Free conduit fill calculator — NEC Chapter 9, done right
Pick the raceway, add every conductor in the run, and see the fill percentage against the NEC limit for that number of conductors. If you are over, it tells you the next size of the same conduit that works. No signup, no email.
The areas come straight from NEC Chapter 9: Table 4 for the conduit, Table 5 for the conductors, Table 1 for the percentage. Every value is listed under "How we calculated this" so you can check it against the book.
How to use it
1. Pick the conduit type and trade size. EMT, PVC Schedule 40 and 80, IMC and rigid metal, from ½" to 4".
2. Add conductors. Size, insulation type and quantity. Add grounds and neutrals — they count toward fill. Mixed sizes are fine; add a row per size.
3. Nipples. If the raceway is 24 inches or less between boxes or enclosures, tick the box and the limit goes to 60% under Chapter 9 Note 4.
How the math works
- Conduit area is the total internal cross-section from Table 4 — for example ¾" EMT is 0.533 sq in.
- Conductor area is from Table 5 and includes the insulation: #12 THHN is 0.0133 sq in, #12 XHHW is 0.0181. The insulation type matters more than most people expect.
- The limit comes from Table 1: 53% for one conductor, 31% for two, 40% for three or more. The odd-looking 31% for two conductors exists because two round objects pack a circle badly and can jam.
- Fill % is total conductor area ÷ conduit area. The limit applies to the sum, so nine small conductors and one big one are judged together.
Worked example: how many #12 THHN in ¾" EMT?
¾" EMT has 0.533 sq in. At 40% that is 0.213 sq in available. Each #12 THHN is 0.0133 sq in, so 0.213 ÷ 0.0133 = 16.0 — 16 conductors, which matches the Annex C table. Switch to XHHW and the answer drops to 11. A 20 A circuit with three #12s and a ground is at 10% fill in ¾" EMT, so the conduit is nowhere near the limiting factor; ambient temperature and the number of current-carrying conductors are.
What this tool does not do
It does not derate for more than three current-carrying conductors (310.15), it does not size the conduit for pulling tension on long runs with bends, and it does not cover compact conductors, cables in raceway, or multi-conductor assemblies, which use other tables. It is a fill check, and the AHJ's interpretation and your adopted code edition govern.
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FAQ
What is the maximum conduit fill percentage?
40% for three or more conductors, 31% for two, and 53% for a single conductor, from NEC Chapter 9 Table 1. Nipples of 24 inches or less between enclosures may be filled to 60%.
How many #12 THHN wires can go in 3/4 EMT?
16. Three-quarter EMT has 0.533 sq in, 40% of that is 0.213 sq in, and each #12 THHN is 0.0133 sq in. In ½-inch EMT the answer is 9, and in 1-inch EMT it is 26.
How many #10 THHN in 1 inch EMT?
16. One-inch EMT has 0.864 sq in, 40% is 0.346, and #10 THHN is 0.0211 sq in each. The same conduit takes 26 #12s or 35 #14s.
Do ground wires count toward conduit fill?
Yes. Every conductor in the raceway counts, including equipment grounding conductors and neutrals. Bare grounds use the smaller Table 8 areas; this calculator uses the insulated figure, which is conservative.
Why is the fill limit for two conductors only 31%?
Two round conductors pack poorly inside a circle and tend to jam against each other and the wall during a pull, so the code allows less area than for three or more, where the conductors settle into a bundle.
Does conduit fill change with PVC vs EMT?
Yes, because the internal area differs. ¾-inch PVC Schedule 40 has 0.508 sq in against 0.533 for EMT, and Schedule 80 drops to 0.409 — its thicker wall costs you about a fifth of the fill. Pick the actual conduit type in the calculator.
Is this calculator free?
Yes, completely. No account, no email, no download. It runs in your browser and we never see your numbers.