Junction Box Fill Calculator
NEC Article 314.16 requires that every electrical box have sufficient volume to safely contain all conductors, devices, and fittings without overcrowding. To calculate required volume, count each conductor, device, internal clamp, and grounding conductor group using the allowances from NEC Table 314.16(B), then multiply the total conductor count by the volume per conductor for the largest wire gauge in the box. This calculator walks through the standard fill calculation so you can verify that a proposed box is large enough or find the minimum box volume needed.
Box fill calculation formula (NEC 314.16)
Equivalent conductors = current-carrying conductors + (1 if grounds present) + (1 if internal clamps) + (devices × 2)
Minimum volume (cu in) = Equivalent conductors × volume per conductor (NEC Table 314.16(B))
All equipment grounding conductors together count as one conductor of the largest ground wire size. Internal cable clamps (where present) count as one conductor of the largest wire in the box. Each device yoke (switch or outlet) counts as two conductors of the largest wire size connected to it. Conductors running through the box that are not spliced still count as one each.
Box fill quick reference
- 14 AWG conductor fill allowance: 2.00 cubic inches each.
- 12 AWG conductor fill allowance: 2.25 cubic inches each.
- 10 AWG conductor fill allowance: 2.50 cubic inches each.
- 8 AWG conductor fill allowance: 3.00 cubic inches each.
- 6 AWG conductor fill allowance: 5.00 cubic inches each.
- A standard 2x4 switch box (2.5 in deep) is typically rated 22 cubic inches, sufficient for 9 to 11 conductors of 12 AWG.
Junction box fill: frequently asked questions
What is box fill and why does the NEC regulate it?
Box fill is the total volume occupied by conductors, devices, fittings, and clamps inside an electrical box. The NEC (Article 314.16) limits fill to prevent overcrowding, which causes heat buildup, insulation damage, and difficulty making connections. The box must have sufficient cubic-inch volume to safely contain all conductors and devices.
How do I count conductors for box fill?
Each current-carrying conductor entering the box counts as one. A conductor that passes through the box (looped without a splice) counts as one. Conductors that originate inside the box (from a device) do not count. Equipment grounding conductors collectively count as one regardless of how many there are.
How do device yokes count toward box fill?
Each single-gang device yoke (switch, outlet) counts as two conductors of the largest wire size connected to it. A duplex receptacle mounted on one yoke counts as two conductors. Two switches on a two-gang plate each count as two conductors (four total). This accounts for the space the device occupies.
How do I find the cubic inch rating of a box?
Listed electrical boxes have their cubic-inch volume marked on the box or packaging per UL and NEC requirements. For standard 2x4 single-gang boxes: a 2-inch-deep box is typically 18 to 20 cubic inches; a 2.5-inch-deep box is about 22 cubic inches; a 3.5-inch-deep box is about 32 cubic inches. Deep boxes and extension rings add volume.
What is the volume per conductor from NEC Table 314.16(B)?
NEC Table 314.16(B) lists the volume allowance per conductor by wire size: 14 AWG = 2.00 cu in; 12 AWG = 2.25 cu in; 10 AWG = 2.50 cu in; 8 AWG = 3.00 cu in; 6 AWG = 5.00 cu in. Use the size of the largest conductor in the box for device and clamp fill calculations.
Official sources
- NFPA 70 (NEC): National Electrical Code Article 314.16 and Table 314.16(B).
- U.S. DOE: Home Electrical Systems.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.