Electrical Box Fill Calculator

Code profile:

Use the edition adopted for the installation; local amendments and the inspector remain controlling.

Box volume preset:

Confirm the exact marked capacity before relying on the result.

Marked box volume: {{ formatNumber(box_volume_cu_in, 1) }} in³

The calculator compares this marked capacity with the summed allowances below.

in³

Insulated conductors:

Supported sizes are 18, 16, 14, 12, 10, 8, and 6 AWG.

Rows describe one box, not a batch of boxes.

Equipment grounding conductors:

grounds

Largest grounding conductor:

Used only when one or more grounding conductors are entered.

Device yokes or straps:

yokes

Largest device conductor:

Used only when one or more device yokes or straps are entered.

Internal cable clamps:

Support fitting types present:

Zero is neutral; choose one or two only for the fitting types actually present.

Additional wide-device gangs:

Zero is neutral; use only extra gang positions beyond the ordinary yokes already entered.

gangs

Largest wide-device conductor:

Used only when one or more additional wide-device gangs are entered.

Terminal block assemblies:

Enter zero unless the selected edition and installation include this provision.

assemblies

Largest terminal-block conductor:

Used only when one or more terminal-block assemblies are entered.

Display precision:

Two decimal places is the neutral default; calculations retain full precision.

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

Planning option	Marked volume	Margin	Status	Copy
{{ row.label }}	{{ row.volumeDisplay }}	{{ row.marginDisplay }}	{{ row.status }}

Electrical box fill is based on assigned volume, not on whether conductors can be forced behind a device. Each conductor size has a cubic-inch allowance, and clamps, equipment grounding conductors, device yokes, support fittings, wide equipment, and terminal blocks can add allowances of their own. The total is compared with the box's marked usable volume.

The count is intentionally not one-for-one for every item. An insulated conductor that enters and is spliced or terminated normally counts once, a yoke generally counts as two conductor volumes, and one or more internal clamps share a single allowance. Equipment grounding conductor treatment changed in the 2020 National Electrical Code (NEC), while terminal-block assembly fill appears in the 2023 profile represented here.

Common box fill sources and counting questions
Fill source	Question to settle	Frequent mistake
Insulated conductors	Which sizes enter, splice, or terminate?	Counting cables instead of individual conductors
Grounding conductors	Which NEC edition or local rule applies?	Using the legacy one-allowance rule automatically
Devices and fittings	Which yokes, clamps, supports, or wide gangs are inside?	Counting only wires and omitting hardware
Box capacity	What volume is marked on the installed box or listed assembly?	Assuming boxes of similar dimensions have equal volume

American Wire Gauge (AWG) numbers run opposite to physical conductor size: a lower gauge number represents a larger conductor and a larger volume allowance. Mixed-gauge boxes therefore need each group calculated at its own allowance, while several hardware rules use the largest conductor involved.

A pass at exactly the marked volume satisfies the arithmetic comparison, but it leaves no allowance for a missed conductor or fitting. Crowded boxes are also harder to splice, fold, inspect, and service. A larger box can be the safer practical choice even when the calculation barely passes.

Box fill is only one installation requirement. Conductor ampacity, terminal ratings, grounding and bonding, cable bending space, enclosure listing, raceway fill, local amendments, and workmanship still need separate review by a qualified person and the authority having jurisdiction.

How to Use This Tool:

Use the adopted code edition and the actual marked box volume, then build the allowance ledger from everything that occupies the box.

Choose Code profile for the jurisdiction and edition being applied. Terminal-block entries require the NEC 2023 profile.
Select a box example or enter Marked box volume. Treat a preset as a starting point until the installed product's marking is confirmed.
Enter insulated conductor groups as one AWG,count row per size, such as 12,4. Correct an invalid row before relying on the result.
Add equipment grounding conductors, device yokes, and internal clamps with the conductor sizes that set their allowances.
Open Advanced for support fitting types, extra wide-device gangs, terminal-block assemblies, and display precision.
Review Fill audit line by line before accepting the pass status, then compare the margin with the listed box options.

Interpreting Results:

Required fill volume is the sum of all counted allowances. The result passes when required volume is less than or equal to marked box volume. Spare volume is positive for a pass, zero at exact capacity, and negative for an overfilled result.

Use Fill percentage as a ratio, not as a substitute for the ledger. Missing hardware can make a low percentage wrong.
Check the AWG basis on every row, especially clamps, support fittings, yokes, and mixed conductor groups.
Treat a small positive margin as a prompt to recount the actual box and consider the next larger listed volume.

A passing volume comparison does not approve the complete installation. Verify the adopted NEC edition, local amendments, product listing, conductor use, and field conditions.

Technical Details:

NEC-style box fill converts each counted source into conductor-volume equivalents. Each equivalent is multiplied by the cubic-inch allowance for its AWG basis, and the products are added without rounding. Display precision from zero to three decimal places changes presentation only.

Formula Core:

The volume sum and capacity margin determine the pass result.

\begin{array}{lcl} V_{required} & = & \sum_{i = 1}^{k} n_{i} v_{i} \\ V_{margin} & = & V_{box} - V_{required} \\ F & = & \frac{V_{required}}{V_{box}} \times 100 % \end{array}

n_i is the allowance count for a ledger row, v_i is its AWG volume, and F is fill percentage. Pass is defined by V_margin ≥ 0, so equality belongs to the passing side of the boundary.

Rule Core:

The count assigned to each source depends on its role and the selected code profile.

Electrical box fill allowance rules
Source	Allowance count	AWG basis
Insulated conductor	1 for each entering conductor that is spliced or terminated	That conductor's AWG
Equipment grounding conductors, NEC 2020/2023	1 for the first 1 to 4, then 0.25 for each additional ground	Largest grounding conductor entered
Equipment grounding conductors, legacy profile	1 total when any are present	Largest grounding conductor entered
Device yoke or strap	2 per yoke	Largest conductor connected to the device
Additional wide-device gang	2 per extra gang	Largest conductor connected to the wide equipment
One or more internal clamps	1 total	Largest conductor present
Luminaire stud or hickey	1 per fitting type present, up to 2 types	Largest conductor present
Terminal-block assembly, NEC 2023	1 per assembly	Largest conductor terminated to the assemblies

Conductor volume allowances by American Wire Gauge
AWG	Volume allowance
18	1.50 in³
16	1.75 in³
14	2.00 in³
12	2.25 in³
10	2.50 in³
8	3.00 in³
6	5.00 in³

Four #12 insulated conductors use 9.00 in³, two #14 conductors use 4.00 in³, two #12 grounds use 2.25 in³ under the modern rule, one #12 yoke uses 4.50 in³, and internal clamps use 2.25 in³. The 22.00 in³ total fits a 22.50 in³ box with 0.50 in³ spare, or about 97.78% fill.

The conductor parser accepts 18, 16, 14, 12, 10, 8, and 6 AWG with whole counts from 0 through 200 per row. Counts for grounds, devices, fittings, and terminal blocks are also whole numbers within their stated form limits.

Safety Notes:

Incorrect box-fill work can contribute to damaged insulation, poor terminations, overheating, and unsafe maintenance conditions.

De-energize and verify the circuit before opening or changing an enclosure.
Use the edition adopted by the authority having jurisdiction; the newest profile is not automatically the governing rule.
Have a qualified electrician verify close margins, unusual fittings, local amendments, and the completed installation.

References:

NFPA 70, National Electrical Code, 2023 edition, National Fire Protection Association.
NFPA 70 code-development report for Article 314, National Fire Protection Association, 2022.