Box Fill Calculator
Enter every current-carrying conductor, every grounding conductor, how many switches or receptacles are in the box, and whether it has an internal cable clamp. You get the total volume NEC 314.16(B) requires, checked against your box's real volume.
Current-carrying conductors (hots and neutrals)
Grounding conductors
All grounds together only count as one allowance, sized by the largest ground present.
Each device counts as 2 allowances.
Stamped on the box; pick a common size or enter your own.
Required box volume
15.75 cu in
sum of every allowance below
Fits
✓ within box volume
| Allowance | Count | Per unit | Subtotal |
|---|---|---|---|
| Current-carrying conductors | 4 | 2.25 cu in | 9.00 cu in |
| Grounding conductors (all count as 1, sized by largest: 12 AWG) | 1 | 2.25 cu in | 2.25 cu in |
| Devices (2x each, sized by largest conductor: 12 AWG) | 1 | 4.50 cu in | 4.50 cu in |
Common box scenarios
Typical residential device-box setups and the volume they actually require, against the most common real box sizes.
| Setup | Required volume | Fits 18 cu in box? |
|---|---|---|
| Single switch, one cable in/out (12/2 w/ ground) | 11.25 cu in | Yes |
| Duplex receptacle, feed-through (2x 12/2 w/ ground) | 15.75 cu in | Yes |
| 3-way switch, two cables (12/3 + 12/2, both w/ ground) | 18.00 cu in | Yes |
Common standard box volumes: Single-gang plastic device box (18 cu in); Single-gang plastic device box, deep (22.5 cu in); 4" square metal box, 1-1/2" deep (21.0 cu in); 4" square metal box, 2-1/8" deep (30.3 cu in); 4-11/16" square metal box, 2-1/8" deep (42.0 cu in).
Not applied
Fixture-specific allowances for equipment that isn't a switch or receptacle, ganged multi-device boxes (each additional device in a multi-gang box still adds its own 2x allowance, which this calculator handles, but a physically ganged box's combined volume needs its own stamped rating, not just each single-gang volume added together), and local amendments. Those are exactly what the electrician and the inspector check beyond this calculator.
Frequently Asked Questions
Why do grounding conductors only count once, no matter how many there are?
NEC 314.16(B)(5): all the equipment grounding conductors in a box together count as a single volume allowance, sized by the largest one present. It's a deliberate exception -- without it, a box with several grounds would fill up almost entirely on grounds alone.
Why does a device count as two allowances instead of one?
NEC 314.16(B)(4): each strap containing one or more devices (a switch, a receptacle) counts as two volume allowances, sized by the largest conductor connected to that device. A device physically displaces more room than a single conductor termination, and the code accounts for that with the extra allowance.
Do conductors that just pass through the box without a splice still count?
Yes. Any conductor that runs through the box counts toward fill whether or not it's spliced or terminated there -- it still physically occupies space inside the box. Only conductors that never enter the box at all are excluded.
Where do I find my box's actual volume?
It's usually stamped inside the box itself, or printed on the packaging if it's still available. NEC Table 314.16(A) lists the standard volume for common manufactured box sizes, which is where this calculator's common-size dropdown comes from -- but the number stamped on your specific box always wins if it differs.
What if my box has a fixture (not a switch or receptacle)?
A light fixture or other listed equipment that isn't a switch or receptacle generally doesn't add its own volume allowance the way a device does -- only the conductors feeding it count, the same as any other conductors in the box. This calculator's "devices" field is specifically for switches and receptacles.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.