NEC Junction Box Fill Calculator
Check if an electrical box is big enough under NEC 314.16. Count wires, grounds, clamps and devices, see a visual fill meter and find the smallest box that fits.
1. Box
Volume from NEC Table 314.16(A) for metal boxes.
Optional. Add the volume marked on it.
2. Conductors
Count each insulated wire that enters the box and ends or is spliced inside it, and each wire that passes through. Do not count pigtails or wire nuts.
| Wire size | Wires | Loops 12 in+ |
|---|---|---|
| 18 AWG1.50 in³ | ||
| 16 AWG1.75 in³ | ||
| 14 AWG2.00 in³ | ||
| 12 AWG2.25 in³ | ||
| 10 AWG2.50 in³ | ||
| 8 AWG3.00 in³ | ||
| 6 AWG5.00 in³ |
An unbroken loop of 12 in (300 mm) or more counts as two.
3. Grounding conductors
Up to 4 grounding wires count as one allowance. Each one after that adds ¼.
4. Clamps and fittings
Each type counts once, based on the largest wire in the box.
All clamps inside the box count once together. Connectors fixed outside the box do not count.
5. Devices
Each device yoke counts as two allowances, based on the largest wire connected to it.
- 1Largest wire on it
The box is big enough
18.00 in³ needed, 21.00 in³ available
Box fill
3.00 in³ spare
Breakdown
| Item | Allowances | Each | Volume |
|---|---|---|---|
| 12 AWG conductors | 4 | 2.25 in³ | 9.00 in³ |
| 12 AWG grounding wires | 1 | 2.25 in³ | 2.25 in³ |
| Cable clamps | 1 | 2.25 in³ | 2.25 in³ |
| Device, 1 gang (12 AWG) | 2 | 2.25 in³ | 4.50 in³ |
| Total needed | 18.00 in³ | ||
| Box volume | 21.00 in³ | ||
Smallest standard metal boxes that fit
Click a box to use it.
What counts toward box fill
- AInternal cable clamps: 1 for all clamps
- BWire that enters and ends on a device: 1
- CWire spliced inside the box: 1 each
- DPigtails and wire nuts: 0
- EUnbroken loop 12 in or longer: 2
- FGrounding wires: 1 for up to 4, then ¼ each
- GDevice on a yoke: 2
A short unbroken wire that passes straight through counts as 1. Clamps and fittings use the volume of the largest wire in the box.
Based on NEC 2023, section 314.16, for wire sizes 18 to 6 AWG. Check which code edition your area uses.
Disclaimer: We are not licensed electricians or electrical inspectors, and this calculator is not legal or professional advice. Results follow NEC 2023 section 314.16 and are for general information only. Local codes and code editions vary, so always confirm with a licensed electrician and your local inspection authority before you install.
Frequently Asked Questions
About this calculator
A box fill calculator checks whether an electrical box is big enough for the wires, devices and fittings inside it. In the United States, the National Electrical Code (NEC) sets these rules in section 314.16. Each wire and part takes up a set amount of space, called a volume allowance, and the total must not be more than the volume of the box. An overfilled box is hard to wire safely, traps heat, can damage wire insulation and will fail an inspection. This junction box fill calculator follows the 2023 NEC and updates the result as you type.
How to calculate box fill
- Count the wires. Each insulated wire that enters the box and ends or is spliced inside counts as 1. A wire that passes through without a break counts as 1, and an unbroken loop of 12 in or more counts as 2. Pigtails and wire nuts do not count.
- Add the grounding wires. Up to 4 grounding wires count as 1, and each extra one adds ¼.
- Add 1 for internal cable clamps, and 1 for each type of luminaire fitting (stud or hickey). Both use the largest wire in the box.
- Add 2 for each device yoke, such as a switch or a receptacle. This uses the largest wire connected to that device.
- Multiply each count by the volume for its wire size and add everything up. The box volume must be equal to or larger than this total.
Box fill chart: volume per wire size
NEC Table 314.16(B) gives the space that one wire of each size takes up:
- 18 AWG: 1.50 in³ (24.6 cm³)
- 16 AWG: 1.75 in³ (28.7 cm³)
- 14 AWG: 2.00 in³ (32.8 cm³)
- 12 AWG: 2.25 in³ (36.9 cm³)
- 10 AWG: 2.50 in³ (41.0 cm³)
- 8 AWG: 3.00 in³ (49.2 cm³)
- 6 AWG: 5.00 in³ (81.9 cm³)
Example: a receptacle in a device box
Two 12/2 NM cables come into a 3 × 2 × 3½ in metal device box (18.0 in³) with internal clamps, and one receptacle is installed. The 4 insulated wires count as 4 × 2.25 = 9.00 in³. The 2 grounding wires count as one allowance: 2.25 in³. The clamps add 2.25 in³, and the receptacle adds 2 × 2.25 = 4.50 in³. The total is 18.00 in³, which is exactly the box volume, so the box is big enough.
Example: a junction box with splices
Four 12/2 NM cables meet in a 4 × 1½ in square box (21.0 in³) with clamps, and the wires are spliced together. The 8 insulated wires need 18.00 in³, the 4 grounding wires need 2.25 in³ and the clamps need 2.25 in³. The total is 22.50 in³, which is more than 21.0 in³, so the box is too small. A 4¹¹⁄₁₆ × 1¼ in square box (25.5 in³) or a 4 × 2⅛ in square box (30.3 in³) would work.
How many wires can go in a junction box?
If a box only holds wires of one size and nothing else, divide the box volume by the volume for that size and round down. A 4 × 1½ in square box (21.0 in³) can hold 10 wires of 14 AWG or 9 wires of 12 AWG. A 3 × 2 × 3½ in device box (18.0 in³) can hold 9 wires of 14 AWG or 8 wires of 12 AWG. Clamps, grounding wires and devices also take space, so in a real box the number of wires you can add is usually lower.
Plastic boxes and extension rings
NEC Table 314.16(A) only lists metal boxes. Plastic (nonmetallic) boxes have their volume marked inside, so choose Plastic / other and enter that number. If you add a plaster ring, an extension ring or a domed cover that is marked with a volume, you can add that volume to the box. The calculator has a field for this.
Grounding wires: the 2020 and 2023 rule
In the 2017 NEC and earlier, all grounding wires in a box counted together as a single allowance. Since the 2020 NEC, up to four grounding wires still count as one, but each extra grounding wire adds a quarter of an allowance. For example, six 12 AWG grounding wires count as 1.5 allowances, which is 3.375 in³. The 2023 NEC keeps this rule. If your area still uses an older edition, your grounding count may be a little lower.
- Every NEC 314.16 rule — Counts conductors, long loops, grounding wires, clamps, luminaire fittings and devices, each with the correct allowance.
- All standard metal boxes — Includes every box in NEC Table 314.16(A), plus your own volume for plastic boxes and extension rings.
- Visual fill meter — See how full the box is, which parts take the most space and how far over the limit you are.
- Box suggestions — Shows the smallest standard boxes that fit your wiring, and you can switch to one with a click.