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How the Conduit Fill Calculator works
This calculator checks how full a conduit is under NEC Chapter 9. Pick EMT, IMC, rigid or PVC (Schedule 40 or 80), the trade size, THHN/THWN-2 or XHHW-2 wire, the wire size and the number of wires. It shows the fill percentage, the limit that applies, the most wires of that size that fit and the smallest conduit that takes them.
Fill % = number of wires × area of one wire (Chapter 9 Table 5) ÷ inside area of the conduit (Table 4). The limit comes from Chapter 9 Table 1: 53% for one wire, 31% for two, 40% for three or more. For the most wires that fit, Note 7 lets the count round up to the next whole wire when its decimal part is 0.8 or more.
Example: four 10 AWG THHN in 1/2" EMT. One wire is 0.0211 in², four are 0.0844 in², and 1/2" EMT has 0.304 in² inside, so the fill is 0.0844 ÷ 0.304 = 27.76%, under the 40% limit. The most that fit: 40% of 0.304 is 0.1216 in², and 0.1216 ÷ 0.0211 = 5.76; the decimal is under 0.8, so the answer is 5.
Count the equipment grounding conductor too (Chapter 9 Note 3). The wire areas are for stranded THHN/THWN-2 and XHHW-2; compact conductors, cables and mixed sizes are not covered, and the 60% allowance for nipples of 24" or less (Note 4) is not applied. Values are from the 2017 NEC; check the edition your AHJ enforces. Fill is only one rule: the ampacity adjustment for more than three current-carrying conductors (310.15) still applies, and this tool does not decide whether an installation passes inspection.
Frequently asked questions
How many 12 AWG THHN wires fit in 1/2 inch EMT?
9. 40% of 1/2" EMT's 0.304 in² is 0.1216 in²; one 12 AWG THHN is 0.0133 in², and 0.1216 ÷ 0.0133 = 9.14, so 9. In 3/4" EMT it is 16 (0.2132 ÷ 0.0133 = 16.03).
What is the 40 percent conduit fill rule?
NEC Chapter 9 Table 1 limits three or more conductors to 40% of the conduit's inside area. One conductor may fill 53% and two conductors 31%. The percentages use the conductor areas in Table 5 and the conduit areas in Table 4.
Is there a conduit fill chart for THHN?
NEC Annex C has tables with the maximum number of same-size conductors for each conduit type (Table C.1 is EMT). This calculator works the same numbers out from Chapter 9 Tables 4 and 5, and also checks a count you choose. For THHN in 1/2" EMT: 12 of 14 AWG, 9 of 12 AWG, 5 of 10 AWG, 3 of 8 AWG.
Do I count the ground wire in conduit fill?
Yes. Chapter 9 Note 3 says equipment grounding and bonding conductors count, using their actual size, insulated or bare. Add the ground to the number of wires.
What does Note 7 mean for conduit fill?
When you work out the maximum number of same-size conductors, Chapter 9 Note 7 rounds the result up to the next whole number if its decimal is 0.8 or more. 14 AWG THHN in 3/4" EMT: 0.2132 ÷ 0.0097 = 21.98, so 22 are allowed even though they fill 40.04%.