TryBuildCalcElectrical Box Fill Calculator (NEC 314.16)
Check whether your box has enough volume instantly.
🕒 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.
ℹ️Not the box's overall largest conductor — only the largest conductor actually connected to the device(s) on this specific strap.
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:
10.25 of 21 cu in (48.8%)Spare capacity: 10.75 cu in
Volume Allowance Breakdown
Box: 4" x 1-1/2" Square (21 cu in)
Conductor fill: 4 cu in (2× 14 AWG)
Device/equipment strap fill: 4 cu in (1× strap @ 14 AWG, each strap × double allowance based on the conductor connected to it)
Equipment grounding conductor fill: 2.25 cu in (1× 12 AWG, 1 allowance)
Total required volume: 10.25 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.
What Is an Electrical Box Fill Calculator?
An electrical box fill calculator checks whether an outlet, switch, or junction box has enough internal volume for everything packed inside it — conductors, internal cable clamps, fixture studs or hickeys, device straps, and equipment grounding conductors — per NEC 314.16. Each item gets a specific volume allowance from Table 314.16(B), based on the largest conductor present; the sum must not exceed the box's actual cubic-inch capacity from Table 314.16(A) (or its manufacturer-marked capacity for nonmetallic and other unlisted boxes).
Overfilling a box is a genuinely common code violation — it's easy to add "just one more" device or conductor without re-checking the volume math, especially in remodel work where an existing box is reused for a heavier load than it was originally sized for.
Why the calculation has several distinct parts:
- Each conductor is counted at its OWN size's allowance — mixed conductor sizes aren't collapsed into one number
- Clamps and support fittings use a shared allowance based on the LARGEST conductor anywhere in the box — but device/equipment straps use the largest conductor connected to THAT specific strap, which can be smaller
- A fixture stud and a hickey each get their own allowance if both are present — but multiple studs alone still only count once
- The equipment grounding conductor rule changed in the 2020 NEC — up to 4 EGCs share one allowance, with only a 1/4 allowance for each one beyond that
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 1-1/2" Square | Sets the available cubic-inch capacity |
| Conductors | 2× 14 AWG | Each sized at its own Table 314.16(B) allowance |
Step 1 — Conductor Fill (NEC 314.16(B)(1))
| Calculation | Result |
|---|---|
| 2 × 14 AWG conductor(s) × 2.00 cu in | 4 cu in |
| Conductor fill subtotal | 4 cu in |
Step 2 — Clamp & Support Fitting Fill (NEC 314.16(B)(2)-(3))
| Calculation | Result |
|---|---|
| No internal clamps, fixture stud, or hickey present — no allowance needed | |
| Clamp + fitting fill subtotal | 0 cu in |
Step 3 — Device/Equipment Strap Fill (NEC 314.16(B)(4))
| Calculation | Result |
|---|---|
| 1 strap × 2 × largest conductor CONNECTED TO THAT STRAP (14 AWG, 2.00 cu in) | 4 cu in |
| Device/equipment strap fill subtotal | 4 cu in |
Step 4 — Equipment Grounding Conductor Fill (NEC 314.16(B)(5), 2020+)
| Calculation | Result |
|---|---|
| 1 × 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 | 4 |
| Step 2 — Clamp + fitting fill | 0 |
| Step 3 — Device/equipment strap fill | 4 |
| Step 4 — Equipment grounding conductor fill | 2.25 |
| Total required volume vs. box capacity | 10.25 of 21 cu in (48.8%) |
Therefore, this box requires 10.25 cu in against its 21 cu in capacity (48.8% fill) — it fits, with room to spare.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Box & Conductor Inputs+-
- Box's actual (or manufacturer-marked) cubic-inch capacity used, not its nominal size
- Every conductor entering the box and terminating or splicing there is counted
- Mixed conductor sizes each use their own Table 314.16(B) allowance, not the largest size for all
- Local code edition and any local amendments confirmed before finalizing
✓Clamp, Fitting & Device Fill+-
- Device/equipment strap count correctly reflects straps, not individual devices
- Each device/equipment strap group's conductor size reflects what is ACTUALLY connected to it, not the box's overall largest conductor
✓Equipment Grounding Conductor Fill+-
- Total equipment grounding/bonding conductor count confirmed for this specific box
- 2020+ NEC quarter-allowance rule applied correctly for more than 4 EGCs
✓Installation & Code Compliance+-
- Box confirmed to fit the calculated total fill, or upsized to one that does
- Permit and inspection requirements confirmed for the scope of the electrical work
- Box selection reviewed or performed by a licensed electrician
Full QC Checklist+−
Verification checklist for electrical box fill sizing — covering box and conductor inputs, clamp/fitting/device fill, equipment grounding conductor fill, and installation code compliance. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Box & Conductor Inputs+-
- Box's actual (or manufacturer-marked) cubic-inch capacity used, not its nominal size
- Every conductor entering the box and terminating or splicing there is counted
- Conductors passing through the box without splicing are still counted
- Mixed conductor sizes each use their own Table 314.16(B) allowance, not the largest size for all
- Local code edition and any local amendments confirmed before finalizing
✓Clamp, Fitting & Device Fill+-
- Internal clamp fill correctly counted as ONE allowance regardless of clamp count
- Fixture stud and hickey fill correctly counted per TYPE of fitting, not per instance
- Device/equipment strap count correctly reflects straps, not individual devices
- Largest-conductor determination for clamp/fitting fill includes the EGC, not just circuit conductors
- Each device/equipment strap group's conductor size reflects what is ACTUALLY connected to it, not the box's overall largest conductor
✓Equipment Grounding Conductor Fill+-
- Total equipment grounding/bonding conductor count confirmed for this specific box
- 2020+ NEC quarter-allowance rule applied correctly for more than 4 EGCs
- EGC size entered matches the actual largest equipment grounding conductor in the box
- EGC volume confirmed as ADDED on top of conductor/clamp/fitting fill, not substituted for it
- Isolated-ground EGCs (per NEC 250.146(D)) included in the same unified EGC count, not omitted
✓Installation & Code Compliance+-
- Box confirmed to fit the calculated total fill, or upsized to one that does
- Box remains accessible after installation, per NEC 314.29
- Box fill re-checked after any remodel work that adds conductors, devices, or EGCs to an existing box
- Permit and inspection requirements confirmed for the scope of the electrical work
- Box selection reviewed or performed by a licensed electrician
NEC Table 314.16(B) Reference
Free space volume allowance required per conductor, clamp, fitting, or device-strap allowance unit, by AWG size.
| Conductor Size | Volume Allowance (cu in) |
|---|---|
| 6 AWG | 5.00 |
| 8 AWG | 3.00 |
| 10 AWG | 2.50 |
| 12 AWG | 2.25 |
| 14 AWG | 2.00 |
| 16 AWG | 1.75 |
| 18 AWG | 1.50 |
Standard Box Volumes (NEC Table 314.16(A))
| Box Type | Capacity (cu in) |
|---|---|
| 4" x 1-1/4" Round/Octagonal | 12.5 |
| 4" x 1-1/2" Round/Octagonal | 15.5 |
| 4" x 2-1/8" Round/Octagonal | 21.5 |
| 4" x 1-1/4" Square | 18.0 |
| 4" x 1-1/2" Square | 21.0 |
| 4" x 2-1/8" Square | 30.3 |
| 4-11/16" x 1-1/4" Square | 25.5 |
| 4-11/16" x 1-1/2" Square | 29.5 |
| 4-11/16" x 2-1/8" Square | 42.0 |
| 3" x 2" x 1-1/2" Device | 7.5 |
| 3" x 2" x 2" Device | 10.0 |
| 3" x 2" x 2-1/4" Device | 10.5 |
| 3" x 2" x 2-1/2" Device | 12.5 |
| 3" x 2" x 2-3/4" Device | 14.0 |
| 3" x 2" x 3-1/2" Device | 18.0 |
| 4" x 2-1/8" x 1-1/2" Device | 10.3 |
| 4" x 2-1/8" x 1-7/8" Device | 13.0 |
| 4" x 2-1/8" x 2-1/8" Device | 14.5 |
| 3-3/4" x 2" x 2-1/2" Masonry | 14.0 |
| 3-3/4" x 2" x 3-1/2" Masonry | 21.0 |
When should you use this box fill calculator?
- Checking whether a standard outlet, switch, or junction box has enough volume for a new or existing installation.
- Planning a remodel that adds conductors, a second device, or an EGC set to an existing box.
- Selecting the right box size before roughing in new construction wiring.
- Cross-checking an electrician's box selection against an independent estimate.
- Verifying a nonmetallic or unlisted box's marked capacity is actually sufficient for the planned fill.
Quick Box Fill Tips
- Count every conductor that enters the box and is spliced or terminated there — including pigtails and jumpers, but not a conductor that both starts and ends inside the box without ever leaving.
- A duplex receptacle or a multi-gang switch on ONE strap still counts as a single device-strap allowance, not one per device.
- Multiple internal clamps together still only count as one allowance — but external cable connectors (clamping outside the box) need no allowance at all.
- When remodeling, re-run this calculation before adding anything to an existing box — a box that fit the original installation may not fit the added load.
- Nonmetallic boxes rely entirely on their manufacturer-marked capacity — there is no NEC table lookup for those.
- When in doubt about a large or unusual fill, size up — the cost difference between box sizes is minor compared to a code violation.
Common Mistakes
- Using the box's nominal size instead of its actual Table 314.16(A) or marked cubic-inch capacity.
- Sizing clamp or support-fitting allowances off the conductor actually connected to them, instead of the largest conductor present anywhere in the box — clamps and fittings use the box's overall largest conductor.
- Sizing a device/equipment strap's allowance off the box's overall largest conductor instead of the largest conductor actually connected to that specific strap — this is the opposite basis from clamps and fittings, and can overstate the required volume.
- Forgetting equipment grounding conductors entirely, or assuming any number of them still only need one allowance (only true up to 4, under the 2020+ NEC).
- Counting a multi-device strap (e.g. a duplex switch) as two device allowances instead of one.
- Adding a device, conductor, or EGC to an existing box during a remodel without re-checking the total fill.
- Assuming a nonmetallic box follows Table 314.16(A) — it doesn't; only its own marked capacity applies.
Limitations
- NEC-based only (US) — other regions use different enclosure-fill rules (IEC/BS 7671 and similar) and this calculator does not model them.
- Device/equipment strap fill relies on you correctly identifying the largest conductor actually connected to each strap group — the calculator doesn't trace individual wiring connections, so an incorrectly entered strap size will misstate that group's allowance.
- Does not model barrier-divided boxes, where each side of an internal barrier is calculated as its own separate volume with its own clamp/fitting allowances.
- Assumes the 2020+ NEC equipment grounding conductor rule (up to 4 EGCs share one allowance, each beyond that adds 1/4) — pre-2020 editions gave any number of EGCs a single allowance.
- Does not size conduit or raceway fill (NEC Chapter 9) — that is a separate calculation, covered by the Conduit Fill Calculator.
- Assumes standard box depths and shapes from Table 314.16(A); a box with an unusual or irregular shape must use its own manufacturer-marked capacity instead.
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.