Box Fill Calculator adds the NEC 314.16 volume for every wire, device, clamp and ground in an electrical box. It also checks your box and names the smallest standard box that fits.
Count each insulated wire that enters the box. Pigtails and wire connectors are not counted.
More Wire Sizes
Add the conductors of any other wire size that enter the same box.
Loops, Fittings and Wide Devices
An unbroken loop 12 in or longer counts twice. Enter those loops here, not under Conductors.
Box Fill Worksheet
| Item | Allowances | Volume |
|---|---|---|
| 12 AWG conductors | 4 | 9.00 in³ |
| Device yokes, two each | 2 | 4.50 in³ |
| Internal cable clamps | 1 | 2.25 in³ |
| Ground wires, all together | 1 | 2.25 in³ |
| Total box fill | 8 | 18.00 in³ |
Box Fill Calculator Rules Under NEC 314.16
Every electrical box has a volume in cubic inches, and everything inside it uses some of that space. NEC 314.16 sets how much. The Box Fill Calculator applies those rules to boxes with conductors of 6 AWG and smaller, which covers nearly every outlet, switch and junction box in a house.
The code does not measure the parts. It counts them in volume allowances, where one allowance is the space set aside for one conductor of a given size. The table shows what each item counts for and which wire size sets its volume.
| Item in the Box | Allowances | Sized By |
|---|---|---|
| Each wire that enters and ends or is spliced | 1 | Its own size |
| Each wire that passes through unbroken | 1 | Its own size |
| Each unbroken loop 12 in or longer | 2 | Its own size |
| Each switch or receptacle yoke | 2 | Largest wire on the device |
| Internal cable clamps, any number | 1 | Largest wire in the box |
| Fixture studs, any number | 1 | Largest wire in the box |
| Hickeys, any number | 1 | Largest wire in the box |
| Up to four ground wires | 1 | Largest ground wire |
| Each ground wire past four | 1/4 | Largest ground wire |
| Each terminal block assembly | 1 | Largest wire on the block |
Box Fill Calculation Formula
Each allowance is worth a fixed volume from the conductor table in 314.16(B). Multiply the allowances of each wire size by its volume and add the results. The total is the smallest box volume the code allows. The Box Fill Calculator does this sum and shows each line of it in the Box Fill Worksheet under the results.
| Wire Size | Volume per Allowance |
|---|---|
| 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 |
The default entries are a receptacle fed by two 12 AWG cables. Four insulated wires count 4. The receptacle yoke counts 2. The internal clamps count 1, and the two grounds together count 1. That is 8 allowances at 2.25 in³ each, so the box needs 18.00 in³.
The same outlet on 14 AWG needs 8 × 2.00, or 16.00 in³. A three-way switch box with one 2-wire and one 3-wire cable has five insulated wires, so it counts 9 allowances and needs 18.00 in³ on 14 AWG.
What Does Not Count in Box Fill
Several things in a box take up room but add nothing to the count. Mike Holt's summary of 314.16 in his Code question and answer newsletter lists them. Pigtails are not counted, because they start and end inside the box. Wire connectors have no allowance in the code either.
Cable connectors with the clamp outside the box do not count, and neither do locknuts or bushings. That is why the Internal Cable Clamps entry has a No option. Fixture wires from a domed canopy get an exception too. Ground wires and up to four fixture wires of 16 AWG or smaller that enter from the canopy can be left out.
Ground Wires After the 2020 Code
For decades all the ground wires in a box counted as one allowance, however many there were. The 2020 NEC changed that. Up to four still count as one allowance. Each ground past four adds a quarter allowance, based on the largest ground wire in the box.
A box with six 12 AWG grounds now counts 1.5 allowances, which is 3.38 in³ where the old rule gave 2.25 in³. A Mike Holt forum thread works the case of ten 12 AWG grounds. Four make one allowance and the other six make 1.5, for 2.5 allowances and 5.625 in³. The Box Fill Calculator gives the same figure when Ground Wires is set to 10.
Box Fill Chart for Standard Metal Boxes
Standard metal boxes have fixed volumes in NEC Table 314.16(A), and the Box Fill Calculator uses that table for its box list. The chart lists the common ones with the most wires each holds when nothing else is inside. A 4 in square box is 4 in on each side and comes 1-1/4, 1-1/2 or 2-1/8 in deep.
| Box | Volume | Most 14 AWG | Most 12 AWG |
|---|---|---|---|
| Device 3 × 2 × 2-1/2 in | 12.5 in³ | 6 | 5 |
| Device 3 × 2 × 3-1/2 in | 18.0 in³ | 9 | 8 |
| Octagon 4 × 1-1/2 in | 15.5 in³ | 7 | 6 |
| Octagon 4 × 2-1/8 in | 21.5 in³ | 10 | 9 |
| Square 4 × 1-1/4 in | 18.0 in³ | 9 | 8 |
| Square 4 × 1-1/2 in | 21.0 in³ | 10 | 9 |
| Square 4 × 2-1/8 in | 30.3 in³ | 15 | 13 |
| Square 4-11/16 × 1-1/4 in | 25.5 in³ | 12 | 11 |
| Square 4-11/16 × 1-1/2 in | 29.5 in³ | 14 | 13 |
| Square 4-11/16 × 2-1/8 in | 42.0 in³ | 21 | 18 |
The box fill of a 4-11/16 in square box is therefore 25.5, 29.5 or 42.0 in³, depending on depth. The volumes are less than length times width times depth, because the table allows for the metal of the box itself. Plastic boxes are not in the table. They go by the volume molded into the box, which you enter under Custom or plastic box.
Checking a Box With a Plaster Ring
A plaster ring, extension ring or domed cover adds to the box when it is marked with a volume. Mike Holt's worked example is a 4 in square box 2-1/8 in deep with a 3.6 in³ ring. The assembly holds 30.3 plus 3.6, or 33.9 in³.
The box contains two receptacles, five 12 AWG wires and two 12 AWG grounds. That is 4 for the yokes, 5 for the wires and 1 for the grounds, or 10 allowances and 22.50 in³. Enter the same items in the Box Fill Calculator with a custom volume of 33.9 in³ and no internal clamps. It returns 22.50 in³ used and 11.40 in³ left. That is room for five more 14 AWG wires, which is the answer Holt gives.
Leave Box to Check on Find the smallest box when you have not bought the box yet. The result names the smallest listed box and gives the smallest device, square and octagon boxes that hold the fill.
Terminal Blocks, Loops and Wide Devices
The 2023 NEC added an allowance for terminal blocks. Electrical Contractor Magazine describes the new 314.16(B)(6). Each terminal block assembly counts as one allowance, based on the largest conductor terminated on it.
Two older rules are easy to miss. A wire that loops through the box unbroken counts once, but a loop at least 12 in long counts twice. The 12 in is twice the 6 in of free conductor that the code requires at each box. A device too wide for a single-gang box, such as a range receptacle, counts two allowances for each gang it covers.
All three are under Loops, Fittings and Wide Devices in the Box Fill Calculator. The same section sets a different size for the ground wires or the device wires when they are not the same as the main wire size.
Boxes With 4 AWG and Larger Wire
Box fill by volume stops at 6 AWG. Boxes that hold 4 AWG and larger conductors are sized by the conduit entering them, under NEC 314.28, and not by cubic inches. The Pull Box Sizing Calculator handles those. To choose the conductor size before you count box fill, use the Wire Size Calculator.