How this box fill calculator works
NEC 314.16 counts volume allowances, not wires: each thing in the box uses a share of the largest conductor’s volume, and the total can’t exceed the box:
fill = allowances × volume-per-conductor ≤ box volume
At the defaults — 12 AWG, 4 conductors, one device, a ground, no clamps — that is 7 allowances, 15.75 in³, which fits an 18 in³ box with 2.25 in³ to spare; the full chain is in the worked example. Size the box to the fill, then round up to the next box.
What counts, and how much
The trick is that the four things in a box each count differently — get the rules straight and the arithmetic is easy:
- Conductors: each current-carrying wire (hot, neutral) counts as one, by its own gauge. A pigtail that stays in the box doesn’t count.
- Devices: each yoke or strap — a receptacle or switch — counts as two of the largest conductor.
- Clamps: all internal cable clamps together count as one.
- Grounds: all equipment grounds together count as one, however many enter.
- Not counted: wire connectors, device screws, and the raised plaster ring’s marked volume adds to the box.
Reading the box, and the largest conductor
Two things trip people up — where the volume number comes from, and which wire sets the allowance:
- Box volume: stamped inside the box in cubic inches; a plaster ring or extension adds its own marked volume.
- Largest conductor rules: devices, clamps and grounds all use the biggest wire in the box — one 10 AWG raises every allowance.
- Standard sizes: a single-gang plastic box is often 18–22.5 in³; a metal device box near 12.5.
- Round up: if the fill is over, step to a deeper box or a larger gang.
- This is fill, not conduit: conduit fill (raceways) is a different NEC calculation.
When the fill comes out over, you have two honest moves: a deeper box of the same gang, or a plaster ring that adds its stamped volume to the box below it. A raised device ring can add several cubic inches — sometimes enough to turn an over-filled box legal without opening the wall wider. What you can’t do is ignore it: an overstuffed box traps heat, strains the connections, and is one of the first things an inspector checks.
Volume per conductor, and common box sizes
Table 1 is the allowance per conductor by wire size; Table 2 is the volume of common boxes.
| Wire | in³ each |
|---|---|
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
| Box | in³ |
|---|---|
| Metal device 3×2×2.5 | 12.5 |
| Plastic single-gang | 18–22.5 |
| 4×4×1.5 square | 21 |
| 4×4×2.125 square | 30.3 |
How to check box fill
- Read the box volume off the inside stamp, and add a plaster ring’s marked volume if there is one.
- Pick the largest wire size in the box — it sets the device, clamp and ground allowances.
- Count the current-carrying conductors — hots and neutrals through and landing.
- Add the devices, clamps and grounds — each by its rule.
- Compare the fill to the box, and step up a size if it’s over.
Worked example: a device box on a 12 AWG circuit
The default, worked line by line — 12 AWG (2.25 in³ each), 4 conductors, one receptacle, a ground, no clamps, in an 18 in³ box:
fill = 7 × 2.25 = 15.75 in³
box = 18 in³ → fits, 2.25 in³ spare
add a second device = 9 × 2.25 = 20.25 → needs a bigger box
The spare is thin: add one more cable (two conductors) or a second device and the 18 in³ box is over, and you step to a 22.5 in³ box or a two-gang. That is the whole value of running the number — a box that’s legal on paper before the wires are ever crammed in.
What this box fill calculation doesn’t cover
Box fill is one NEC rule; a compliant install has others:
- Conduit fill. The percentage-fill rule for conductors in a raceway — a different calculation.
- Conductor bending space. Minimum wire-bending room at terminals in larger boxes.
- Ampacity and derating. Wire sizing and bundling adjustments — see the voltage drop calculator for the run.
- Pigtail counting. Conductors that originate and terminate inside the box are not counted.
- Local amendments. Jurisdictions can adopt different NEC editions — build to yours.
Box fill FAQ
How do you calculate box fill?
Add the NEC 314.16 volume allowances and compare to the box volume. Each current-carrying conductor counts as one allowance for its wire size; each device (receptacle or switch) counts as two of the largest conductor; all internal clamps together count as one; all equipment grounds together count as one. The total must not exceed the box’s cubic inches. The default — 4 twelve-gauge conductors, a device and a ground — is 7 allowances, 15.75 in³, which fits an 18 in³ box.
How many wires can I put in a box?
It depends on the wire size and what else is in the box. A plain 18 in³ box with 12 AWG holds 8 conductors (18 ÷ 2.25); add a receptacle and a ground and it drops to 5, because the device eats two allowances and the grounds one. Bigger wire takes more room per conductor, so fewer fit.
What counts toward box fill?
Conductors, devices, clamps and grounds — but each in its own way. A conductor passing through or landing counts once; a pigtail that stays inside the box does not count. A strap (device) counts as two. All cable clamps together are one allowance; all grounds together are one. Wire connectors and the device’s own screws do not count.
How much volume does each wire take in box fill?
Per NEC Table 314.16(B): 14 AWG is 2.0 in³, 12 AWG 2.25, 10 AWG 2.5, 8 AWG 3.0, and 6 AWG 5.0. Devices, clamps and grounds all use the allowance of the largest conductor in the box — so mixing a big wire in raises the cost of every device and clamp count.
How many 12 gauge wires fit in an 18 cubic inch box?
Eight if the box holds nothing but conductors — 18 ÷ 2.25. In a real device box with one receptacle and a ground, subtract two allowances for the device and one for the grounds, leaving room for five 12 AWG conductors. The calculator does the subtraction so you can see what actually fits.
Do ground wires count in box fill?
Yes, but all of them together count as a single allowance of the largest ground, no matter how many grounds enter the box — one bundle, one allowance. A second, separate set of isolated grounds counts as one more. Grounds are easy to forget, and they are the difference between a box that passes and one that doesn’t.
Sources and standards
The whole method is one code section:
- NEC 314.16(A): box volumes and the fill-not-to-exceed rule.
- NEC 314.16(B): the volume allowances — conductor, device (×2), clamp (1), grounds (1).
- Table 314.16(B): the cubic-inch allowance per conductor by wire size.
- The AHJ governs: the adopted NEC edition and any local amendment — build to your jurisdiction.
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