type of fitting (Emerson's reprint, below); the page follows the code
- Minnesota Department of Labor and Industry, Electrical Inspection Checklist for One-Family Dwellings based on the 2023 NEC (revised 2023-05-06), item 046, NEC 314.16: each insulated wire one allowance, all grounding wires combined one, each switch or receptacle two, all internal cable clamps one; more than four equipment grounding conductors add a quarter allowance each; four 14/2 cables with a switch, a receptacle and clamps need 28 in³
- City of San Antonio Development Services, 2020 National Electrical Code training (November 2022): 314.16(B)(5), new in 2020 — an additional ¼ volume allowance for equipment grounding conductors exceeding four
- Emerson (Appleton and O-Z/Gedney), Capacity of Steel Boxes and Covers, commercial and industrial fittings catalogue page 444 (February 2017), which reprints NEC 314.16 from the 2008 edition with NFPA's permission: 314.16(A), a box's volume including plaster rings, domed covers and extension rings that are marked with their volume or made from a box size listed in Table 314.16(A); 314.16(B)(3), a single allowance for each type of support fitting, luminaire studs or hickeys, at the largest conductor in the box; 314.16(B)(5), one allowance for the equipment grounding conductors and, for an additional set of isolated ones under 250.146(D), one more; and Tables 314.16(A) and 314.16(B) in cm³ with in³ in brackets, a 4 × 2⅛ in square box at 497 cm³ (30.3 in³) and a 14 AWG conductor at 32.8 cm³ (2.00 in³)
- Leviton, Captain Code 2020 NEC change summary, 314.16(B)(5): from 2020 a quarter allowance for each equipment grounding conductor or bonding jumper beyond four, a change that "eliminates the need to specifically address" the isolated-ground conductors of 250.146(D)
Which documents these citations point at
- International Residential Code — Table E3905.12.2.1 (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
- National Electrical Code (NFPA 70) — 314.16(B)(1), 314.16 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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