Box fill calculator
Count the wires, grounds and devices going into an electrical box and see whether it's big enough, or the smallest box that is.
The wires need
18.00 cubic inches
Fits, with 0.00 cu in to spareToo full: over by 0.00 cu in
- Box volume
- 18.00 cu in (100% used)
- Smallest box of this kind that fits
- 3 x 2 in device box, 3-1/2 in deep (18.0 cu in)
- Insulated wires
- 9.00 cu in
- Grounds
- 2.25 cu in
- Devices
- 4.50 cu in
- Clamps and fittings
- 2.25 cu in
An estimate to plan with. Follow your local code (the NEC or your state's version); your inspector has the final say.
How to calculate box fill
Every wire and part in the box takes up a set amount of space, based on wire size. Add them up and compare the total with the box's volume, plus any mud ring, extension ring or domed cover that's marked with its volume.
- Insulated wires: each one that comes into the box and ends or is spliced there counts once, and so does one that passes straight through. Short pigtails that never leave the box don't count.
- Grounds: one wire's worth for up to 4 ground wires, at the largest ground's size, plus a quarter for each one after that (2020 NEC and later). Under the 2017 NEC and earlier, all the grounds together count once.
- Each device (a switch or receptacle on one strap): two wires' worth, at the largest wire connected to it. A device wider than one gang counts that much for each gang.
- Internal cable clamps: one wire's worth in total, at the largest wire in the box. Each kind of fixture stud or hickey also counts one.
Worked example
A receptacle in a 3 × 2 × 3-1/2 in box (18.0 cu in) with two 12/2 cables: four 12 AWG wires at 2.25 cu in each is 9.00 cu in; the two grounds count once, 2.25; the receptacle counts twice, 4.50; and the clamps once, 2.25. That's 18.00 cu in, exactly the box's 18.0, so it fits with nothing to spare.
Box fill chart
The most wires of one size each standard metal box holds with nothing else in it. Take off two for each device, one for the grounds and one for clamps, at the same size.
| Box | Cu in | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|---|
| 4 in round/octagon, 1-1/4 in deep | 12.5 | 6 | 5 | 5 | 4 | 2 |
| 4 in round/octagon, 1-1/2 in deep | 15.5 | 7 | 6 | 6 | 5 | 3 |
| 4 in round/octagon, 2-1/8 in deep | 21.5 | 10 | 9 | 8 | 7 | 4 |
| 4 in square, 1-1/4 in deep | 18.0 | 9 | 8 | 7 | 6 | 3 |
| 4 in square, 1-1/2 in deep | 21.0 | 10 | 9 | 8 | 7 | 4 |
| 4 in square, 2-1/8 in deep | 30.3 | 15 | 13 | 12 | 10 | 6 |
| 4-11/16 in square, 1-1/4 in deep | 25.5 | 12 | 11 | 10 | 8 | 5 |
| 4-11/16 in square, 1-1/2 in deep | 29.5 | 14 | 13 | 11 | 9 | 5 |
| 4-11/16 in square, 2-1/8 in deep | 42.0 | 21 | 18 | 16 | 14 | 8 |
| 3 x 2 in device box, 1-1/2 in deep | 7.5 | 3 | 3 | 3 | 2 | 1 |
| 3 x 2 in device box, 2 in deep | 10.0 | 5 | 4 | 4 | 3 | 2 |
| 3 x 2 in device box, 2-1/4 in deep | 10.5 | 5 | 4 | 4 | 3 | 2 |
| 3 x 2 in device box, 2-1/2 in deep | 12.5 | 6 | 5 | 5 | 4 | 2 |
| 3 x 2 in device box, 2-3/4 in deep | 14.0 | 7 | 6 | 5 | 4 | 2 |
| 3 x 2 in device box, 3-1/2 in deep | 18.0 | 9 | 8 | 7 | 6 | 3 |
| 4 x 2-1/8 in handy box, 1-1/2 in deep | 10.3 | 5 | 4 | 4 | 3 | 2 |
| 4 x 2-1/8 in handy box, 1-7/8 in deep | 13.0 | 6 | 5 | 5 | 4 | 2 |
| 4 x 2-1/8 in handy box, 2-1/8 in deep | 14.5 | 7 | 6 | 5 | 4 | 2 |
| 3-3/4 x 2 in masonry box, 2-1/2 in deep | 14.0 | 7 | 6 | 5 | 4 | 2 |
| 3-3/4 x 2 in masonry box, 3-1/2 in deep | 21.0 | 10 | 9 | 8 | 7 | 4 |
Volume per wire
| Wire size | Cubic inches each |
|---|---|
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
Mud ring volumes
Typical marked volumes for raised single-device covers on a 4 in square box. Use the number stamped on yours; an unmarked ring adds nothing.
| Raised | Cubic inches |
|---|---|
| 1/4 in | 1.5 |
| 1/2 in | 3.5 |
| 5/8 in | 4.5 |
| 3/4 in | 5.5 |
| 1 in | 7.5 |
| 1-1/4 in | 9.5 |
| 1-1/2 in | 11.3 |
| 2 in | 14.5 |
Sources: volume per wire and box and ring volumes from the Hubbell RACO, Steel Boxes, Covers and Accessories catalog, Box Fill table (the same per-wire figures appear in other box makers' catalogs and the City of Republic, MO, Electrical Box Fill Calculations handout); the 2020 ground rule from EC&M, Code Quiz: Box Volume Calculations and EGCs (2020 NEC change). Plastic boxes and many others are marked with their own volume; use that number.
Box fill questions
How many wires can go in a 4 inch square box?
A 4 in square box 2-1/8 in deep (30.3 cu in) holds 15 wires of 14 AWG or 13 of 12 AWG with nothing else in it. Grounds, devices and clamps take room too, so enter everything above. A mud ring marked with its volume adds space: a 1/2 in raised ring is typically 3.5 cu in.
How many wires can go in a single gang box?
It depends on its depth and volume. A 3 × 2 × 3-1/2 in metal box (18.0 cu in) holds 9 wires of 14 AWG or 8 of 12 AWG alone, but a switch or receptacle takes two of those spaces and the grounds one. Plastic boxes are stamped with their volume inside; choose "Other" and enter it.
What size junction box do I need?
Enter your wires, grounds, devices and clamps, and the calculator shows the smallest standard box of the same kind that holds them. For example, a switch with 14/2 cables in and out needs 16.00 cu in, which is more than a 2-1/2 in deep device box (12.5 cu in); the smallest device box that fits is the 3 x 2 in device box, 3-1/2 in deep (18.0 cu in). A box that's more than full is hard to close up and can damage the insulation.
Do ground wires count in box fill?
Yes, but not one for one. Under the 2020 NEC and later, up to 4 grounds count as one wire of the largest ground's size, and each one past 4 adds a quarter. With eight 12 AWG wires, six grounds and two receptacles, that's 3.38 cu in for the grounds, enough to put a 4 in square 2-1/8 in box over by 0.07 cu in; under the 2017 rule the grounds count 2.25 and it fits. Choose the rule your area uses.
Does a mud ring count toward box fill?
Only if it's marked with its volume. Add the marked cubic inches of the ring, an extension ring or a domed cover to the box. A ring with no volume stamped on it adds nothing.
What about wire size and conduit fill?
Box fill only checks space. The wire still has to be the right size for the breaker; see the wire size calculator. For wires in a pipe between boxes, use the conduit fill calculator.