Box Fill Calculator
Add up the NEC 314.16(B) volume allowances for the conductors, clamps, support fittings, device yokes, and grounds in an outlet, switch, or junction box, and check the total against the box's cubic-inch capacity. Built on Table 314.16(B), default NEC 2020 (values unchanged 2017 to 2023).
How to use this calculator
- Enter the box volume in cubic inches, or pick a standard metal box from the list (Table 314.16(A)). Nonmetallic boxes have the volume stamped inside.
- Enter how many conductors of each size run through or terminate in the box. Do not count pigtails that never leave the box.
- Set the number of device yokes and support fittings, and check whether internal clamps and grounding conductors are present.
- Read the required volume and the adequate/too-small status. The breakdown shows exactly where each cubic inch goes.
NEC reference
Fill is computed per NEC 2020 Article 314.16(B). Each conductor takes the free-space allowance from Table 314.16(B) at its own size; internal clamps add one allowance at the largest conductor (314.16(B)(2)); each luminaire stud or hickey adds one (314.16(B)(3)); each device yoke adds a double allowance (314.16(B)(4)); and the equipment grounding conductors add one allowance as a group, with a separate isolated-ground set adding another (314.16(B)(5)). Standard metal box volumes are from Table 314.16(A). The Table 314.16(B) allowances are unchanged across the 2017, 2020, and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Table 314.16(B) volume allowances
| Conductor size (AWG) | Free-space allowance (in³) |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
Table 314.16(B) stops at 6 AWG. Boxes containing 4 AWG and larger conductors are sized by NEC 314.28 (pull and junction boxes), a different, dimension-based calculation.
Worked example: two 14/2 cables into a switch box
A single-pole switch is fed by two 14/2 NM cables landing in a 4" × 1¼" square box (18.0 in³) with internal clamps.
- Conductors: two blacks + two whites = 4 × 14 AWG × 2.00 = 8.00 in³ (314.16(B)(1)).
- Clamps: one allowance at 14 AWG = 2.00 in³ (314.16(B)(2)).
- Switch yoke: double allowance = 2 × 2.00 = 4.00 in³ (314.16(B)(4)).
- Grounds: the two bare grounds count once = 2.00 in³ (314.16(B)(5)).
Total required = 8.00 + 2.00 + 4.00 + 2.00 = 16.00 in³. The 18.0 in³ box has 2.00 in³ to spare, so it is adequate. Add a second switch on its own yoke and the requirement jumps to 20.00 in³, which the 18.0 in³ box can no longer hold.
The counting rules that trip people up
- Grounds count once, not per wire. Every equipment grounding conductor in the box shares a single allowance at the largest ground size (314.16(B)(5)).
- All internal clamps count once. One or ten cable clamps still add just one allowance, based on the largest conductor in the box (314.16(B)(2)).
- Devices are a double allowance per yoke. A single receptacle or switch strap counts twice the largest conductor (314.16(B)(4)); a two-gang box with two yokes counts four times.
- Pigtails do not count. A conductor that both starts and ends inside the box is excluded (314.16(B)(1)).
This calculator bases the clamp, support, device, and grounding allowances on the largest conductor in the box. That is exact for clamps and support fittings; for device yokes and grounds the code uses the largest conductor connected to that yoke or the largest ground, so basing them on the largest conductor in the box can only over-estimate the requirement; it never undersizes the box.
Frequently asked questions
How do you calculate box fill?
Total the Table 314.16(B) allowances for every counted conductor, plus one allowance for all internal clamps, one per support fitting, a double allowance per device yoke, and one for the grounding conductors as a group. The sum is the minimum box volume; compare it to the box's rated cubic inches.
Does a cable connector or locknut count?
No. Only internal cable clamps trigger the 314.16(B)(2) allowance. Fittings on the outside of the box, such as a two-screw connector clamped outside with only a locknut inside, do not count as clamp fill.
What about 4 AWG and larger conductors?
Table 314.16(B) only covers 18 through 6 AWG. Once a box contains 4 AWG or larger conductors, it is sized by NEC 314.28 (pull and junction boxes), which sets minimum box dimensions from the raceway sizes and the pull geometry rather than a volume total. This calculator flags those sizes instead of guessing.
Do extension rings and plaster rings add volume?
Yes. A plaster ring, extension ring, or domed cover marked with a cubic-inch capacity adds that volume to the box (314.16(A)). Add the marked figure to the box-volume field above to include it.
Box fill guides
- Box Fill Calculation Example: 4-Inch Square Box: the full 314.16(B) method worked three ways, including the mixed-size largest-conductor rule.
- How Many Wires Fit in a Single-Gang Box: device-box capacity by depth, and why the device and clamps eat the box first.
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Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
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For deeper NEC training on box fill and rough-in wiring, Mike Holt's NEC courses are the industry standard.
Working from the printed code? NEC code books and tab sets are available from Mike Holt.
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