Box Fill Calculator
Size an electrical box to NEC 314.16(B). Add conductors by gauge, devices, clamps, and grounds for a required volume and a recommended box.
- Pick the NEC edition - choose the code edition your jurisdiction enforces. It changes how grounding conductors are counted.
- Add conductors by size - add a row for each wire gauge entering the box and set the count. Each conductor is counted at its own size per 314.16(B)(1).
- Add devices and fittings - enter switches/receptacles (each counts double), internal clamps, and fixture studs or hickeys.
- Enter grounds - set the number and largest size of equipment grounding conductors. Wire nuts and pigtails are not counted.
- Read the analysis - the panel shows required volume, the minimum box that fits, and a pass/fail if you select your box size.
Box fill is the total volume your conductors, devices, clamps, and grounds require inside an electrical box under NEC 314.16(B). Count each conductor at its own size from Table 314.16(B), add a double allowance for each device, a single allowance for clamps and for each fixture stud or hickey, and a grounding allowance, then choose a box from Table 314.16(A) with at least that volume. A common single-gang device box holds 18 cubic inches; this tool computes your exact requirement and recommends the smallest box that passes.
What Counts Toward Box Fill
NEC Table 314.16(B) Volume Allowances
| Conductor Size (AWG) | Volume Allowance (in³) | Volume (cm³) |
|---|---|---|
| 18 AWG | 1.50 | 24.6 |
| 16 AWG | 1.75 | 28.7 |
| 14 AWG | 2.00 | 32.8 |
| 12 AWG | 2.25 | 36.9 |
| 10 AWG | 2.50 | 41.0 |
| 8 AWG | 3.00 | 49.2 |
| 6 AWG | 5.00 | 81.9 |
Allowances apply to 18 AWG through 6 AWG. Conductors 4 AWG and larger that enter a box must be sized by NEC 314.28, not 314.16.
NEC Table 314.16(A) Common Box Capacities
| Box (trade size) | Volume (in³) | Common Name | Max 12 AWG |
|---|---|---|---|
| 3 × 2 × 2 device | 10.0 | single-gang | 4 |
| 3 × 2 × 2½ device | 12.5 | single-gang deep | 5 |
| 3 × 2 × 3½ device | 18.0 | single-gang deepest | 8 |
| 4 × 4 × 1¼ square | 18.0 | 4-square | 8 |
| 4 × 4 × 1½ square | 21.0 | 1900 box | 9 |
| 4 × 4 × 2⅛ square | 30.3 | 4-square deep | 13 |
| 4-11/16 × 1½ square | 29.5 | 4-11/16 | 13 |
| 4-11/16 × 2⅛ square | 42.0 | 4-11/16 deep | 18 |
A box marked with a volume larger than the table value may be used at its marked capacity. The "Max 12 AWG" column counts conductors only, before clamps, devices, or grounds.
Worked Examples
Each example is computed with the same per-gauge method this calculator uses. A 14-2 cable means two insulated conductors plus a bare ground; a 12-2 means two 12 AWG conductors plus a ground.
| Scenario | What Is Counted | Required Volume | Minimum Box |
|---|---|---|---|
| Single switch, one 14-2 cable, internal clamp | 2 × #14 + clamp + 1 device + 1 ground | 12.0 in³ | 3 × 2 × 2½ (12.5 in³) |
| Receptacle, two 12-2 cables, internal clamps | 4 × #12 + clamp + 1 device + 2 grounds | 18.0 in³ | 4-square or 1900 box |
| Junction box, three 12-3 cables spliced | 9 × #12 + clamp + 3 grounds | 24.75 in³ | 4-11/16 × 1½ (29.5 in³) |
The receptacle case is the textbook NEC example: four conductors, a clamp, a device, and two grounds all reduce to eight 12 AWG allowances, which is 18 cubic inches.
How the Calculation Works
All allowances come from NEC Table 314.16(B). Conductors use their own size; every other item uses the largest conductor in the box, except grounds, which use the largest grounding conductor.
Clamp fill = 1 × largest-conductor allowance if any internal clamp is present
Support fitting = 1 × largest-conductor allowance for each fixture stud or hickey
Device fill = 2 × largest-conductor allowance for each device yoke
Ground fill 2020 = 1 allowance for up to 4 grounds, then +¼ allowance per ground over 4
Ground fill 2017 = 1 allowance for all grounds, plus 1 more for an isolated set
Required volume = conductor + clamp + fitting + device + ground fill
The minimum box is the smallest entry in Table 314.16(A) whose volume is at least the required volume. All volumes are in cubic inches.
Code References
- Conductor fill counts each conductor that enters and terminates, is spliced, or passes through the box once, at its own size from Table 314.16(B). A conductor coiled at least twice the free-conductor length counts twice.NEC (NFPA 70) §314.16(B)(1)
- One or more internal cable clamps count as a single allowance based on the largest conductor in the box. Connectors whose clamping mechanism is outside the box are not counted.NEC (NFPA 70) §314.16(B)(2)
- Each fixture stud or hickey counts as one allowance based on the largest conductor in the box.NEC (NFPA 70) §314.16(B)(3)
- Each device yoke or strap counts as a double allowance based on the largest conductor connected to that device.NEC (NFPA 70) §314.16(B)(4)
- In the 2020 and later editions, up to four grounding conductors count as one allowance and each additional ground adds one-quarter allowance, based on the largest grounding conductor. In the 2017 edition, all grounds count as one allowance, with an additional allowance for an isolated set per 250.146(D).NEC (NFPA 70) §314.16(B)(5)
- Box volumes are taken from Table 314.16(A) or the volume marked on the box. Wire connectors and conductors that do not leave the box are not assigned a volume allowance.NEC (NFPA 70) §314.16(A)
Next Steps
Box fill is one check in a rough-in. These tools cover what usually comes next:
Related Calculators
Box fill is governed by NEC Article 314. Browse the full electrical calculator collection.