Box fill calculator
A box with four 12 AWG wires, two grounds, one switch or outlet and internal clamps needs 18.00 in³. Each wire takes a fixed volume by size: 18 AWG 1.50 in³, 16 AWG 1.75 in³, 14 AWG 2.00 in³, 12 AWG 2.25 in³, 10 AWG 2.50 in³, 8 AWG 3.00 in³, 6 AWG 5.00 in³. Add the switches, clamps and grounds and compare the total with the volume of the box.
Bare or green, all together
Fits: the 3 × 2 × 3½ in. device box holds 18 in³.
- Box volume
- 18 in³
- Needed
- 18 in³
- Room left
- 0 in³
How it is calculated
- 12 AWG conductors: 4 × 2.25 in³ = 9 in³
- Internal cable clamps: 1 × 2.25 in³ = 2.25 in³
- Devices (2 each): 1 × 4.5 in³ = 4.5 in³
- Grounding conductors (2, counted together): 1 × 2.25 in³ = 2.25 in³
How to calculate box fill
Box fill adds a volume for every wire, device, clamp and ground in a box and compares the total with the volume of the box. The worked numbers use the calculator's default: 4 wires of 12 AWG, 2 grounds, 1 device and internal clamps.
Step 1: volume of the wires
Wire volume = number of wires × volume per wire of that size
Volume per wire: 18 AWG 1.5 in³, 16 AWG 1.75 in³, 14 AWG 2 in³, 12 AWG 2.25 in³, 10 AWG 2.5 in³, 8 AWG 3 in³, 6 AWG 5 in³
Example 4 × 2.25 in³ (12 AWG) = 9 in³
Step 2: clamps and fixture fittings
Internal cable clamps: 1 × the volume of the largest wire, however many clamps
Fixture studs or hickeys: 1 × the largest wire for each kind
Example Clamps: 1 × 2.25 in³ = 2.25 in³
Step 3: switches and receptacles
Each device (per strap): 2 × the volume of the largest wire connected to it
Example 1 × 2 × 2.25 in³ = 4.5 in³
Step 4: ground wires
All ground wires together: 1 × the volume of the largest wire, plus ¼ of it for each ground beyond four
Example 2 grounds: 1 × 2.25 in³ = 2.25 in³
Step 5: total and box size
Required volume = wires + clamps and fittings + devices + grounds
The box volume, as marked or as listed for a standard metal box, must be at least the required volume
Example 9 + 2.25 + 4.5 + 2.25 = 18 in³.
Example A device box 3 × 2 × 3½ in. holds 18 in³, so it fits.
How many wires fit in an electrical box?
A device box 3 × 2 × 3½ in. (18 in³) holds up to 9 wires of 14 AWG or 8 wires of 12 AWG when nothing else is in it. Every device, clamp set and group of grounds takes room from these counts, as in the steps above.
| Box | Volume, in³ | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|---|
| Device 3 × 2 × 1½ in. | 7.5 | 3 | 3 | 3 | 2 | 1 |
| Device 3 × 2 × 2 in. | 10 | 5 | 4 | 4 | 3 | 2 |
| Device 4 × 2⅛ × 1½ in. | 10.3 | 5 | 4 | 4 | 3 | 2 |
| Device 3 × 2 × 2¼ in. | 10.5 | 5 | 4 | 4 | 3 | 2 |
| Device 3 × 2 × 2½ in. | 12.5 | 6 | 5 | 5 | 4 | 2 |
| Device 4 × 2⅛ × 1⅞ in. | 13 | 6 | 5 | 5 | 4 | 2 |
| Device 3 × 2 × 2¾ in. | 14 | 7 | 6 | 5 | 4 | 2 |
| Device 4 × 2⅛ × 2⅛ in. | 14.5 | 7 | 6 | 5 | 4 | 2 |
| Device 3 × 2 × 3½ in. | 18 | 9 | 8 | 7 | 6 | 3 |
| Masonry 3¾ × 2 × 2½ in. | 14 | 7 | 6 | 5 | 4 | 2 |
| Masonry 3¾ × 2 × 3½ in. | 21 | 10 | 9 | 8 | 7 | 4 |
| Round or octagonal 4 × 1¼ in. | 12.5 | 6 | 5 | 5 | 4 | 2 |
| Round or octagonal 4 × 1½ in. | 15.5 | 7 | 6 | 6 | 5 | 3 |
| Round or octagonal 4 × 2⅛ in. | 21.5 | 10 | 9 | 8 | 7 | 4 |
| Square 4 × 4 × 1¼ in. | 18 | 9 | 8 | 7 | 6 | 3 |
| Square 4 × 4 × 1½ in. | 21 | 10 | 9 | 8 | 7 | 4 |
| Square 4 11/16 × 4 11/16 × 1¼ in. | 25.5 | 12 | 11 | 10 | 8 | 5 |
| Square 4 11/16 × 4 11/16 × 1½ in. | 29.5 | 14 | 13 | 11 | 9 | 5 |
| Square 4 × 4 × 2⅛ in. | 30.3 | 15 | 13 | 12 | 10 | 6 |
| Square 4 11/16 × 4 11/16 × 2⅛ in. | 42 | 21 | 18 | 16 | 14 | 8 |
What counts toward box fill
Every insulated wire that enters the box counts once, and so does a wire that passes through without a splice. Short pigtails that stay inside the box do not count.
The calculator uses the largest wire in the box for the clamps, devices and grounds, which errs on the safe side.
Plastic and marked boxes
Plastic boxes and many steel boxes have their volume stamped inside. Use the marked volume when it is there; the sizes in the list are standard metal boxes.