Check how full a conduit is. Choose the conduit type and trade size, add up to four wire sizes with counts, and see the fill percentage against the NEC limit (53%, 31% or 40%), the smallest conduit that passes and how many more wires would fit.
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Wires (size and count)
Enter your values to see the result.
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How to use the Conduit Fill Calculator
Choose the conduit type and trade size.
Choose the wire insulation (THHN/THWN-2 is the most common in conduit).
Enter each wire size and count, including the equipment ground.
Tick the nipple box for a conduit section of 24 in or less.
Read the fill % and pass/fail, then check the table for other sizes.
NEC conduit fill limits
Number of conductors
Maximum fill
Reference
1
53%
NEC Chapter 9, Table 1
2
31%
NEC Chapter 9, Table 1
3 or more
40%
NEC Chapter 9, Table 1
Nipple 24 in (600 mm) or shorter
60%
NEC Chapter 9, Note 4
Fill % = Σ (wire count × wire area) ÷ conduit internal area × 100. Wire areas are from NEC Chapter 9, Table 5 and conduit areas from Table 4.
Maximum THHN wires per EMT size (40% fill)
EMT size
40% area in²
14 AWG
12 AWG
10 AWG
8 AWG
6 AWG
1/2"
0.122
12
9
5
3
2
3/4"
0.213
22
16
10
5
4
1"
0.346
35
26
16
9
6
1-1/4"
0.598
61
45
28
16
11
1-1/2"
0.814
84
61
38
22
16
2"
1.342
138
100
63
36
26
Counts are for conductors of a single size; a 2-wire run is limited to 31% and a single wire to 53%. Where the division gives a decimal of 0.8 or more, NEC Chapter 9 Note 7 lets you round up to the next whole conductor (for example 0.213 ÷ 0.0097 = 21.96 → 22), which is how the NEC Annex C tables are built; the calculator's pass/fail uses the strict percentage. Always confirm with the code edition adopted where you work.
Frequently asked questions
What is the NEC conduit fill limit?
NEC Chapter 9, Table 1 limits the total cross-section of conductors to 53% of the conduit's internal area for one conductor, 31% for two, and 40% for three or more. Nipples 24 inches or shorter may be filled to 60%.
How do I calculate conduit fill?
Add the cross-sectional area of every conductor (from NEC Chapter 9, Table 5 for the insulation type), divide by the internal area of the conduit (Table 4) and multiply by 100. Compare the result with the 53/31/40% limit.
How many 12 AWG THHN wires fit in 3/4 inch EMT?
3/4" EMT has 0.533 in² of internal area, so 40% is 0.213 in². A 12 AWG THHN wire is 0.0133 in², which allows 16 conductors. 1/2" EMT (0.122 in² at 40%) allows 9.
Do I count the ground wire?
Yes. Equipment grounding conductors count toward conduit fill and must be included with their actual area. They do not count as current-carrying conductors for ampacity derating.
Why is the 2-wire limit only 31%?
Two round conductors side by side can jam in a bend when they are close to the conduit diameter. The lower limit leaves room to pull them without damaging the insulation.
Is passing conduit fill enough?
Not always. More than three current-carrying conductors in one raceway also require ampacity derating (NEC 310.15(C)(1)), and long pulls with many bends may need a larger conduit to stay within pulling tension limits.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.