Box Fill Chart
Use this quick lookup chart to compare common box volumes, conductor-only capacity, and practical box choices before you run the full NEC 314.16 calculation.
A box fill chart lists the maximum number of conductors an electrical box can legally hold under NEC 314.16, by box volume and wire gauge. As a quick answer, a standard 18 cu in single-gang device box holds up to 9 × 14 AWG, 8 × 12 AWG, or 7 × 10 AWG conductors before deductions for devices, clamps, and grounds. Use the box × gauge chart below for every common box size.
Sizing a box instead (NEC box size)? Multiply your conductor count by the per-gauge volume — 2.00 cu in (14 AWG), 2.25 (12 AWG), 2.50 (10 AWG) — then choose a box with at least that many cubic inches, or let the box fill calculator size it and apply the device, clamp, and ground deductions for you.
What This Box Fill Chart Is Good For
This page is a fast planning reference for electricians, estimators, and contractors who want to compare common box sizes before they open the full calculator. It shows conductor-only capacity by box volume so you can rule out boxes that are obviously too small.
It is not a substitute for a full NEC 314.16 calculation. Devices, internal clamps, support fittings, grounding conductors, and terminal blocks can all reduce the real capacity of a box, which is why this chart works best as a quick screening tool.
Electrical box volume calculator: box capacity vs. required fill
Box capacity is the nominal or marked volume in cubic inches (in³). The box-fill check passes when required fill, the sum of all applicable volume allowances, is less than or equal to that capacity. The table below gives standard nominal volumes; use the box volume calculator to calculate the required fill for your wiring.
Two numbers, two different jobs
Available volume comes from Table 314.16(A) for the matching standard metal box, or the manufacturer's volume marking for the actual box. Match the shape and depth, not just the face size: a square box and an octagonal box can have different capacities. For a plastic or special box, use its marked volume rather than borrowing a similar metal box's number. Adding wires changes required fill; it does not change the marked capacity.
Required fill adds these separate items, using the counting rules in the NEC 314.16 code reference:
- Counted conductors: each wire uses its own AWG allowance; 14 AWG is 2.00 in³ and 12 AWG is 2.25 in³.
- Internal cable clamps: one allowance total when present, based on the largest conductor in the box.
- Device yokes: two allowances per yoke under 314.16(B)(4), based on the largest conductor connected to that yoke. A duplex receptacle on one strap is one yoke; see the device-yoke box-fill guide.
- Grounding conductors: this site's calculator groups them into one allowance. The reference uses the largest grounding conductor; the single-gauge calculator uses the selected AWG.
- Other fittings: the calculator adds one allowance per support-fitting type (stud or hickey), plus its separate terminal-block allowance when applicable.
Standard metal-box volumes: Table 314.16(A)
The 14 AWG maximum is volume ÷ 2.00, rounded down. These are pure-conductor counts with no other deductions: no yokes, grounds, internal clamps, or fittings. Use the existing multi-gauge charts below when comparing wire sizes.
| Box type | Nominal volume (in³) | 14 AWG maximum (conductors only) |
|---|---|---|
| 4" x 1-1/4" Round/Octagonal | 12.5 | 6 |
| 4" x 1-1/2" Round/Octagonal | 15.5 | 7 |
| 4" x 2-1/8" Round/Octagonal | 21.5 | 10 |
| 4" x 1-1/4" Square | 18.0 | 9 |
| 4" x 1-1/2" Square | 21.0 | 10 |
| 4" x 2-1/8" Square | 30.3 | 15 |
| 4-11/16" x 1-1/4" Square | 25.5 | 12 |
| 4-11/16" x 1-1/2" Square | 29.5 | 14 |
| 4-11/16" x 2-1/8" Square | 42.0 | 21 |
| 3" x 2" x 1-1/2" Device Box | 7.5 | 3 |
| 3" x 2" x 2" Device Box | 10.0 | 5 |
| 3" x 2" x 2-1/4" Device Box | 10.5 | 5 |
| 3" x 2" x 2-1/2" Device Box | 12.5 | 6 |
| 3" x 2" x 2-3/4" Device Box | 14.0 | 7 |
| 3" x 2" x 3-1/2" Device Box | 18.0 | 9 |
Worked example: 21.0 in³ square box
A 4" × 1-1/2" square box contains three counted 12 AWG insulated conductors, one switch or receptacle yoke connected to 12 AWG, one 12 AWG ground, and internal clamps. No support fittings or terminal blocks are present. Every allowance here uses 2.25 in³, matching the calculator's single-gauge inputs.
- Conductors: 3 × 2.25 = 6.75 in³.
- One yoke: 2 × 2.25 = 4.50 in³.
- Ground: 1 × 2.25 = 2.25 in³.
- Internal clamps, together: 1 × 2.25 = 2.25 in³.
- Support fittings + terminal blocks: 0.00 + 0.00 = 0.00 in³.
Required fill = 15.75 in³ ≤ 21.00 in³: COMPLIANT for box fill, with 5.25 in³ remaining.
The 22.5 in³ boundary: keep the same yoke, ground, and clamps, but use six counted 12 AWG conductors in a box marked 22.5 in³. Required fill is (6 + 2 + 1 + 1) × 2.25 = 22.50 in³: COMPLIANT, exactly at capacity. A seventh counted conductor raises it to 24.75 in³: NON-COMPLIANT, exceeding the marking by 2.25 in³. The conductors alone still fit; the complete allowance total does not.
Quick Capacity Chart
The counts below are conductor-only reference values. Real installations usually need a larger box once devices, grounds, clamps, or splices are included.
| Box Type | Volume | 14 AWG | 12 AWG | 10 AWG | Best Use |
|---|---|---|---|---|---|
| Handy Box 1-7/8" Deep 4" x 2-1/8" x 1-7/8" | 13.0 cu.in. | 6 | 5 | 5 | Surface mount devices |
| Single Gang Device Box 18 cu.in. 3" x 2" x 3-1/2" | 18.0 cu.in. | 9 | 8 | 7 | Light switches |
| 2-Gang Device Box 2-3/4" Deep 4" x 3" x 2-3/4" | 28.0 cu.in. | 14 | 12 | 11 | Dual switches |
| 4" Square x 1-1/4" Deep 4" x 4" x 1-1/4" | 18.0 cu.in. | 9 | 8 | 7 | Junction boxes |
| 4" Square x 1-1/2" Deep 4" x 4" x 1-1/2" | 21.0 cu.in. | 10 | 9 | 8 | Junction boxes |
| 4" Square x 2-1/8" Deep 4" x 4" x 2-1/8" | 30.3 cu.in. | 15 | 13 | 12 | High-capacity junction |
| 4-11/16" Square x 1-1/2" Deep 4-11/16" x 4-11/16" x 1-1/2" | 29.5 cu.in. | 14 | 13 | 11 | Commercial installations |
| 4-11/16" Square x 2-1/8" Deep 4-11/16" x 4-11/16" x 2-1/8" | 42.0 cu.in. | 21 | 18 | 16 | High-capacity commercial |
Maximum Conductors by Box Size and Wire Gauge
The number below each gauge is how many same-size conductors a bare box holds under NEC 314.16 — the box volume divided by the Table 314.16(B) allowance (14 AWG = 2.00, 12 AWG = 2.25, 10 AWG = 2.50, 8 AWG = 3.00, 6 AWG = 5.00 cu.in.), rounded down.
| Box (bare) | Volume | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|---|
| Round Pan Box 1/2" Deep 4" Round x 1/2" | 4.0 cu.in. | 2 | 1 | 1 | 1 | 0 |
| Handy Box 1-1/2" Deep 4" x 2-1/8" x 1-1/2" | 10.3 cu.in. | 5 | 4 | 4 | 3 | 2 |
| Handy Box 1-7/8" Deep 4" x 2-1/8" x 1-7/8" | 13.0 cu.in. | 6 | 5 | 5 | 4 | 2 |
| FS Box Single Gang 4" x 2" x 2" | 13.5 cu.in. | 6 | 6 | 5 | 4 | 2 |
| Single Gang Device Box 2-3/4" Deep 3" x 2" x 2-3/4" | 14.0 cu.in. | 7 | 6 | 5 | 4 | 2 |
| Handy Box 2-1/8" Deep 4" x 2-1/8" x 2-1/8" | 14.5 cu.in. | 7 | 6 | 5 | 4 | 2 |
| Octagon Box 1-1/2" Deep 4" Octagon x 1-1/2" | 15.5 cu.in. | 7 | 6 | 6 | 5 | 3 |
| 4" Square x 1-1/4" Deep 4" x 4" x 1-1/4" | 18.0 cu.in. | 9 | 8 | 7 | 6 | 3 |
| Single Gang Device Box 18 cu.in. 3" x 2" x 3-1/2" | 18.0 cu.in. | 9 | 8 | 7 | 6 | 3 |
| PVC Single Gang Box 18 cu.in. 3" x 2-1/4" x 3-3/4" | 18.0 cu.in. | 9 | 8 | 7 | 6 | 3 |
| FD Box Single Gang 4" x 2" x 2-5/8" | 18.0 cu.in. | 9 | 8 | 7 | 6 | 3 |
| PVC Single Gang Box 20 cu.in. 3" x 2-1/4" x 4" | 20.0 cu.in. | 10 | 8 | 8 | 6 | 4 |
| PVC Round Ceiling Box 20 cu.in. 4" Round x 2-1/4" | 20.0 cu.in. | 10 | 8 | 8 | 6 | 4 |
| 4" Square x 1-1/2" Deep 4" x 4" x 1-1/2" | 21.0 cu.in. | 10 | 9 | 8 | 7 | 4 |
| Masonry Box 3-1/2" Deep 3-3/4" x 4" x 3-1/2" | 21.0 cu.in. | 10 | 9 | 8 | 7 | 4 |
| Octagon Box 2-1/8" Deep 4" Octagon x 2-1/8" | 21.5 cu.in. | 10 | 9 | 8 | 7 | 4 |
| PVC Single Gang Box 22 cu.in. 3" x 2-1/4" x 4-1/4" | 22.5 cu.in. | 11 | 10 | 9 | 7 | 4 |
| FS Box 2-Gang 4" x 4" x 2" | 24.0 cu.in. | 12 | 10 | 9 | 8 | 4 |
| 4-11/16" Square x 1-1/4" Deep 4-11/16" x 4-11/16" x 1-1/4" | 25.5 cu.in. | 12 | 11 | 10 | 8 | 5 |
| 2-Gang Device Box 2-3/4" Deep 4" x 3" x 2-3/4" | 28.0 cu.in. | 14 | 12 | 11 | 9 | 5 |
| 4-11/16" Square x 1-1/2" Deep 4-11/16" x 4-11/16" x 1-1/2" | 29.5 cu.in. | 14 | 13 | 11 | 9 | 5 |
| 4" Square x 2-1/8" Deep 4" x 4" x 2-1/8" | 30.3 cu.in. | 15 | 13 | 12 | 10 | 6 |
| PVC 2-Gang Box 32 cu.in. 4" x 3-1/4" x 3" | 32.0 cu.in. | 16 | 14 | 12 | 10 | 6 |
| Old Work Box 2-Gang 32 cu.in. 4" x 3" x 3" | 32.0 cu.in. | 16 | 14 | 12 | 10 | 6 |
| 2-Gang Device Box 3-1/2" Deep 4" x 3" x 3-1/2" | 36.0 cu.in. | 18 | 16 | 14 | 12 | 7 |
| 4-11/16" Square x 2-1/8" Deep 4-11/16" x 4-11/16" x 2-1/8" | 42.0 cu.in. | 21 | 18 | 16 | 14 | 8 |
| 3-Gang Device Box 2-3/4" Deep 5-5/8" x 3" x 2-3/4" | 42.0 cu.in. | 21 | 18 | 16 | 14 | 8 |
| PVC 3-Gang Box 44 cu.in. 5-3/4" x 3-1/4" x 3" | 44.0 cu.in. | 22 | 19 | 17 | 14 | 8 |
| 4-Gang Device Box 2-1/8" Deep 7-1/4" x 3" x 2-1/8" | 50.0 cu.in. | 25 | 22 | 20 | 16 | 10 |
| PVC 4-Gang Box 60 cu.in. 7-1/2" x 3-1/4" x 3" | 60.0 cu.in. | 30 | 26 | 24 | 20 | 12 |
How many wires fit in common boxes?
How many wires fit in a 4-inch square, 1-1/2 in deep?
A 21.0 cu.in. box holds 9 twelve-gauge or 10 fourteen-gauge conductors in a bare box (21.0 ÷ 2.25 = 9; 21.0 ÷ 2.00 = 10). Subtract two per device and one for clamps or grounds.
How many wires fit in a 4-inch square, 2-1/8 in deep?
A 30.3 cu.in. box holds 13 twelve-gauge or 15 fourteen-gauge conductors in a bare box (30.3 ÷ 2.25 = 13; 30.3 ÷ 2.00 = 15). Subtract two per device and one for clamps or grounds.
How many wires fit in a 3 x 2 x 3-1/2 in device box?
A 18.0 cu.in. box holds 8 twelve-gauge or 9 fourteen-gauge conductors in a bare box (18.0 ÷ 2.25 = 8; 18.0 ÷ 2.00 = 9). Subtract two per device and one for clamps or grounds.
How many wires fit in a 4-inch octagon, 1-1/2 in deep?
A 15.5 cu.in. box holds 6 twelve-gauge or 7 fourteen-gauge conductors in a bare box (15.5 ÷ 2.25 = 6; 15.5 ÷ 2.00 = 7). Subtract two per device and one for clamps or grounds.
Volume Comparison
These bars make it easier to compare how fast box volume grows when you move from a single-gang device box to square and large-format junction boxes.
Handy Box 1-7/8" Deep
4" x 2-1/8" x 1-7/8"
14 AWG
6 max
12 AWG
5 max
10 AWG
5 max
Single Gang Device Box 18 cu.in.
3" x 2" x 3-1/2"
14 AWG
9 max
12 AWG
8 max
10 AWG
7 max
2-Gang Device Box 2-3/4" Deep
4" x 3" x 2-3/4"
14 AWG
14 max
12 AWG
12 max
10 AWG
11 max
4" Square x 1-1/4" Deep
4" x 4" x 1-1/4"
14 AWG
9 max
12 AWG
8 max
10 AWG
7 max
4" Square x 1-1/2" Deep
4" x 4" x 1-1/2"
14 AWG
10 max
12 AWG
9 max
10 AWG
8 max
4" Square x 2-1/8" Deep
4" x 4" x 2-1/8"
14 AWG
15 max
12 AWG
13 max
10 AWG
12 max
4-11/16" Square x 1-1/2" Deep
4-11/16" x 4-11/16" x 1-1/2"
14 AWG
14 max
12 AWG
13 max
10 AWG
11 max
4-11/16" Square x 2-1/8" Deep
4-11/16" x 4-11/16" x 2-1/8"
14 AWG
21 max
12 AWG
18 max
10 AWG
16 max
Fast Field Picks
Single switch or receptacle box
Typical quick-check target: around 14.0 to 18.0 cu.in. depending on conductor size and device fill.
Practical choice: 18 cu.in. single-gang device box
It gives more room than a shallow box when the location has feed-through conductors, a GFCI, or a smart device body.
Feed-through GFCI or smart receptacle
Typical quick-check target: around 15.75 to 18.0 cu.in. on 12 AWG layouts.
Practical choice: 18 cu.in. single-gang box or deeper
A GFCI yoke, grounding bundle, and onward cable quickly consume the reserve that smaller one-gang boxes appear to offer.
Standard junction box with multiple splices
Typical quick-check target: around 18.0 to 21.0 cu.in. for common 12 AWG splice layouts.
Practical choice: 4-inch square x 1-1/2-inch deep box
The 21.0 cu.in. square box is a reliable baseline when a box is doing real splice work instead of only supporting one device.
Large junction, feeder splice, or crowded transition box
Typical quick-check target: 30.3 cu.in. and up once 10 AWG, 8 AWG, or multiple circuits are involved.
Practical choice: 4-inch square x 2-1/8-inch deep or 4-11/16-inch square box
Larger boxes leave reserve for bending space, connector bulk, and future re-termination instead of forcing an exact-limit install.
Important Notes Before You Use the Chart
- This chart is based on conductor volume allowances and box volume only. Device yokes, internal clamps, support fittings, grounds, and terminal blocks can change the final answer.
- The 14 AWG and 12 AWG counts shown in the table are conductor-only capacities from the marked box volume. They are not permission to fill a working box with that many conductors plus devices.
- 10 AWG counts on this page are quick reference values calculated from box volume at 2.50 cu.in. per conductor. Always verify the exact conductor mix and fittings in the real box.
- Use this chart to narrow your box choice fast, then use the main box fill calculator to confirm compliance on the actual layout.
Box Fill Chart FAQ
Is this box fill chart enough to prove NEC compliance?
No. It is a quick planning chart, not a final compliance tool. NEC 314.16 requires you to count the real conductors, grounds, clamps, devices, and other fittings in the actual box.
Why can the marked box volume look sufficient but still fail box fill?
The marking can exceed the wire-only subtotal while being smaller than the complete required fill. Add yokes, grounds, internal clamps, and applicable fittings before comparing. For example, seven 12 AWG conductors need 15.75 in³ alone, but one yoke, one ground allowance, and internal clamps raise the total to 24.75 in³, exceeding a 22.5 in³ box.
Can I use the conductor-only maximum shown in the chart for a receptacle box?
Not usually. A receptacle or switch yoke adds two conductor allowances by itself, and feed-through conductors, grounds, and clamps also consume box volume. The chart is only a first-pass screen.
What should I open after I use this chart?
Use the main Box Fill Calculator for the exact count, then cross-check conductor allowances in the Wire Gauge Chart and code rules in the NEC Code Reference.
What does electrical box capacity mean?
Box capacity is the available volume in cubic inches, taken from the applicable standard-box table or the volume marked on the box. Required fill is the separate sum of conductor and component allowances; it must not exceed that capacity.
How do I use the cubic inches marked on my box?
Enter that number as the box volume in the calculator, then enter the actual conductor gauge, conductor count, yokes, grounds, internal clamps, and fittings. The marking supplies the capacity side of the comparison; it does not tell you how much fill your wiring requires.
Use the Chart for Speed, Then Run the Exact Count
The chart helps you reject obviously undersized boxes in seconds. Use the full calculator before rough-in is finalized so conductor size, yokes, grounds, clamps, and other allowances are all counted correctly.
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