Free tool · Electrical
Electrical Box Fill Calculator
Choose the box, add the conductors, devices, clamps and grounds that go in it, and the calculator totals the volume required under NEC 314.16 and tells you whether the box is big enough. If it is not, it names the smallest standard metal box that is.
The numbers stay in your browser: we do not store or receive them.
Conductors entering the box
Count each wire that comes in from outside and is spliced or terminated, and each wire that runs through. Do not count short jumpers that stay inside the box.
Devices, clamps and grounds
Fill required
18.00 cu in
Box volume
21.00 cu in
4 in square, 1-1/2 in deep
Result
Fits
3.00 cu in to spare (85.7% full)
Smallest standard metal box that holds it: 3 × 2 × 3-1/2 in device box or 4 in square, 1-1/4 in deep (18.0 cu in).
| Item | Allowances | Cubic inches |
|---|---|---|
| 4 × 12 AWG conductors | 4 × 2.25 | 9.00 |
| Internal cable clamps (one allowance) | 1 × 2.25 | 2.25 |
| 1 device yoke with up to 12 AWG (two allowances each) | 2 × 2.25 | 4.50 |
| 2 grounding conductors, largest 12 AWG (one allowance) | 1 × 2.25 | 2.25 |
| Total | 18.00 |
How box fill is counted
Each wire size has a volume allowance in Table 314.16(B): 14 AWG is 2.00 cu in, 12 AWG is 2.25, 10 AWG is 2.50, 8 AWG is 3.00 and 6 AWG is 5.00. The box volume, from Table 314.16(A) or marked on the box, must be at least the total.
Conductors: each wire that comes in from outside and is spliced or terminated counts once, and a wire that runs through unbroken counts once. A jumper that starts and ends inside the box does not count. An unbroken loop of at least 12 inches counts twice.
Cable clamps: one or more internal clamps count as one allowance, based on the largest wire in the box. Fixture studs and hickeys: one allowance for each type, based on the largest wire.
Devices: each yoke or strap counts two allowances, based on the largest wire connected to it. A device wider than one gang counts two for each gang.
Grounds: the grounding and bonding conductors together take one allowance, based on the largest one, for up to four of them. Under the 2020 NEC each one beyond four adds a quarter allowance. Before 2020 all of them together counted once, and a box on that edition is figured that way.
A worked example
A 4 in square box 1-1/2 in deep holds 21.0 cu in. Two 12/2 cables with ground bring in four 12 AWG wires (4 × 2.25 = 9.00), the box has internal clamps (2.25), one receptacle (2 × 2.25 = 4.50) and the grounds (2.25). The total is 18.00 cu in, so it fits with 3.00 to spare.
Add a second device and the total is 22.50, which is 1.50 over. A 4-11/16 in square box 1-1/4 in deep (25.5 cu in) is the smallest standard box that holds it.
A common tight case is a 3 × 2 × 2-1/2 in device box (12.5 cu in) with two 14 AWG cables in and out (4 × 2.00 = 8.00), a receptacle (4.00) and the grounds (2.00). That is 14.00, which is too full. The 3 × 2 × 2-3/4 in box (14.0 cu in) holds it exactly.
What this does not do
The built-in boxes are the standard metal boxes in Table 314.16(A). A plastic box, an extension ring or a listed special box has its own marked volume, so choose Other and enter it. It does not count terminal blocks (added in the 2023 NEC), the exceptions for small luminaire wires, or box depth and free-conductor length rules. Conductors 4 AWG and larger are sized under 314.28 for pull and junction boxes. Check the result against the code edition your jurisdiction has adopted.
FAQ
How do I calculate box fill?
Add up the volume allowance for every conductor, clamp, fitting, device and ground in the box using Table 314.16(B), then compare the total to the box volume. The calculator does the counting and the comparison.
Does a receptacle count as a wire?
It counts as two wires of the largest size connected to it, one yoke at a time. A device wider than one gang counts two for each gang.
Do the ground wires count?
Yes. Under the 2020 NEC up to four grounding conductors share one allowance based on the largest, and each extra one adds a quarter allowance. Before 2020 they all counted as one.
Do pigtails count?
A conductor that starts and ends inside the box does not count. The wires that come in from outside and are spliced to it do.
What volume does a plastic box have?
Nonmetallic boxes are marked with their volume in cubic inches. Choose Other box and enter the number printed on it.
Is anything I type saved?
No. The calculation runs in your browser and nothing is stored or sent to us.
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