Electrical Box Fill Calculator

Box volume preset:

Loads a marked cubic-inch capacity for the check; edit the exact volume below when the box marking differs.

Marked box volume:

Enter the usable box volume, including any listed extension ring volume you intend to use.

cu in

Insulated conductors:

One row per conductor size, using AWG,count. Example: 12,4 for four #12 insulated conductors.

Pigtails, locknuts, bushings, and wire connectors are normally not counted in box fill.

Equipment grounding conductors:

Enter how many grounding conductors enter the box, then select the largest grounding conductor size.

Grounding fill rule:

Choose the equipment-grounding-conductor rule used by your adopted code cycle.

Device yokes or straps:

Count switches, receptacles, dimmers, or similar device straps mounted in the box.

Internal cable clamps:

Set to Yes only for clamps inside the box that occupy fill space.

Fixture studs or hickeys:

Use 0 for ordinary switch or receptacle boxes with no support fitting allowance.

fittings

{{ error }}

Wide device gang positions:

Leave at 0 for ordinary switches, receptacles, and dimmers counted above.

Terminal block assemblies:

Leave at 0 when the box has only ordinary splices, connectors, switches, or receptacles.

Decimal places:

Use 0-4 places for displayed cubic-inch values and exported tables.

places

Metric	Value	Check note	Copy
{{ row.metric }}	{{ row.value }}	{{ row.note }}

Fill source	Count basis	AWG basis	Volume	Rule note	Copy
{{ row.source }}	{{ row.countBasis }}	{{ row.awgBasis }}	{{ row.volume }}	{{ row.note }}

Box option	Volume	Spare or short	Sizing status	Copy
{{ row.option }}	{{ row.volume }}	{{ row.margin }}	{{ row.status }}

Embed:

Customize

Include current inputs

Size

Advanced

Width

Height

Aspect ratio

Max height

Collapsible embed

Allow fullscreen

Referrer policy

Sandbox tokens

Electrical boxes are sized by usable volume, not by how neatly the wires appear to fit. Box fill compares the marked cubic-inch capacity of an outlet, device, or junction box with the volume allowances assigned to the conductors and fittings inside it. The rule matters because crowded boxes are harder to splice, fold, inspect, and service without nicking insulation, stressing terminals, or leaving devices pressed hard against wiring.

The calculation is not a physical measurement of each conductor. NEC-style box fill converts box contents into conductor-equivalent allowances, then converts those allowances into cubic inches using the American Wire Gauge (AWG) size involved. A #12 AWG allowance uses more cubic inches than a #14 AWG allowance, so the same count can pass in one branch-circuit size and fail in another.

Electrical box fill counting factors
Box content	Why it matters	Common mistake
Insulated conductors	Each supported AWG size has its own cubic-inch allowance.	Counting cable runs but not the individual conductors that enter the box.
Equipment grounding conductors	Grounds are grouped by the selected rule and use the largest grounding conductor size.	Counting every ground as a full allowance when the adopted rule groups them differently.
Device yokes or straps	Switches, receptacles, dimmers, and similar yokes usually add two allowances.	Adding a bulky device without adding the yoke allowance.
Internal clamps, supports, and terminal blocks	Some hardware inside the box uses fill space and may require an allowance.	Treating external clamps and internal clamps as if they were the same thing.

Small residential changes often create the tightest calls. A deeper dimmer, an added GFCI-style receptacle, a new cable entering an old-work box, or a switch from #14 to #12 conductors can consume the remaining margin. The box trade description is only a starting clue. The enforceable capacity is the marked cubic-inch volume of the actual box and any listed extension ring being used.

Conductors, clamps, and device yokes converted into required box fill volume

Several items are easy to overcount or miss. Pigtails that originate and end inside the box are treated differently from conductors that enter the box. Wire connectors, locknuts, and bushings are usually not counted as separate fill items. Equipment grounding conductors, internal clamps, and device yokes have special rules that can change the answer even when the visible number of wires looks unchanged.

A passing box-fill result is still not a complete electrical design. Conductor ampacity, overcurrent protection, cable or raceway method, box support, grounding continuity, device ratings, required conductor length at the box, and local amendments still need their own checks. The adopted code cycle and the authority having jurisdiction decide what is enforceable for a specific installation.

How to Use This Tool:

Begin with the box volume, then enter the conductors and hardware that consume fill space. The result appears only after all required counts and AWG bases are valid.

Choose Box volume preset for a common marked capacity or select Custom marked volume. Edit Marked box volume when the installed box or listed extension ring shows a different cubic-inch value.
Enter Insulated conductors as one size and count per line using the visible AWG,count pattern. For example, 12,4 means four #12 AWG insulated conductors.
Set Equipment grounding conductors, choose the largest grounding conductor size, and select the Grounding fill rule that matches the adopted code cycle or local requirement.
Count Device yokes or straps, choose the largest conductor connected to those devices, and set Internal cable clamps to Yes only when clamps occupy fill space inside the box.
Add Fixture studs or hickeys when support fittings are inside the box. Open Advanced for wide device gang positions, terminal block assemblies, or a different display precision.
Read Fill Snapshot first. A valid result shows pass or overfilled status, required fill volume, spare volume or shortage, fill percentage, grounding rule, and the largest AWG basis.
Use Allowance Ledger to audit the counted sources and Box Size Options to compare common capacities. If a validation message appears, correct the unsupported AWG size, non-whole count, missing conductor basis, or box volume before using the result.

Advanced Tips:

Use the actual marked volume of the product being installed. A preset is only a starting point, and a listed extension ring should be counted only when it is part of the final installation.
Choose the NEC 2020/2023 grounding rule only when that rule applies locally. The legacy/local option is useful for jurisdictions or worksheets that still group all equipment grounding conductors as one allowance.
Leave Wide device gang positions at zero for ordinary switches, receptacles, and dimmers already counted as device yokes. Use it for large devices that occupy more than one gang position.
Enter Terminal block assemblies only when the adopted rule and product situation call for terminal-block fill. Each entered assembly uses the selected terminal AWG basis.
Raise Decimal places when the margin is tight. A result rounded to a whole cubic inch can hide a small shortage or a small spare volume.
Use Fill Volume Stack to see which source consumes the most space before deciding whether a larger box, extension ring, or layout change is the cleanest correction.

Interpreting Results:

Status is the main pass or overfill call. Pass means the required fill volume is less than or equal to the marked box volume entered. Overfilled means the counted conductors and fittings require more cubic inches than the box provides.

Required fill volume is the sum of all counted allowances in cubic inches.
Spare volume or Volume shortage is the margin between marked capacity and required volume.
Fill percentage is required volume divided by marked box volume.
Equivalent allowances restates the required volume using the largest conductor basis so the count is easier to review.
Box Size Options marks which common volumes pass and which are too small for the same contents.

Treat a narrow pass cautiously. One missed yoke, internal clamp, terminal block, conductor size, or local amendment can turn a small spare margin into a shortage. When the margin is close, recheck the ledger and consider a larger marked box or listed extension ring.

Technical Details:

NEC-style small-conductor box fill is based on Article 314.16 concepts for outlet, device, and junction boxes containing conductors in the #18 AWG through #6 AWG range. The method assigns allowance counts to conductors and certain internal hardware, multiplies those counts by the cubic-inch allowance for the relevant conductor size, and compares the total with the marked box volume.

The AWG basis matters because lower gauge numbers represent larger conductors. The largest conductor present supplies the allowance basis for internal clamps and support fittings. Device yokes use the largest conductor connected to the yoke. Terminal block assemblies use the largest conductor terminated to the assembly when the adopted rule includes that fill item.

Formula Core:

The pass boundary is inclusive: required fill volume may equal the marked box volume, but it may not exceed it.

\begin{array}{lcl} V_{required} & = & \sum_{i = 1}^{n} (A_{i} \times U_{i}) \\ V_{margin} & = & V_{box} - V_{required} \\ Fill percent & = & 100 \times \frac{V_{required}}{V_{box}} \end{array}

Electrical box fill formula symbols
Symbol	Meaning	Unit
`V`_required	Total fill required by all counted sources.	cu in
`A`_i	Allowance count for one source, such as conductors, yokes, grounds, clamps, supports, or terminal blocks.	allowances
`U`_i	Cubic inches per allowance for the AWG basis used by that source.	cu in per allowance
`V`_box	Marked box volume, including any listed extension volume being counted.	cu in

Conductor Allowance Table:

Electrical box fill conductor allowances
Conductor size	Volume per allowance	Typical meaning
#18 AWG	1.50 cu in	Small conductor allowance basis
#16 AWG	1.75 cu in	Small conductor allowance basis
#14 AWG	2.00 cu in	Common 15 amp branch-circuit conductor basis
#12 AWG	2.25 cu in	Common 20 amp branch-circuit conductor basis
#10 AWG	2.50 cu in	Larger branch-circuit conductor basis
#8 AWG	3.00 cu in	Large supported conductor basis
#6 AWG	5.00 cu in	Largest supported conductor basis

Rule Core:

Electrical box fill allowance rules
Counted source	Allowance rule modeled	AWG basis
Insulated conductors	Each entered conductor that is spliced or terminated counts once.	That conductor size.
Equipment grounds, NEC 2020/2023 profile	Up to four equipment grounding conductors count as one allowance; each additional ground adds one-quarter allowance.	Largest grounding conductor entered.
Equipment grounds, legacy/local profile	All entered equipment grounding conductors count together as one allowance.	Largest grounding conductor entered.
Device yokes or straps	Each yoke counts as two allowances.	Largest conductor connected to the device yoke.
Wide device gang positions	Each mounting gang position counts as two allowances.	Largest conductor connected to the wide device.
Internal clamps	One or more internal clamps count as one allowance.	Largest conductor present in the box.
Support fittings	Each fixture stud or hickey counts as one allowance.	Largest conductor present in the box.
Terminal block assemblies	Each assembly counts once when the adopted rule includes terminal-block fill.	Largest conductor terminated to the assembly.

Validation and Precision:

Supported conductor sizes are #18, #16, #14, #12, #10, #8, and #6 AWG. Counts must be non-negative whole numbers, and the marked box volume must be greater than zero. Clamp and support allowances require at least one conductor-size basis because their cubic-inch allowance depends on the largest conductor present.

Display precision affects shown and exported numbers. The arithmetic uses the allowance values before rounding, so a result rounded to zero or one decimal place may hide a small margin that appears at a higher precision.

Safety Notes:

Electrical work can create fire, shock, and equipment hazards. Box fill is a planning calculation, not a permit, inspection, or full design review.

Use the adopted code edition, product markings, local amendments, and the authority having jurisdiction for the actual installation.
Confirm conductor count rules for pigtails, grounds, clamps, terminal blocks, and unusual devices before treating a close pass as acceptable.
Do not use a passing fill result to skip ampacity, breaker, device-rating, grounding, box-support, cable-method, working-space, or workmanship checks.

Worked Examples:

Extra-deep single-gang branch circuit

Use a 22.5 cu in marked volume, enter 12,4 and 14,2, add two #12 equipment grounding conductors under the NEC 2020/2023 profile, count one #12 device yoke, and set internal clamps to Yes. The required fill is 22.00 cu in, so Fill Snapshot reports a pass with 0.50 cu in spare.

Same contents in an 18 cu in box

Move the same contents to an 18.0 cu in preset. The required fill remains 22.00 cu in, so the result reports a 4.00 cu in shortage. Box Size Options can show which common preset volumes pass, but the installed product still needs an actual marking that meets or exceeds the requirement.

Rejected conductor row

A row such as 12 AWG four will not calculate because the count is not numeric. Change it to 12,4. Unsupported sizes, negative counts, fractional counts, and missing counts keep the result from appearing until corrected.

FAQ:

Do pigtails count as insulated conductors?

Pigtails that originate and end inside the box are normally not entered as separate insulated conductors. Count conductors that enter the box and are spliced or terminated there, then verify unusual cases against the adopted code.

Why does one device yoke count as two allowances?

A device yoke represents the switch, receptacle, dimmer, or similar strap mounted in the box. The modeled rule counts each yoke as two allowances based on the largest conductor connected to it.

Which grounding fill rule should I choose?

Choose the rule used by the adopted code cycle and local enforcement for the job. The NEC 2020/2023 option counts up to four equipment grounding conductors as one allowance and adds one-quarter allowance for each extra ground; the legacy/local option counts all entered grounds together as one allowance.

Why does an internal clamp need a conductor size?

Internal clamps use one allowance at the largest conductor size present. If no conductor, ground, device, wide device, or terminal size is entered, there is no AWG basis for the clamp allowance.

Can a passing result be used as code approval?

No. A passing result only means the entered allowances do not exceed the entered marked volume. Final approval depends on the full installation, product listing, local rules, and inspection.

Glossary:

Allowance: A rule-based conductor equivalent that is converted into cubic inches.
AWG: American Wire Gauge, the conductor size system used by the volume allowance table.
Device yoke: The mounting strap for a switch, receptacle, dimmer, or similar device.
Equipment grounding conductor: The grounding or bonding conductor counted under the selected equipment-grounding-conductor rule.
Marked box volume: The usable cubic-inch capacity marked on the box or listed extension being installed.
Terminal block assembly: A terminal block inside the box that may require its own fill allowance under NEC 2023-style rules.

References:

NFPA 70 National Electrical Code, 2023 edition, National Fire Protection Association.
Box Sizing Tips, EC&M, February 16, 2015.
Box-Fill Calculations, Part VIII, Electrical Contractor Magazine, February 15, 2005.
314.16(B)(6) Terminal Block Fill, ElectricalLicenseRenewal.com.