Conduit fill calculator.
Pick your conduit and conductors. Get the fill percentage, a PASS/FAIL against NEC Chapter 9, the maximum number of conductors of that size — and the smallest conduit that passes. Flip to reverse mode to size the conduit for the wires you already have.
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How the math works
Conduit fill is a simple ratio of areas. Every number below comes straight from NEC Chapter 9:
- Total conductor area — number of conductors × the area of one conductor. Conductor areas come from NEC Chapter 9, Table 5 (square inches per insulation type and wire size).
- Conduit internal area — from NEC Chapter 9, Table 4 for EMT, PVC Sch 40 and rigid metal conduit (RMC), per trade size.
- Allowable fill — from NEC Chapter 9, Table 1: 53% for one conductor, 31% for two, and 40% for three or more conductors.
- Fill % — total conductor area ÷ conduit internal area × 100. It passes when fill % ≤ the allowable limit.
- Max conductors — allowable area ÷ one conductor area, rounded down. (The limit shifts to 40% once you cross three.)
- Smallest conduit that passes — the calculator walks up the trade sizes for your chosen conduit type and reports the first one where your conductors fit at the correct limit.
Worked example: three 12 AWG THHN in 1/2″ EMT. Conductor area 0.0133 sq in × 3 = 0.0399 sq in. 1/2″ EMT internal area is 0.304 sq in; at the 40% limit that allows 0.1216 sq in. 0.0399 ≤ 0.1216, so it passes at 13.1% fill.
This is the easy 20%. The other 80% is on your drawings.
A single conduit check is quick. What eats an electrical bid is the count: every homerun, every feeder, every device, every fixture and the conduit and wire that connect them, across a 60-sheet set with the conduit schedule on one page and the panel schedule on another. That’s the part that leaks money — and it’s exactly what Pilars AI takeoff does from your actual plans: it reads the symbols, counts devices and fixtures, follows homeruns, and returns conduit, wire, boxes and fittings by type. $100 per trade, per plan.
Questions electricians actually ask
What is the maximum conduit fill allowed by the NEC?
NEC Chapter 9, Table 1 sets the limits by conductor count: one conductor up to 53% of internal area, two up to 31%, and three or more up to 40%. This calculator applies the right limit automatically based on the count you enter.
How is conduit fill calculated?
Sum the cross-sectional area of every conductor (Table 5 gives sq in per insulation type and size), divide by the conduit’s internal area (Table 4), and compare to the allowable percentage for that count.
Do THHN, THWN and XHHW use the same fill area?
THHN and THWN share the same approximate areas in Table 5, so they’re treated together. XHHW is slightly larger for some sizes; this tool uses the THHN/THWN column, the most common building-wire case. For XHHW above 8 AWG, confirm against your code book.
How many THHN conductors fit in 3/4″ EMT?
Switch on reverse mode. 3/4″ EMT has 0.533 sq in internal area; at 40% that’s 0.213 sq in, which fits about sixteen 12 AWG THHN. The tool shows the exact maximum for any size.
Is this conduit fill calculator NEC compliant?
It uses standard published NEC Chapter 9 values (Table 4, Table 5, Table 1). It’s a reference tool — always verify against the NEC edition adopted in your jurisdiction and account for derating, short nipples and grounding conductors.
Let Pilars take off your whole set.
Upload your plans. Pilars reads the symbols, counts devices and fixtures, follows homeruns, and returns conduit, wire, boxes and fittings by type. $100 per trade.