Box fill calculator
Enter what goes in the box. You get the volume the NEC requires, line by line, and every standard metal box that holds it. For plastic boxes, enter the volume stamped inside.
Inputs
Result
| Item | Counts as | cu in |
|---|
Metal box volumes
NEC Table 314.16(A). Green rows hold what you entered.
| Box (inches) | Volume, cu in | 14 AWG max | 12 AWG max |
|---|
Volume per conductor
NEC Table 314.16(B).
| Wire size | cu in 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 |
How box fill is counted
NEC 314.16 gives every wire a volume by its size, then counts everything in the box as a number of wires:
- Each insulated wire that comes into the box counts once. Pigtails that start and end inside the box do not count.
- All the ground wires together count as one wire of the largest ground. From the 2020 NEC, add a quarter for each ground beyond four.
- Each device, a switch or receptacle on one strap, counts as two wires of the largest wire connected to it.
- Internal cable clamps, however many, count as one wire of the largest in the box.
- Each fixture stud or hickey counts as one of the largest wire.
A single-gang box with three 12/2 cables feeding a receptacle: six 12 AWG wires (6), three grounds (1), the receptacle (2) and the clamps (1) make 10 wires of 12 AWG at 2.25 cu in: 22.5 cu in. A 4 x 1-1/2 in square box at 21.0 is too small; a 4 x 2-1/8 in at 30.3 works.
Questions
How many wires can go in a box?
It depends on the box volume and wire size. A 4 x 2-1/8 in square box holds 15 wires of 14 AWG or 13 of 12 AWG, before devices, clamps and grounds are counted. The calculator counts all of it.
How do I count box fill?
Count each wire entering the box as one, all grounds together as one, each switch or receptacle as two, and any internal clamps as one. Multiply by 2.00 cu in for 14 AWG or 2.25 for 12 AWG and compare with the box volume.
Do ground wires count in box fill?
Yes. All the equipment grounding conductors together count as one wire of the largest ground, and under the 2020 and 2023 NEC each ground beyond four adds a quarter more.
Do wire nuts count in box fill?
No. Wire connectors, splicing devices and pigtails that start and end in the box are not counted.
What is the volume of a 4 inch square box?
18.0 cu in at 1-1/4 in deep, 21.0 at 1-1/2 in and 30.3 at 2-1/8 in, by NEC Table 314.16(A). A plaster ring or extension adds the volume marked on it.
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