How to use the box fill calculator
Pick your box from the list of standard metal boxes, or choose “Plastic or other box” and enter the volume stamped inside it. Count the wires of each gauge that come into the box (hots, neutrals and switch travelers, but not the grounds) and enter them by size. Then enter the number of ground wires and the size of the largest one, the switches or outlets, any fixture studs or hickeys, and whether the box has clamps inside.
This is a planning check. Your local code edition and the inspector have the final say.
The formula (NEC 314.16(B))
Each item takes a volume allowance from Table 314.16(B):
| Wire size | Allowance per wire |
|---|---|
| 14 AWG | 2.00 cu in |
| 12 AWG | 2.25 cu in |
| 10 AWG | 2.50 cu in |
| 8 AWG | 3.00 cu in |
| 6 AWG | 5.00 cu in |
- Wires: one allowance for each wire that enters the box and is spliced or ends there, or passes through
- Clamps: one allowance for all internal cable clamps, at the largest wire size in the box
- Fixture studs and hickeys: one allowance for each type, at the largest wire size
- Devices: two allowances for each yoke (switch or outlet)
- Grounds: one allowance for up to four, at the largest ground’s size, plus a quarter for each one past four
- Volume needed = the sum, which must not exceed the box’s volume
The code sizes a device’s allowance by the largest wire connected to it. The calculator uses the largest wire in the box, which is the same or larger, so it never undercounts.
Worked example: two 12/2 cables and one 14/2 in a 4 in square box
- Wires: 4 × 2.25 + 2 × 2.00 = 13 cu in
- Clamps 2.25 + fixture stud 2.25 = 4.5 cu in
- One device: 2 × 2.25 = 4.5 cu in
- Three grounds, largest 12 AWG: 2.25 cu in
- Total 24.25 cu in. The 4 × 1-1/2 in square box holds 21 cu in, so it’s too full by 3.25 cu in. A 4-11/16 × 1-1/4 in square box (25.5 cu in) fits.
Standard metal box volumes
Volumes from NEC Table 314.16(A). The wire counts assume nothing else in the box: no devices, clamps or grounds.
| Box | Volume | 14 AWG wires | 12 AWG wires |
|---|---|---|---|
| 3 × 2 × 2-1/2 in device | 12.5 cu in | 6 | 5 |
| 3 × 2 × 3-1/2 in device | 18 cu in | 9 | 8 |
| 4 × 2-1/8 × 1-7/8 in handy box | 13 cu in | 6 | 5 |
| 4 × 1-1/2 in round/octagon | 15.5 cu in | 7 | 6 |
| 4 × 2-1/8 in round/octagon | 21.5 cu in | 10 | 9 |
| 4 × 1-1/2 in square | 21 cu in | 10 | 9 |
| 4 × 2-1/8 in square | 30.3 cu in | 15 | 13 |
| 4-11/16 × 2-1/8 in square | 42 cu in | 21 | 18 |
Common mistakes
- Forgetting the devices. A switch or outlet takes two allowances, as much room as two wires.
- Counting every ground separately. Up to four grounds count as one wire; only those past four add a quarter each.
- Counting pigtails. Wires that start and end inside the box don’t count.
- Guessing a plastic box’s volume. Read the number stamped inside it; similar-looking boxes differ.
Frequently asked questions
How many wires can go in a single-gang box?
It depends on the box's volume. A 3 × 2 × 3-1/2 in metal device box (18 cu in) holds 9 wires of 14 AWG or 8 of 12 AWG with nothing else in it. Add a switch (2 allowances) and the grounds (1) and it holds 6 of 14 AWG or 5 of 12 AWG.
Do ground wires count toward box fill?
Yes. Up to four grounds together count as one wire of the largest ground's size. Since the 2020 NEC, each ground past four adds a quarter allowance, so 8 grounds of 12 AWG count 2 × 2.25 = 4.5 cu in.
Do pigtails and jumpers count?
No. Only wires that come into the box from outside count, plus any that pass through. A pigtail that starts and ends inside the box takes no allowance.
How do I find the volume of a plastic box?
Read it off the box: the code requires every nonmetallic box, and any box of 100 cu in or less that isn't in Table 314.16(A), to be marked with its volume. Pick "Plastic or other box" and enter that number.
What if my box is too full?
Use a deeper or bigger box, or add volume on top of it. Extension rings, plaster rings and domed covers marked with their volume add to the box's volume. Moving a splice to another box also works.
Why does a switch count as two wires?
NEC 314.16(B)(4) gives each yoke or strap a double allowance, based on the largest wire connected to it. A device wider than a single 2 in device box counts double for every gang it needs.
Can I use this for my inspection?
Use it to plan the box before you buy it. Your local code edition and the inspector have the final say, and some areas still use editions without the quarter allowance for extra grounds.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.