Free calculator
Box Fill Calculator
Count what goes in the box to get the volume it needs under NEC 314.16, itemized by rule. Check it against a standard metal box or the volume stamped in yours, and see the smallest box that works.
- Itemized by 314.16(B) rule
- 2017, 2020 and 2023 NEC
- Smallest box that fits
- No email required
Box
Standard metal boxes use the 314.16(A) volume. For a nonmetallic box, a box not listed or ganged sections, choose Other and enter the volume stamped inside, or the sections added up. Add the volume stamped on any plaster ring, extension ring or domed cover.
Conductors and devices
Conductors: every hot, neutral, traveler or switch leg that enters the box, spliced, terminated or passing through. Enter an unbroken loop of 12 in or more as 2. Leave out pigtails that never leave the box, and enter grounds below. Device gangs: 1 for each yoke, on the row of the largest conductor connected to it, or 2 for a device that needs two gangs.
Grounds and fittings
Count the equipment grounds and bonding jumpers that enter the box. Isolated grounds for isolated-ground receptacles get their own allowance under the 2017 NEC and count with the other grounds under the 2020 and 2023 NEC. Clamps whose clamping part sits outside the box count nothing.
How it’s calculated
Volume per conductor. Each conductor allowance is the free space in 314.16(B): 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 cu in. The rule covers 6 AWG and smaller. Boxes with 4 AWG and larger conductors are sized under 314.28.
Conductors, 314.16(B)(1). Each conductor that enters from outside the box counts once at its own size, whether it is spliced, terminated or passes straight through. An unbroken loop at least twice the 6 in free length of 300.14, so 12 in or more, counts twice. A conductor no part of which leaves the box, like a pigtail, counts zero. Up to four fixture wires smaller than 14 AWG and a ground that enter from a domed luminaire canopy and end in the box can be left out.
Clamps and fittings, 314.16(B)(2) and (B)(3). One allowance covers all internal cable clamps, at the largest conductor in the box. Fixture studs and hickeys take one allowance for each type present, also at the largest conductor.
Device yokes, 314.16(B)(4). Two allowances for each yoke or strap, at the largest conductor connected to a device on it. A device wider than a single 2 in device box takes two for each gang it needs to mount.
Equipment grounds, 314.16(B)(5). 2020 and 2023 NEC: one allowance for up to 4 grounds and bonding jumpers entering the box, at the largest of them, plus 0.25 of that allowance for each one after the fourth. Isolated grounds count with the rest. 2017 NEC: one allowance for all of them, at the largest, plus one more for an isolated ground set under 250.146(D), at the largest conductor in that set.
Terminal blocks, 314.16(B)(6). New in the 2023 NEC: one allowance for each terminal block assembly, at the largest conductor terminated on it. Earlier editions have no such rule, so the calculator leaves them out when you pick 2017 or 2020.
Box volume. Standard metal boxes take their volume from the 314.16(A) table, in cu in: 4 × 1¼ in round/octagonal 12.5; 4 × 1½ in round/octagonal 15.5; 4 × 2⅛ in round/octagonal 21.5; 4 in square × 1¼ in 18.0; 4 in square × 1½ in 21.0; 4 in square × 2⅛ in 30.3; 4-11/16 in square × 1¼ in 25.5; 4-11/16 in square × 1½ in 29.5; 4-11/16 in square × 2⅛ in 42.0; 3 × 2 × 1½ in device 7.5; 3 × 2 × 2 in device 10.0; 3 × 2 × 2¼ in device 10.5; 3 × 2 × 2½ in device 12.5; 3 × 2 × 2¾ in device 14.0; 3 × 2 × 3½ in device 18.0; 4 × 2⅛ × 1½ in device 10.3; 4 × 2⅛ × 1⅞ in device 13.0; 4 × 2⅛ × 2⅛ in device 14.5; 3¾ × 2 × 2½ in masonry, per gang 14.0; 3¾ × 2 × 3½ in masonry, per gang 21.0; FS single gang, 1¾ in deep 13.5; FD single gang, 2⅜ in deep 18.0; FS multiple gang, 1¾ in deep 18.0; FD multiple gang, 2⅜ in deep 24.0. Nonmetallic boxes and other boxes up to 100 cu in are marked with their volume. Plaster rings, extension rings and domed covers add their marked volume, or the volume of the listed box they are made to. Ganged sections add together.
Result. The box fits when the volume needed is no more than the box volume plus the rings. Room for more is the spare volume ÷ the allowance of the largest conductor, rounded down. The smallest box that works is the smallest box of the same type in the table whose volume plus your rings covers the volume needed. For an other box, it is the smallest round, square or device box that does.
Each space in a box with a barrier is figured separately, and conduit bodies follow 314.16(C), which this calculator does not cover. Check the edition your inspector enforces: states and cities adopt the NEC on their own schedules and some amend it.
Pricing the job? The electrical estimate template has a device takeoff that turns openings into labor hours and boxes, grouped the way electricians price the work.
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Frequently asked questions
How many 12 AWG wires fit in a 4 in square box, 1½ in deep?
9, if nothing else is in the box. The box is 21.0 cu in (314.16(A)) and each 12 AWG conductor takes 2.25 cu in, so 21.0 ÷ 2.25 rounds down to 9. The grounds take one more allowance, which leaves room for 8, and internal cable clamps take another, which leaves 7. In 14 AWG the same box holds 10. A device on a plaster ring takes two allowances, and the volume stamped on the ring is added to the box.
Does the ground wire count in box fill?
Yes. Under the 2020 and 2023 NEC, up to 4 grounds entering the box count as one conductor, sized by the largest ground, and each ground after the fourth adds a quarter (314.16(B)(5)). Six 14 AWG grounds count as 1.5, or 3.00 cu in. The 2017 NEC counted all of them as one, 2.00 cu in for the same six, plus one more for an isolated ground set. Ground pigtails that never leave the box do not count.
How does a switch or receptacle count in box fill?
As two conductors for each yoke, sized by the largest conductor connected to it (314.16(B)(4)). A single-pole switch on 14 AWG takes 4.00 cu in and a 20 A receptacle on 12 AWG takes 4.50. Two devices on one yoke, like a double switch, still count as one yoke. A device too wide for a single 2 in gang counts two for each gang it needs: on 6 AWG, one that takes two gangs is 20.00 cu in.
Do wire nuts count in box fill?
No. Box fill counts the conductors and the items listed in 314.16(B)(2) through (B)(6): internal cable clamps, fixture studs and hickeys, device yokes, equipment grounds and, under the 2023 NEC, terminal blocks. Twist-on wire connectors are not on that list, and 314.16(B) says small fittings such as locknuts and bushings need no allowance. Pigtails and jumpers that never leave the box do not count either.
What is the box fill for a single-gang receptacle box?
Take one receptacle fed by two 12/2 cables, one in and one out, in a 3 × 2 × 3½ in metal device box with internal clamps. The four 12 AWG conductors take 9.00 cu in, the clamps 2.25, the receptacle 4.50 and the two grounds 2.25. That is 18.00 cu in, exactly the box’s 18.0, so it fits with nothing to spare. A 3 × 2 × 2¾ in box at 14.0 cu in is too small. With 14/2 cable the same box needs 16.00 cu in.
Do plaster rings and extension rings add to the box volume?
Yes, when they are marked with their volume or made to the size of a box in the 314.16(A) table. A 4 in square extension ring 1½ in deep adds 21.0 cu in, the same as the box. Plaster rings and domed covers add the volume stamped on them, and an unmarked ring adds nothing. Nonmetallic boxes, and boxes of 100 cu in or less that are not in the table, are marked with their volume by the maker: choose “Other box” and enter it.
Is anything I enter sent to BuildWorkPro?
The calculator sends nothing: the math runs in your browser. The box and what goes in it are kept in the page address so you can bookmark or share a result; like any page address, it is recorded by our site analytics if you accept analytics cookies.