Box Fill Calculator
This box fill calculator checks whether a junction box or device box is large enough for the wires, devices, and clamps inside it, using the NEC 314.16 counting rules. Enter the largest wire size, the conductors, devices, grounds, and clamps, pick the box, and the box fill calculator returns the volume required, the fill percentage, and whether the box passes.
Based on NEC 314.16. Your inspector and local amendments have the final say.
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The Box Fill Calculator is part of our Cable & Protection collection. Browse more cable & protection to size, convert, and check your work alongside this tool.
What the box fill calculator does
Every electrical box has a volume in cubic inches, and every wire, device, and clamp inside it uses part of that volume. Overfilled boxes trap heat and damage insulation when the wires are folded in. The box fill calculator counts each item the way NEC 314.16 does, multiplies by the volume allowance for the wire size, and compares the total with the box you picked. The same rules apply to a junction box, an outlet box, or a switch box.
NEC box fill rules (314.16)
- Each insulated conductor that starts or ends in the box counts once. A wire that passes through without a splice also counts once.
- Each device yoke (switch or receptacle) counts twice, based on the largest wire connected to it.
- All internal cable clamps together count once.
- Equipment grounding conductors: up to four count once together; each additional one adds a quarter.
- Each fixture stud or hickey counts once.
- Pigtails and wires that stay entirely inside the box do not count.
Allowances per conductor: 18 AWG 1.50 in³, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, and 6 AWG 5.00.
Box fill calculator example
A receptacle box with two 12/2 cables: four insulated conductors, two grounds, one receptacle, and internal clamps:
A 4 in square box 1-1/2 in deep holds 21.0 in³, so it passes at 85.7% fill with 3.0 in³ to spare. A 3 x 2 x 2-1/2 in device box at 12.5 in³ would be too small.
How to use the junction box fill calculator
Choose the largest conductor size in the box. Enter the insulated conductors that enter the box, both hots and neutrals. Add the number of switches or receptacles, the grounding conductors, whether the box has internal cable clamps, and any fixture studs or hickeys. Pick the box size, or choose Other and type its marked volume. Press Calculate.
Reading your electrical box fill calculation
A fill of 100% or less passes the code; above 100% you need a larger box, an extension ring, or fewer conductors. Plaster rings, extension rings, and raised covers marked with a volume add to the box volume. Leaving some room spare makes the wiring easier to fold in and cooler in service, especially with 12 AWG and larger wire or bulky dimmers.
When to use an electrical box size calculator
- Choosing a box for a new outlet or switch
- Adding a circuit through an existing junction box
- Checking a box before a rough-in inspection
- Fitting a smart switch or dimmer into an old box
- Sizing a box for a light fixture with a stud
Common box fill calculator mistakes
- Counting each ground separately. Up to four count once together.
- Counting a device once. Each yoke counts twice.
- Forgetting internal clamps. They count once in total.
- Using the box size instead of its marked volume. Use the stamped cubic inches when shown.
Tips for NEC box fill
- Choose a deeper box for dimmers and smart switches.
- Add an extension ring when an old box is too small.
- Use the volume stamped inside the box when it differs from the table.
Box fill rules for mixed wire sizes
When a box holds more than one wire size, each conductor counts at the allowance for its own size, while devices, clamps, and grounds use the largest conductor tied to them. A box with a 12 AWG circuit and a 14 AWG circuit counts the 12s at 2.25 in³ and the 14s at 2.00 in³. This calculator applies the largest size to everything, which errs on the safe side. Larger wire also means a larger breaker, so check the circuit with the breaker size calculator.
Junction box size for pull boxes and large conductors
For 4 AWG and larger, NEC 314.28 sizes boxes by the raceway size instead of volume: a straight pull needs a length of at least eight times the largest raceway, and an angle pull six times. Between boxes, the raceway itself has fill limits, which the conduit fill calculator checks. Choose the conductor gauge first with the wire size calculator, since it sets every allowance in the box.
Box fill chart: maximum conductors by box size
Conductor allowances that fit with no devices or clamps:
| Box | Volume (in³) | 14 AWG | 12 AWG | 10 AWG |
|---|---|---|---|---|
| Device box 3 x 2 x 2-1/2 in | 12.5 | 6 | 5 | 5 |
| Device box 3 x 2 x 3-1/2 in | 18.0 | 9 | 8 | 7 |
| 4 in octagon x 1-1/2 in | 15.5 | 7 | 6 | 6 |
| 4 in octagon x 2-1/8 in | 21.5 | 10 | 9 | 8 |
| 4 in square x 1-1/2 in | 21.0 | 10 | 9 | 8 |
| 4 in square x 2-1/8 in | 30.3 | 15 | 13 | 12 |
| 4-11/16 in square x 2-1/8 in | 42.0 | 21 | 18 | 16 |
Box Fill Calculator FAQ
How many wires can go in a junction box?
Divide the box volume by the allowance for the wire size. A 4 in square box 1-1/2 in deep (21.0 in³) holds 10 allowances of 14 AWG or 9 of 12 AWG, before devices and clamps.
How do grounds count in box fill?
Up to four equipment grounding conductors count as one allowance together. Each one beyond four adds a quarter allowance.
Do wire nuts count in box fill?
No. Wire connectors, pigtails that stay in the box, and the ground screw do not count.
How does a device count in box fill?
Each switch or receptacle yoke counts as two allowances, based on the largest wire connected to it.