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.
How to Use
- Pick the NEC edition - choose the edition adopted by your jurisdiction. The current national edition may not be the locally adopted edition, and grounding or terminal-block rules differ by edition.
- Add conductors by size - add each conductor gauge and count. Ordinary terminating, spliced, and pass-through conductors count once at their own size; qualifying long unbroken loops or coils belong under Advanced and count twice.
- Add devices and fittings - enter device yokes or required mounting gangs, internal clamps, and any luminaire stud or hickey types that are present.
- Enter grounds and terminal blocks - set the number and largest size of entering equipment grounding conductors. For NEC 2023/2026, add terminal-block assemblies by the largest conductor terminated to each group.
- Compare box volume - use a listed Table 314.16(A) box, a custom/marked volume, and any qualifying marked ring, cover, or extension volume. The result is a 314.16 volume check, not a complete installation approval.
Quick Answer
Box fill is the volume required by conductors, device yokes or mounting gangs, internal clamps, support-fitting types, grounding conductors, and applicable terminal blocks under NEC 314.16(B). Count ordinary conductors at their own size, count qualifying long unbroken loops or coils twice, add the applicable component allowances, then compare the required volume with usable box volume under 314.16(A). This tool performs that volume check and can show the smallest listed Table 314.16(A) base-box capacity that meets it.
What Counts Toward Box Fill
NEC Box-Fill Volume Allowances by Conductor Size
| 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 |
In the 2026 NEC these values appear in Table 314.16(B)(4); earlier editions use earlier table numbering. This calculator supports 18 AWG through 6 AWG. Boxes involving 4 AWG and larger conductors can also require NEC 314.28 dimensional checks outside this tool.
Common NEC Table 314.16(A) 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 |
Selected common entries are shown here; the calculator box picker includes the complete 2026 Table 314.16(A) metal-box list. The "Max 12 AWG" values apply before allowances for clamps, devices, grounds, support fittings, or terminal blocks.
Worked Examples
Each example uses the same per-gauge and component rules as the calculator. 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 | Example Listed Capacity / Box |
|---|---|---|---|
| Single switch, one 14-2 cable, internal clamp | 2 × #14 + clamp + 1 yoke + 1 ground | 12.0 in³ | 12.5 in³ · 3 × 2 × 2½ device box |
| Receptacle, two 12-2 cables, internal clamp | 4 × #12 + clamp + 1 yoke + 2 grounds | 18.0 in³ | 18.0 in³ · 4 × 4 × 1¼ square box |
| Junction box, three 12-3 cables spliced | 9 × #12 + clamp + 3 grounds | 24.75 in³ | 25.5 in³ · 4-11/16 × 1¼ square box |
| Wide device using two mounting gangs, #12 conductors | 4 × #12 + 2 mounting gangs + 2 grounds | 20.25 in³ | 21.0 in³ · 4 × 4 × 1½ square box |
These are volume examples only. A box that meets the 314.16 volume requirement can still be subject to separate depth, clearance, listing, and installation requirements.
How the Calculation Works
NEC 314.16(B) adds the applicable volume categories. The calculator keeps each conductor-bearing group at its own gauge so mixed sizes are not collapsed into one largest-wire estimate.
Conductor fill = sum of (count × allowance) for each gauge; terminating, spliced, and pass-through conductors each count once
Long loop / coil = 2 × allowance at that gauge for each qualifying unbroken loop or coil
Clamp fill = 1 × largest-conductor-present allowance if one or more internal cable clamps are present
Support fittings = 1 × largest-conductor-present allowance if luminaire stud type is present, plus 1 more if hickey type is present
Device fill = 2 × each yoke or required mounting gang × the allowance for that group’s largest connected conductor
Ground fill 2020+ = 1 allowance for up to 4 entering grounds, then +¼ allowance per additional ground, based on the largest entering EGC
Ground fill 2017 = 1 allowance for the entering ground set; an additional isolated-ground set receives its own allowance based on that set’s largest conductor
Terminal block 2023+ = 1 allowance per terminal-block assembly, based on the largest conductor terminated to each group
Marked added volume = qualifying marked ring, cover, or extension volume is added to a specifically selected or custom box capacity, not to required fill
Required volume = conductor + loop + clamp + support-fitting + device + ground + applicable terminal-block fill
Long loop / coil = 2 × allowance at that gauge for each qualifying unbroken loop or coil
Clamp fill = 1 × largest-conductor-present allowance if one or more internal cable clamps are present
Support fittings = 1 × largest-conductor-present allowance if luminaire stud type is present, plus 1 more if hickey type is present
Device fill = 2 × each yoke or required mounting gang × the allowance for that group’s largest connected conductor
Ground fill 2020+ = 1 allowance for up to 4 entering grounds, then +¼ allowance per additional ground, based on the largest entering EGC
Ground fill 2017 = 1 allowance for the entering ground set; an additional isolated-ground set receives its own allowance based on that set’s largest conductor
Terminal block 2023+ = 1 allowance per terminal-block assembly, based on the largest conductor terminated to each group
Marked added volume = qualifying marked ring, cover, or extension volume is added to a specifically selected or custom box capacity, not to required fill
Required volume = conductor + loop + clamp + support-fitting + device + ground + applicable terminal-block fill
Auto mode reports the smallest Table 314.16(A) base-box capacity that meets the calculated volume and lists tied candidates at that capacity. It does not assume that a marked ring, cover, or extension is compatible with every listed box. That is a volume result, not a declaration that every other NEC requirement is satisfied.
Special Box-Fill Cases
- Conductors that terminate, are spliced, or pass through the box each count once; a qualifying long unbroken loop or coil counts twice. A conductor that remains wholly inside the box does not receive a separate conductor-fill allowance.
- Small fittings such as locknuts, splicing connectors, and bushings do not receive a separate allowance. A wholly internal pigtail has no conductor-fill allowance, but its gauge can still matter when it is the conductor connected to a device yoke.
- Devices wider than a single 2-inch device box use the number of mounting gangs required. Barriered boxes must be calculated compartment by compartment; if a securely installed barrier is not marked with its volume, the 2026 NEC treats it as occupying 0.5 in³ when metal or 1.0 in³ when nonmetallic. This calculator does not infer compartment geometry or apportionment.
- Boxes involving 4 AWG and larger conductors can also require NEC 314.28 pull/junction-box dimensional checks outside this calculator.
Code References
- NFPA 70 Article 314 provides the box-volume, box-fill, outlet/device-box dimension, and pull/junction-box provisions used to define this calculator’s scope. The calculator applies 314.16 volume rules for its supported 18 AWG through 6 AWG inputs; applicable 314.24 depth/clearance requirements and 314.28 dimensional requirements remain separate checks.NFPA 70, 2026 Edition · Article 314
Next Steps
Box fill is one check in a rough-in. These tools cover what usually comes next:
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Frequently Asked Questions
How do you calculate box fill?
Add the applicable NEC 314.16(B) volume allowances. Ordinary terminating, spliced, and pass-through conductors count once at their own size; qualifying long unbroken loops or coils count twice. Add device-yoke or mounting-gang allowances, one allowance for internal clamps, one allowance for each support-fitting type present, the applicable grounding allowance, and any terminal-block allowance for the selected edition. Compare the total with usable box volume under 314.16(A).
Do neutrals count for box fill?
Yes. An insulated neutral that enters or leaves the box is counted once at its own conductor size for box-fill purposes. In a 12-2 cable, both the black and white insulated conductors contribute 12 AWG conductor allowances. This box-fill count is separate from whether a neutral is treated as current-carrying for ampacity adjustment rules.
Do grounds count in box fill?
Yes, and the rule depends on the NEC edition. Under the 2020 and later codes, up to four entering equipment grounding conductors use one allowance and each additional ground adds one-quarter allowance, based on the largest entering EGC. Under the 2017 code, the entering ground set uses one allowance; an additional isolated-ground set permitted by that edition receives another allowance based on the largest conductor in that additional set.
Do wire nuts count in box fill?
No separate allowance is added for ordinary small fittings such as locknuts, splicing connectors, or bushings in the 2026 NEC 314.16(B) workflow. A pigtail that remains wholly inside the box also has no separate conductor-fill allowance, although its gauge can still determine a device-yoke allowance when it is connected to that device.
How many wires fit in a 1900 box?
A 1900 box, commonly a 4 × 4 × 1½ inch square box, has a 21.0 cubic-inch Table 314.16(A) volume. The table shows nine 12 AWG conductors or ten 14 AWG conductors only when no additional allowances are required for clamps, devices, grounds, support fittings, or terminal blocks. A full box-fill calculation can reduce the available conductor count.
How many wires can I put in a 4-square box?
A 4 × 4 × 1¼ inch square box has 18.0 cubic inches in Table 314.16(A), while the 4 × 4 × 2⅛ version has 30.3 cubic inches. The table’s raw conductor counts assume no additional clamp, device, ground, support-fitting, or terminal-block allowances, so use the full calculator when those items are present.
What is the difference between box fill and box volume?
Box volume is the usable physical capacity established by the box marking or NEC 314.16(A), including qualifying marked added volume where applicable. Box fill is the volume required by the items counted under 314.16(B). The 314.16 volume check passes when required box-fill volume is less than or equal to usable box volume, but other requirements, including applicable 314.24 depth and clearance rules, can still govern the installation.
Do long wire loops count twice in box fill?
A qualifying unbroken conductor loop or coil counts twice for conductor fill under NEC 314.16(B)(1). Use the Long loops / coils controls under Advanced only for loops that meet the code threshold for double counting; ordinary conductors should stay in the main conductor rows.
Updated Sep 2026 · See our Methodology
This calculator performs an NEC 314.16 box-fill volume check for 18 AWG through 6 AWG conductors, including edition-aware grounding rules and applicable terminal-block fill. It does not fully verify NEC 314.24 device-box depth/clearance requirements, product listings, local amendments, or NEC 314.28 dimensions for boxes involving 4 AWG and larger conductors. Box markings, equipment details, and the adopted NEC edition govern final work. Verify permitted installations with the AHJ or a qualified electrical professional. See our Data Sources and Methodology.