Conduit Fill Calculator (EMT + THHN)
Pick an EMT conduit size, a THHN wire size, and the number of conductors, and this tool computes your fill percentage against the NEC Chapter 9 fill limits.
Conduit Fill Calculator
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Fill limits (NEC Chapter 9, Table 1): 1 conductor = 53%, 2 conductors = 31%, 3 or more conductors = 40% of the conduit’s internal area.
Simplified starting point for planning. Does not apply every NEC adjustment/derating factor. Verify against the current NEC and your local AHJ before installing.
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How conduit fill is calculated
Each wire size has a cross-sectional area (THHN insulation), and each conduit size has a total internal area. Fill percentage is the sum of the wire areas divided by the conduit’s internal area — and it cannot exceed the NEC limit for the wire count.
| Step | Example: 3/4″ EMT, four 10 AWG THHN |
|---|---|
| THHN area, 10 AWG | 0.0211 in² each |
| Total wire area (4 conductors) | 4 × 0.0211 = 0.0844 in² |
| 3/4″ EMT internal area | 0.533 in² |
| Allowable area (40% limit, 3+ wires) | 0.533 × 0.40 = 0.2132 in² |
| Fill percentage | 0.0844 ÷ 0.533 × 100 = 15.8% — PASS |
Fill limits step down as conductor count goes up: a single conductor can fill up to 53% of a conduit (it needs room to be pulled straight), two conductors are capped at 31%, and three or more are capped at 40% to leave room for heat dissipation and pulling.
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Frequently asked questions
Why does the fill limit change with the number of conductors?
A single conductor needs more relative room to be pulled without jamming, so it is allowed up to 53% fill. Two conductors are capped at 31%, and three or more are capped at 40% — the standard NEC Chapter 9 limits that also protect against heat buildup.
Does this include ground wires?
Include every current-carrying and grounding conductor you plan to pull through the conduit in the conductor count for an accurate fill percentage.
What happens if I exceed the fill limit?
Overfilled conduit makes pulling wire difficult or impossible and can cause overheating. Step up to the next conduit size, or split the run across two conduits.
Is this conduit fill calculator free?
Yes — free, no sign-up. Pick your conduit size, wire size, and conductor count, and read the fill percentage instantly. Estimates only; confirm against the current NEC and your local AHJ.