Box Fill Calculator & Chart — NEC 314.16
Work out the minimum electrical box volume for the conductors, devices, clamps and grounds inside it, or check how many wires a standard box holds.
Volume per conductor: 14 AWG = 2.00 in³ · 12 AWG = 2.25 in³ · 10 AWG = 2.50 in³ · 8 AWG = 3.00 in³ · 6 AWG = 5.00 in³. Each device counts as two conductors; all grounds together count as one.
Box fill calculator
Count each insulated conductor entering the box (hots, neutrals, travelers). Do not count pigtails.
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Volume required per conductor
| Conductor size | Volume per conductor (in³) | cm³ |
|---|---|---|
| 18 AWG | 1.50 | 24.6 |
| 16 AWG | 1.75 | 28.7 |
| 14 AWG | 2.00 | 32.8 |
| 12 AWG | 2.25 | 36.9 |
| 10 AWG | 2.50 | 41.0 |
| 8 AWG | 3.00 | 49.2 |
| 6 AWG | 5.00 | 81.9 |
What counts, and for how much
| Item in the box | Counts as |
|---|---|
| Each conductor that enters and is spliced or terminated | 1 allowance, at its own size |
| Conductor that passes straight through unbroken | 1 allowance |
| Pigtails and jumpers that never leave the box | 0 |
| Internal cable clamps (any number) | 1 allowance, based on the largest conductor in the box |
| Each fixture stud or hickey | 1 allowance each, largest conductor |
| Each device yoke (switch, receptacle) | 2 allowances, based on the largest conductor connected to that device |
| Equipment grounding conductors, up to four | 1 allowance in total, based on the largest ground |
| Each additional ground beyond four | ¼ allowance each (2020 NEC and later) |
| Up to four fixture wires smaller than 14 AWG entering from a domed canopy | 0 |
Standard metal box capacities
| Box size (in) | Type | Volume (in³) | #18 | #16 | #14 | #12 | #10 | #8 | #6 |
|---|---|---|---|---|---|---|---|---|---|
| 4 × 1-1/4 | Round / octagonal | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 × 1-1/2 | Round / octagonal | 15.5 | 10 | 8 | 7 | 6 | 6 | 5 | 3 |
| 4 × 2-1/8 | Round / octagonal | 21.5 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 × 1-1/4 | Square | 18 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 × 1-1/2 | Square | 21 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
| 4 × 2-1/8 | Square | 30.3 | 20 | 17 | 15 | 13 | 12 | 10 | 6 |
| 4-11/16 × 1-1/4 | Square | 25.5 | 17 | 14 | 12 | 11 | 10 | 8 | 5 |
| 4-11/16 × 1-1/2 | Square | 29.5 | 19 | 16 | 14 | 13 | 11 | 9 | 5 |
| 4-11/16 × 2-1/8 | Square | 42 | 28 | 24 | 21 | 18 | 16 | 14 | 8 |
| 3 × 2 × 1-1/2 | Device | 7.5 | 5 | 4 | 3 | 3 | 3 | 2 | 1 |
| 3 × 2 × 2 | Device | 10 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 3 × 2 × 2-1/4 | Device | 10.5 | 7 | 6 | 5 | 4 | 4 | 3 | 2 |
| 3 × 2 × 2-1/2 | Device | 12.5 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 3 × 2 × 2-3/4 | Device | 14 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3 × 2 × 3-1/2 | Device | 18 | 12 | 10 | 9 | 8 | 7 | 6 | 3 |
| 4 × 2-1/8 × 1-1/2 | Device | 10.3 | 6 | 5 | 5 | 4 | 4 | 3 | 2 |
| 4 × 2-1/8 × 1-7/8 | Device | 13 | 8 | 7 | 6 | 5 | 5 | 4 | 2 |
| 4 × 2-1/8 × 2-1/8 | Device | 14.5 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3-3/4 × 2 × 2-1/2 | Masonry | 14 | 9 | 8 | 7 | 6 | 5 | 4 | 2 |
| 3-3/4 × 2 × 3-1/2 | Masonry | 21 | 14 | 12 | 10 | 9 | 8 | 7 | 4 |
Nonmetallic boxes are not in the standard table; their volume is molded or stamped inside (typically 18–22.5 in³ for a single-gang, 32–34 in³ for a two-gang). Use that figure with the per-conductor allowances above.
Worked example. A metal 4-inch square box, 1-1/2 deep (21 in³), with internal clamps, where two 12/2 cables and one 14/2 cable meet and feed one receptacle wired with 12 AWG: 12 AWG conductors 4 × 2.25 = 9.00; 14 AWG conductors 2 × 2.00 = 4.00; clamps 1 × 2.25; device 2 × 2.25 = 4.50; grounds 1 × 2.25. Total 22.0 in³ — just over. Add a raised cover marked with its volume, or use the 2-1/8 deep box.
Conductors 4 AWG and larger follow different rules: boxes are sized by raceway entry dimensions under 314.28, not by volume.
Frequently asked questions
How many 12 AWG wires can go in a single-gang box?
It depends on the box volume. Each 12 AWG conductor needs 2.25 in³. A standard 18 in³ plastic single-gang box holding one receptacle fits two 12/2 cables: 4 conductors + 1 for all grounds + 2 for the device = 7 allowances × 2.25 = 15.75 in³.
How many wires can I put in a 4-inch square box?
A 4 × 1-1/2 inch square box is 21 in³: ten 14 AWG or nine 12 AWG conductors before deductions. A 4 × 2-1/8 deep box is 30.3 in³: fifteen 14 AWG or thirteen 12 AWG.
Do wire nuts and pigtails count in box fill?
No. Wire connectors do not count, and neither do pigtails or jumpers that start and end inside the box.
Do ground wires count in box fill?
All the equipment grounding conductors together count as a single conductor of the largest ground size present, for up to four grounds. From the 2020 NEC, each ground beyond four adds a quarter allowance.
How does a switch or receptacle count?
Each yoke or strap counts as two conductors of the largest size connected to it. A device wider than a single 2-inch gang, such as a range receptacle, counts double for each gang it needs.
Does a plaster ring or extension add volume?
Yes, if it is marked with its volume in cubic inches. Add that figure to the box volume.
NECTables.com is an independent reference and is not affiliated with, endorsed by, or a substitute for the National Fire Protection Association (NFPA). NEC® and National Electrical Code® are registered trademarks of the NFPA. Values shown are commonly published engineering data and independent calculations based on the methods of NFPA 70; they are not a reproduction of the Code. Always confirm against the edition adopted in your jurisdiction, including local amendments, and consult a licensed electrician or engineer before doing electrical work.