How to use the conduit fill calculator
Pick the conduit type and trade size. In How many fit mode, choose a wire size and the calculator gives the maximum number of THHN/THWN-2 conductors allowed, following the method in NEC Annex C. In Check a mix mode, list up to three wire sizes with counts (include the grounding conductor) and it reports the percent fill against the limit for that number of wires.
The method
allowed area = conduit area (Table 4) × 53% (1 wire), 31% (2 wires) or 40% (3+ wires)
max same-size wires = allowed area ÷ wire area (Table 5); round up if the decimal is 0.8 or more (Note 7)
mixed wires: Σ wire areas ≤ allowed area
Worked example: 12 AWG THHN in ¾ in EMT
- Table 4: ¾ in EMT has 0.533 in² inside, so 40% is 0.213 in².
- Table 5: one 12 AWG THHN is 0.0133 in².
- 0.213 ÷ 0.0133 = 16.0, so 16 conductors.
Quick reference: THHN in EMT (40% fill)
| Wire | ½ in | ¾ in | 1 in | 1¼ in |
|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 |
| 12 AWG | 9 | 16 | 26 | 45 |
| 10 AWG | 5 | 10 | 16 | 28 |
| 8 AWG | 3 | 6 | 9 | 16 |
| 6 AWG | 1 | 4 | 7 | 12 |
Calculated from Tables 1, 4 and 5 with Note 7, the same way NEC Annex C Table C.1 is built.
Things that catch people out
- Fill isn't derating. Twelve 12 AWG wires may fit in a conduit, but nine or more current-carrying conductors reduce each one's ampacity to 70% or less.
- Different insulation, different areas. XHHW, THW and RHW are bigger or smaller than THHN; this calculator uses THHN/THWN-2 only.
- Short nipples (24 in or less between boxes) may be filled to 60% (Chapter 9 Note 4).
- Pulling is a separate question. A legal fill can still be a hard pull through several bends. Leave room for long runs.
Questions people ask
How many 12 gauge wires can go in 1/2 inch EMT?
Nine 12 AWG THHN. NEC Table 4 gives ½ in EMT 0.122 in² at the 40% fill allowed for three or more wires, and Table 5 gives 12 AWG THHN 0.0133 in² each: 0.122 ÷ 0.0133 = 9.2, so 9.
How many 12 gauge wires in 3/4 EMT?
Sixteen 12 AWG THHN: 0.213 in² ÷ 0.0133 in² = 16.0.
What is the 40% conduit fill rule?
NEC Chapter 9 Table 1 limits conductors to 53% of the conduit's area for one wire, 31% for two wires, and 40% for three or more. The odd 31% for two wires exists because two round wires can jam against each other in a bend.
What is Note 7 for conduit fill?
When every conductor is the same size and the calculation leaves a decimal of 0.8 or more, Chapter 9 Note 7 lets you round up to the next whole number. For example, 8 AWG THHN in ¾ in EMT works out to 5.8, so 6 are allowed.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding conductors count toward fill (Chapter 9 Note 3), even though they don't count as current-carrying for ampacity derating. Include them in the wire list.
Sources and assumptions
- NFPA 70 (NEC) Chapter 9 Table 4: conduit and tubing internal areas (EMT, PVC Sch 40/80, RMC), 2011 table pages
- NEC Chapter 9 Table 5: THHN, THWN, THWN-2 conductor areas
- NEC Chapter 9 Table 1 (53% / 31% / 40%) and Note 7 (round up at 0.8 or more for same-size conductors). Table 4 and 5 values have been stable across recent editions; check your adopted code.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.