TryBuildCalcJunction Box Fill Calculator (NEC 314.16)
Check a junction box's fill.
🕒 Last updated: September 2, 2026
Inputs
ℹ️Standard NEC Table 314.16(A) metal box sizes. Nonmetallic boxes and any box not in the table must be marked by the manufacturer with their actual capacity — use Custom for those.
Conductors
Clamps & Fittings
Device/Equipment Straps
A strap/yoke holding a switch, receptacle, or other device — a duplex receptacle or a 2-gang switch on one strap still counts as ONE strap, not two. Each allowance is based on the largest conductor actually connected to that specific strap, which can be smaller than the box's overall largest conductor.
Equipment Grounding Conductors
ℹ️Total equipment grounding/bonding conductors entering the box. Up to 4 share one volume allowance; each one beyond 4 adds only a 1/4 allowance (NEC 314.16(B)(5), 2020+).
This box fits your fill:
18 of 30.3 cu in (59.4%)Spare capacity: 12.3 cu in
Volume Allowance Breakdown
Box: 4" x 2-1/8" Square (30.3 cu in)
Conductor fill: 13.5 cu in (6× 12 AWG)
Internal clamp fill: 2.25 cu in (1 allowance, largest conductor: 12 AWG)
Equipment grounding conductor fill: 2.25 cu in (3× 12 AWG, 1 allowance)
Total required volume: 18 cu in
Assumptions Used
NEC Table 314.16(A) (standard box volumes), NEC Table 314.16(B) and 314.16(B)(1)-(5) (box fill calculation rules, 2020+ EGC quarter-allowance rule). Clamp and support-fitting fill use the largest conductor present anywhere in the box (314.16(B)(2)/(3)); device/equipment strap fill instead uses the largest conductor actually connected to each specific strap (314.16(B)(4)), entered separately per strap group above. This is a reference estimate — confirm against the code edition adopted in your jurisdiction and a licensed electrician before final installation.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Electrical Box Fill Calculator.
Junction box fill (splices only, no devices)
A pure junction box — used only for splicing conductors, with no switches or receptacles — skips the device-strap allowance entirely, but often has more conductors than a typical device box.
This page is pre-set to a 4x2-1/8" square box with 6x 12 AWG conductors — edit the inputs above to match your actual splicing scenario.
- Set device/equipment straps to 0 for a pure junction box with no switches or receptacles.
- Junction boxes often need a larger size than device boxes simply because they consolidate more conductors from multiple circuits in one place.
- Confirm the junction box remains accessible after installation, per NEC 314.29 — it can't be permanently concealed behind finished surfaces.
Box Fill Formula: How Is It Determined?
Four separate volume allowances, summed and compared against the box's actual capacity.
Step 1 — Conductor Fill (NEC 314.16(B)(1))
For each conductor size present: Volume += Count × Table 314.16(B) allowance for that size
Each conductor that originates outside the box and terminates or is spliced within it counts once, at its own size's allowance — a conductor that both originates and terminates inside the box (never leaving it) is not counted.
Step 2 — Clamp & Support Fitting Fill (NEC 314.16(B)(2)-(3))
Internal clamp(s) present: Volume += 1 × allowance for the largest conductor anywhere in the box
Fixture stud present: Volume += 1 × allowance for the largest conductor anywhere in the box
Hickey present: Volume += 1 × allowance for the largest conductor anywhere in the box
These three allowances are based on the single largest conductor present anywhere in the box (including the equipment grounding conductor, if it happens to be the largest) — a fixture stud and a hickey each get their own allowance if both are present, but multiple studs alone still only count once.
Step 3 — Device/Equipment Strap Fill (NEC 314.16(B)(4))
For each strap group: Volume += Straps × 2 × allowance for the largest conductor CONNECTED TO THAT STRAP
This is a genuinely different basis than Step 2 — 314.16(B)(4) sizes each strap's allowance from the largest conductor actually connected to the device(s) on that specific yoke, not the box's overall largest conductor. A box with a large pass-through conductor that never connects to any device should size its device-strap fill from the smaller, actually-connected conductor instead.
Step 4 — Equipment Grounding Conductor Fill (NEC 314.16(B)(5), 2020+)
Up to 4 EGCs: Volume += 1 × allowance for the largest EGC
Each EGC beyond 4: Volume += 0.25 × allowance for the largest EGC
Total Required Volume = Step 1 + Step 2 + Step 3 + Step 4. The box fits if this total does not exceed its actual (or marked) cubic-inch capacity.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Box type | 4" x 2-1/8" Square | Sets the available cubic-inch capacity |
| Conductors | 6× 12 AWG | Each sized at its own Table 314.16(B) allowance |
Step 1 — Conductor Fill (NEC 314.16(B)(1))
| Calculation | Result |
|---|---|
| 6 × 12 AWG conductor(s) × 2.25 cu in | 13.5 cu in |
| Conductor fill subtotal | 13.5 cu in |
Step 2 — Clamp & Support Fitting Fill (NEC 314.16(B)(2)-(3))
| Calculation | Result |
|---|---|
| Internal clamp(s): 1 × largest conductor in box (12 AWG, 2.25 cu in) | 2.25 cu in |
| Clamp + fitting fill subtotal | 2.25 cu in |
Step 3 — Device/Equipment Strap Fill (NEC 314.16(B)(4))
| Calculation | Result |
|---|---|
| No device/equipment straps present — no allowance needed | |
| Device/equipment strap fill subtotal | 0 cu in |
Step 4 — Equipment Grounding Conductor Fill (NEC 314.16(B)(5), 2020+)
| Calculation | Result |
|---|---|
| 3 × 12 AWG EGC — up to 4 share 1 allowance = 1 allowance × 2.25 cu in | 2.25 cu in |
| Equipment grounding conductor fill subtotal | 2.25 cu in |
Total Required Volume
| Step | Volume (cu in) |
|---|---|
| Step 1 — Conductor fill | 13.5 |
| Step 2 — Clamp + fitting fill | 2.25 |
| Step 3 — Device/equipment strap fill | 0 |
| Step 4 — Equipment grounding conductor fill | 2.25 |
| Total required volume vs. box capacity | 18 of 30.3 cu in (59.4%) |
Therefore, this box requires 18 cu in against its 30.3 cu in capacity (59.4% fill) — it fits, with room to spare.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.