Electrical Box Fill Calculator (NEC 314.16): Cubic Inches and Box Size
Box fill in cubic inches under NEC 314.16 (2020/2023): conductors by size, mixed gauges, devices, internal clamps, fixture studs and grounds (one allowance, plus ¼ for each beyond four), with the standard Table 314.16(A) boxes checked, plaster-ring volume and the smallest box that complies.
How it works
Fill = Σ allowances × volume per conductor (NEC Table 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 in³) · conductors: 1 each (loops ≥ 12″: 2) · devices: 2 per yoke at the largest conductor on the device · internal clamps: 1 total at the largest conductor · studs/hickeys: 1 per type · grounds: 1 for up to four, + ¼ for each beyond four, at the largest ground · Fill ≤ marked box volume + marked ring/extension volume
NEC 314.16 limits how much goes into an outlet, device or junction box so conductors do not overheat, insulation is not damaged when devices are pushed in, and splices stay reliable. Each item is converted into "volume allowances" sized by Table 314.16(B). Every conductor that comes into the box and ends or is spliced there counts once, as does one passing straight through; an unbroken loop at least 12 inches long counts twice. Wires that start and end inside the box (pigtails) and wire connectors are not counted. Each single-gang device yoke (switch, receptacle) counts two allowances at the largest conductor connected to it. Internal cable clamps count once in total, fixture studs and hickeys once per type, both at the largest conductor in the box. Equipment grounding conductors count once for up to four, and since the 2020 NEC each additional ground adds a quarter allowance. Add the allowances, multiply by the cubic inches for each size, and compare with the volume in Table 314.16(A) for metal boxes or the volume marked on plastic boxes, plus any marked plaster ring or extension.
Worked example
With the defaults (12 AWG, 6 conductors, 1 receptacle, 3 grounds, internal clamps): 6 + 2 (device) + 1 (clamps) + 1 (grounds) = 10 allowances × 2.25 = 22.50 in³. The 4″ octagon × 1½″ box (15.5 in³) is 7.00 in³ too small. The smallest standard metal box that fits is a 4-11/16″ square × 1¼″ (25.5 in³, 3.00 in³ to spare); a 4-11/16″ × 1½″ (29.5 in³) or a 4″ square × 2⅛″ (30.3 in³) also works, while a 4″ square × 1½″ (21.0 in³) does not.
Sources: Open Exam Prep: NEC 314.16 box fill, Tables 314.16(A) and (B), EGC ¼-allowance rule and worked examples · EC&M: box fill calculations, counting rules and mixed-size worked examples · Electrical Contractor: box fill calculations, clamp and mixed-size examples
Frequently asked questions
How do I calculate box fill?
Count the allowances: one per conductor entering, two per device yoke, one for all internal clamps, one per type of stud or hickey, and one for the grounds (plus ¼ for each ground beyond four). Multiply by the volume for the wire size (2.00 in³ for 14 AWG, 2.25 in³ for 12 AWG) and compare with the box volume.
How many 12 AWG wires can go in a 4″ square box?
A 4″ × 1½″ square box (21.0 in³) holds 9 allowances of 12 AWG and a 4″ × 2⅛″ square box (30.3 in³) holds 13. Devices, clamps and grounds use some of those, so the number of actual wires is smaller.
Do grounding wires count in box fill?
Yes, but as a group. Up to four equipment grounding conductors count as one allowance at the size of the largest ground. Under the 2020 and 2023 NEC, each ground beyond four adds a quarter allowance, so six grounds count 1.5 allowances.
Do wire nuts and pigtails count?
No. Wire connectors are not counted, and a pigtail that starts and ends inside the box is not counted. A conductor that leaves the box does count.
Does a plaster ring or extension ring add volume?
Yes. If a plaster ring, raised cover or extension ring is marked with its volume, add it to the box volume. If it is not marked, it adds nothing.
What if the box has two wire sizes?
Each conductor uses the volume for its own size. Devices count at the largest conductor connected to them, clamps and studs at the largest conductor in the box, and grounds at the largest ground. Enter the second size under More options.
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Educational estimate. Electrical, plumbing and HVAC work must comply with the local code (NEC/CEC/IEC, IPC/UPC) and be checked by a licensed professional; safety limits (NIOSH, ACGIH, OSHA) are guidance, not a substitute for a workplace assessment.