Conduit Fill Calculator

Conduit type:

Choose the raceway family used for the proposed or installed run.

Trade size:

Select the size to check; the result also identifies the first listed same-type size that fits.

Fill basis:

Use Auto for ordinary complete raceway systems; select an explicit basis only when the adopted code and installation support it.

Nipple length:

Changing the unit preserves the physical length; keep the qualifying nipple at 2 ft / 24 in or less.

Conductor groups:

Examples: 3,6,THHN, 1,#10,THHN, or 2 12 XHHW-2.

Verify conductor markings and the adopted code before using the schedule for installation.

Pull length:

Leave at zero when pull-route guidance is not needed.

Total bend angle:

{{ formatNumber(bend_degrees, 0) }}°

Use 0-720 degrees; zero is neutral.

Planning reserve:

{{ formatNumber(reserve_percent, 0) }}%

Use 0-30%; zero is neutral.

Display precision:

Choose one through four decimal places without changing the result.

Measure	Value	Review note	Copy
{{ row.label }}	{{ row.value }}	{{ row.note }}

Qty	Size	Insulation	Area each	Area total	Limit share	Copy
{{ row.quantity }}	{{ row.size_label }}	{{ row.insulation_label }}	{{ formatArea(row.area_each_sq_in) }}	{{ formatArea(row.area_total_sq_in) }}	{{ formatPercent(row.limit_share_percent) }}

Trade size	Internal area	Allowed area	Fill used	Sizing status	Copy
{{ analysis.conduit_label }} {{ row.trade_size }}	{{ formatArea(row.internal_area_sq_in) }}	{{ formatArea(row.allowed_area_sq_in) }}	{{ formatPercent(row.fill_percent) }}	{{ row.status }}

Where is current fill relative to the selected Chapter 9 limit and optional planning target?

Conduit fill measures how much of a raceway's internal cross-sectional area is occupied by insulated conductors. The calculation uses area rather than wire count. A large conductor consumes more space than a small one, and insulation types with the same American Wire Gauge size can have different outside areas.

Raceway type matters because equal trade sizes do not guarantee equal internal area. Electrical metallic tubing (EMT), intermediate metal conduit (IMC), rigid metal conduit (RMC), and Schedule 40 or Schedule 80 PVC each use their own dimensional table. The conductor schedule must also include equipment grounding or bonding conductors when the adopted rules require them to be counted.

Fill limits are intended to leave room for installation rather than pack circles perfectly inside a larger circle. Under the Chapter 9 rule represented here, one conductor may occupy 53% of the raceway area, two may occupy 31%, and three or more may occupy 40%. A qualifying nipple no longer than 24 inches may use a 60% basis. Those percentages apply to complete raceway systems and do not replace other installation rules.

Factors that change conduit fill
Factor	Why it changes the result	Common error
Raceway family and trade size	They set the internal area	Using EMT dimensions for PVC or RMC
Conductor size and insulation	They set area per conductor	Counting only the metal conductor
Conductor count	It selects the 53%, 31%, or 40% limit	Applying 40% to one or two conductors
Nipple qualification	A run of 24 inches or less may use 60%	Applying 60% to a longer raceway

A passing area check does not prove that the pull is practical. Long routes, many bends, high use of the allowed area, and the geometry of three similar conductors can raise pulling or jamming concerns. A larger raceway or lower fill may be appropriate even when the code percentage is not exceeded.

Conduit fill is only one electrical-design check. Ampacity adjustment, voltage drop, box and conduit-body fill, conductor temperature rating, wet-location suitability, support, burial, grounding, and local amendments still need review. Final work should follow the code edition adopted by the authority having jurisdiction and, where required, a qualified electrician or engineer.

How to Use This Tool:

Match the proposed raceway and every conductor before treating the fill percentage as meaningful.

Select the conduit type and trade size. Use the actual raceway family; the same nominal trade size has different internal area across EMT, IMC, RMC, and PVC.
Choose the fill basis. Auto applies the conductor-count rule. Select the 60% nipple basis only for a qualifying run no longer than 2 ft or 24 in.
Enter one conductor group per line. Use quantity, size, and insulation, such as 3,6,THHN or 2,12,XHHW-2. Correct the line named in any schedule warning before continuing.
Review the selected-size check and first listed size that fits. If planning reserve fails or a pull warning appears, consider a larger raceway or revise the schedule even when the selected code limit passes.

Interpreting Results:

Fill used compares total insulated-conductor area with the full internal raceway area. Allowed area applies the selected percentage limit. A pass means conductor area is at or below that allowed area; a negative spare area means the selection is over the limit.

Planning reserve is a stricter user-selected target. Failing it does not change whether the selected 53%, 31%, 40%, or 60% rule passes.
First listed size that fits searches larger and smaller trade sizes only within the selected raceway family.
Pull planning flags certain bends, long high-fill runs, three-conductor jamming geometry, or near-limit fill. It is guidance, not a pulling-tension calculation or installation approval.

Technical Details:

Each conductor group contributes quantity multiplied by the tabulated insulated area for its size and insulation family. Those group areas are summed, then divided by the selected raceway's tabulated internal area. All area calculations use square inches.

Formula Core

The occupied area and percentage fill are calculated before the conductor-count limit is applied.

\begin{aligned} A_{c} & = \sum_{i = 1}^{k} q_{i} a_{i} \\ F & = \frac{A_{c}}{A_{r}} \times 100 % \\ A_{allowed} & = A_{r} \frac{L}{100} \end{aligned}

Conduit fill formula symbols
Symbol	Meaning	Unit
q_i	Quantity in conductor group i	count
a_i	Insulated area of one conductor in group i	in²
A_c	Total conductor area	in²
A_r	Raceway internal area	in²
L	Selected fill limit	%
F	Fill used	%

Rule Core

Conduit fill limit rules and boundaries
Condition	Limit	Boundary
One conductor	53%	Auto basis and total count = 1
Two conductors	31%	Auto basis and total count = 2
Three or more conductors	40%	Auto basis and total count ≥ 3
Explicit raceway basis	40%	Selected directly
Qualifying nipple	60%	Entered length ≤ 2 ft / 24 in; longer input is rejected

A selected size passes when conductor area is no greater than allowed area, with a tolerance of 0.0000000001 in² for floating-point comparison. The optional planning target subtracts the reserve percentage from the fill limit, never below 0%. The first-fit result scans the supported trade-size list from smallest to largest within the selected conduit family.

Pull-warning order

Only the first matching caution is shown, in this order:

Total bends above 270°.
Run length above 100 ft while fill exceeds 75% of the selected limit.
Exactly three conductors with raceway-to-largest-conductor diameter ratio from 2.8 through 3.2, inclusive.
Fill above 90% of the selected limit.

The supported conductor families are THHN/THWN-2, XHHW-2, and RHH/RHW-2/USE-2. Available sizes depend on insulation, spanning 18 AWG through 500 kcmil across the tables. Schedule rows may use commas, tabs, pipes, semicolons, or compact forms such as 2 12 XHHW-2; blank lines and lines beginning with # are ignored.

Display precision may be one through four decimal places and does not change pass, reserve, first-fit, or warning logic.

Accuracy and Code Notes:

Conduit dimensions, conductor areas, and fill rules must match the code edition and product data accepted for the installation.

Verify the selected raceway family, trade size, conductor construction, and insulation marking against current manufacturer data.
The 60% nipple allowance has installation conditions beyond length; selecting it does not prove that the proposed section qualifies.
Actual cable diameter may be needed for products outside the listed conductor tables and for reliable jamming or pulling analysis.
Local amendments and the authority having jurisdiction control. A calculator result is not an inspection, permit decision, or substitute for qualified design review.

Worked Examples:

EMT feeder schedule

A schedule of three 6 AWG THHN conductors plus one 10 AWG THHN conductor has a total area of 0.1732 in². In 3/4-inch EMT with 0.533 in² internal area, four conductors use about 32.495% of the raceway. The 40% allowance is 0.2132 in², leaving 0.0400 in² and making 3/4-inch EMT the first listed EMT size that fits.

Maximum nipple length

The same feeder schedule in 1/2-inch EMT uses about 56.974% fill. It passes the 60% nipple basis at exactly 2 ft, but is flagged as near the limit because it uses more than 90% of the allowance. At 2.01 ft the 60% basis is rejected rather than silently changed.

References:

NFPA 70, National Electrical Code, Chapter 9 tables, National Fire Protection Association, 2026.
NFPA 70 Chapter 9 Table 1 and notes, National Fire Protection Association.
20 Best Practices for Cable Installation, Southwire.