Free Calculator
Conduit Fill Calculator
Check percent conduit fill against the NEC Chapter 9 limits — enter your conductors and raceway, and get the fill percentage, a pass or fail against the 53% / 31% / 40% rule, and the smallest trade size that will take the pull.
Built by SubcontractorHub — the software electrical contractors use to quote, finance, and run every job.
Book a DemoCheck Your Conduit Fill in Seconds
Pick the raceway, add each group of conductors, and see percent fill, the applicable NEC limit, and the minimum conduit size that works.
Internal area: 0.533 sq in
Conductors in the raceway
Add each group of conductors — phases, neutral, and the equipment grounding conductor all count. Set a quantity to 0 to skip a row.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on the published NEC Chapter 9 tables and should be used for general planning purposes only. They are not a substitute for the National Electrical Code, local amendments, or a professional electrical design. Actual raceway sizing varies with manufacturer jacket dimensions, bare grounding conductors, nipple lengths, and conductors already occupying the raceway. Always confirm conduit sizing with a licensed electrician before purchasing, permitting, or installing.
How NEC Conduit Fill Works
The fill limit depends on how many conductors you pull
Most electricians know the “40% rule,” but NEC Chapter 9, Table 1 actually sets three different limits, and the middle one catches people out. Two conductors are held to a stricter limit than three or more, because two round conductors waste more space inside a round raceway than a tighter bundle does.
| Number of conductors | Maximum fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
| Nipple, 24 in. or shorter | 60% |
Percent fill is always calculated against the conduit's total internal area from Table 4 — you do not derate the conduit area first and then compare.
Insulation type changes the answer as much as wire size
Conductor area comes from Chapter 9, Table 5, and it is the area including insulation. The same 12 AWG copper occupies 0.0133 sq in as THHN but 0.0260 sq in as THW — nearly double. Sizing a raceway from wire gauge alone, without checking the insulation, is one of the most common ways a pull fails inspection.
| Conductor | THHN / THWN-2 | XHHW | THW |
|---|---|---|---|
| 14 AWG | 0.0097 | 0.0139 | 0.0209 |
| 12 AWG | 0.0133 | 0.0181 | 0.0260 |
| 10 AWG | 0.0211 | 0.0243 | 0.0333 |
| 8 AWG | 0.0366 | 0.0437 | 0.0556 |
| 6 AWG | 0.0507 | 0.0590 | 0.0726 |
| 4 AWG | 0.0824 | 0.0814 | 0.0973 |
Areas in square inches, NEC Chapter 9, Table 5.
Trade size is not internal diameter
A 1-inch raceway does not hold 1 inch of anything. Trade size is a nominal label, and the usable area behind it swings by roughly 20% across conduit types. That is why a design that passes in EMT can fail after a field change to Schedule 80 PVC.
| Trade size | EMT | RMC | PVC 40 | PVC 80 |
|---|---|---|---|---|
| 1/2" | 0.304 | 0.314 | 0.285 | 0.217 |
| 3/4" | 0.533 | 0.549 | 0.508 | 0.409 |
| 1" | 0.864 | 0.887 | 0.832 | 0.688 |
| 1-1/4" | 1.496 | 1.526 | 1.453 | 1.237 |
| 1-1/2" | 2.036 | 2.071 | 1.986 | 1.711 |
| 2" | 3.356 | 3.408 | 3.291 | 2.874 |
Total internal area in square inches, NEC Chapter 9, Table 4.
Passing fill does not mean passing inspection
Conduit fill and conductor ampacity are two separate tests, and a pull can clear one while failing the other. Once a raceway carries more than three current-carrying conductors, NEC 310.15(C)(1) requires an ampacity adjustment: 80% for 4 to 6 conductors, 70% for 7 to 9, and 50% for 10 to 20. Bundle enough circuits into one conduit and the wire that fit comfortably is no longer large enough for its breaker.
Check fill here, then size the conductors themselves with the wire size calculator and confirm the run length with the voltage drop calculator.
Why estimators care about fill, not just installers
Conduit size drives material cost, fitting count, labor units, and hanger spacing. Discovering during rough-in that a run needs to step from 3/4" to 1" is a change order at best and absorbed margin at worst. Estimators who check fill during takeoff — rather than after the bid is submitted — protect the number they quoted. That is the same reason it pays to keep takeoff, proposal, and job costing in one system: see how electrical contractor software connects the estimate to the job it becomes.
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Book a Free DemoFrequently Asked Questions
What is the maximum conduit fill allowed by the NEC?
NEC Chapter 9, Table 1 sets the limit by how many conductors are in the raceway: 53% for a single conductor, 31% for exactly two conductors, and 40% for three or more. The two-conductor case is the one people get wrong — it is stricter than the three-or-more case, because two round conductors pack inefficiently inside a round raceway. The calculator applies the correct limit automatically based on the total conductor count you enter.
How do you calculate conduit fill?
Add up the cross-sectional area of every conductor (NEC Chapter 9, Table 5, which varies by insulation type), then divide by the total internal area of the conduit (Table 4, which varies by conduit type and trade size). Multiply by 100 to get percent fill, and compare that against the Table 1 limit for your conductor count. Note that fill is calculated on the conduit's TOTAL internal area, not on the already-derated allowable area.
Does the equipment grounding conductor count toward conduit fill?
Yes. Every conductor in the raceway counts toward fill, including the equipment grounding conductor and the neutral, whether insulated or bare. A bare ground uses the bare-conductor dimensions from Chapter 9, Table 8 rather than Table 5. Add the ground as its own row in the calculator so it is included in both the area total and the conductor count that selects the fill limit.
Why does the same wire fill EMT and PVC differently?
Trade size names the nominal size, not the internal diameter. A 1-inch EMT has 0.864 sq in of internal area while 1-inch PVC Schedule 80 has only 0.688 sq in — about 20% less room — because Schedule 80 has thicker walls. Swapping conduit type late in a job is a common way to turn a compliant pull into a violation, so re-check fill any time the raceway type changes.
Do I have to derate conductors when a conduit has more than three current-carrying wires?
Yes, and it is a separate requirement from fill. NEC 310.15(C)(1) requires an ampacity adjustment once a raceway holds more than three current-carrying conductors — 80% for 4 to 6, 70% for 7 to 9, and 50% for 10 to 20. A pull can pass conduit fill and still fail because the conductors are no longer large enough after derating. Check fill and ampacity adjustment separately.
What is the 40% conduit fill rule?
It is the most commonly cited limit because most real pulls contain three or more conductors, which caps fill at 40% of the conduit's internal area. It exists to leave room for heat dissipation and to keep pulling tension low enough that insulation is not damaged during installation. It is a maximum, not a target — many contractors design to roughly 30% to leave room for future adds.
Why do the NEC Annex C tables sometimes allow one more conductor than 40% appears to?
Because of Note 7 to NEC Chapter 9. Where every conductor in the raceway is the same size, the next higher whole number of conductors is permitted when the fill calculation results in a decimal of 0.80 or larger. That is why Annex C lists 26 conductors of 12 AWG THHN in 1-inch EMT even though 26 conductors work out marginally above 40% by raw area. The allowance applies only to a uniform bundle — a pull with mixed conductor sizes is judged on total area alone. This calculator applies Note 7 automatically when every conductor you enter is the same size and insulation, and tells you when it did.
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Get a Free DemoAll calculations are ballpark estimates based on the published NEC Chapter 9 tables and should be verified against the National Electrical Code, local amendments, and a licensed electrician in your area before making any purchasing or installation decisions. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.