How to use this conduit fill calculator
- Choose EMT or PVC Schedule 40 and a trade size.
- Add each group of conductors: quantity and THHN/THWN-2 size. Use “Add another wire size” for mixed fills, such as 6 AWG feeders with a 10 AWG ground.
- Read the fill percentage against the allowed limit. If it fails, the calculator names the smallest conduit size that passes.
When every conductor is the same size, it also shows the maximum number that fit, applying the Chapter 9 Note 7 rounding rule.
The 40% rule (NEC Chapter 9, Table 1)
The NEC limits how much of a raceway’s cross-section conductors can occupy, so wire can be pulled without damaging insulation and heat can escape:
| Number of conductors | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
Two conductors get the lowest limit because two round wires can wedge side by side across the conduit and jam during a pull. A conduit nipple 24 inches or shorter between boxes may be filled to 60% (Chapter 9, Note 4), and this calculator doesn't apply that exception.
Worked example: nine 12 AWG THHN in 3/4" EMT
Four 20 A circuits share one raceway: four hots, four neutrals and one ground, for nine 12 AWG THHN conductors. Will they fit in 3/4" EMT?
It passes easily. The most 12 AWG THHN that fits in 3/4" EMT is 0.533 × 40% ÷ 0.0133 = 16.03, so 16 conductors. That matches the figure in the NEC’s Informative Annex C tables.
Fitting is not the same as allowed to carry full current. With more than three current-carrying conductors in a raceway, NEC 310.15 requires ampacity adjustment. Here the eight hots and neutrals are current-carrying (the ground isn't), so they're derated to 70% of their table ampacity.
Conduit and wire areas used
Total internal area from NEC Chapter 9, Table 4 (square inches):
| Trade size | EMT | PVC Sch 40 |
|---|---|---|
| 1/2" | 0.304 | 0.285 |
| 3/4" | 0.533 | 0.508 |
| 1" | 0.864 | 0.832 |
| 1-1/4" | 1.496 | 1.453 |
| 1-1/2" | 2.036 | 1.986 |
| 2" | 3.356 | 3.291 |
| 2-1/2" | 5.858 | 4.695 |
| 3" | 8.846 | 7.268 |
| 3-1/2" | 11.545 | 9.737 |
| 4" | 14.753 | 12.554 |
THHN/THWN-2 conductor area from Chapter 9, Table 5 (square inches):
| Size | Area | Size | Area |
|---|---|---|---|
| 14 AWG | 0.0097 | 1/0 | 0.1855 |
| 12 AWG | 0.0133 | 2/0 | 0.2223 |
| 10 AWG | 0.0211 | 3/0 | 0.2679 |
| 8 AWG | 0.0366 | 4/0 | 0.3237 |
| 6 AWG | 0.0507 | 250 kcmil | 0.3970 |
| 4 AWG | 0.0824 | 300 kcmil | 0.4608 |
| 3 AWG | 0.0973 | 350 kcmil | 0.5242 |
| 2 AWG | 0.1158 | 500 kcmil | 0.7073 |
| 1 AWG | 0.1562 |
Tips for easier pulls
- Leave headroom. A conduit at 39% passes on paper, but long runs with several bends pull far more easily at 25–30%.
- Count the bends. The NEC allows no more than 360° of bends between pull points (for example 358.26 for EMT).
- Watch for jamming. When three conductors nearly fill a conduit (a conduit-to-wire diameter ratio of about 2.8 to 3.2), they can line up and jam in bends. Go up a size if you're close.
- Plan for the future. Upsizing one trade size now costs little compared with pulling a second run later.
Frequently asked questions
How many 12 AWG THHN wires fit in 1/2" EMT?
Nine. 0.304 in² × 40% = 0.1216 in² of usable area, divided by 0.0133 in² per conductor = 9.1, which rounds down to 9.
Do grounding conductors count toward conduit fill?
Yes. Equipment grounding and bonding conductors count toward fill (Chapter 9, Note 3). They don’t count as current-carrying conductors for ampacity adjustment.
What is the conduit fill for two wires?
31%. Two conductors have the lowest limit because they can wedge across the conduit during a pull. One conductor may use 53%, and three or more use 40%.
Can I use this for XHHW or other insulation types?
This version uses THHN/THWN-2 areas, the most common building wire. XHHW, RHW and other insulations have different areas in Table 5, and compact conductors are in Table 5A. For those, multiply the actual area from the table by the quantity and compare it with 40% of the conduit area.
Does conduit fill change with the 2023 NEC?
The fill percentages in Chapter 9, Table 1 and the dimensions in Tables 4 and 5 have been stable for many code cycles, but always confirm against the edition your jurisdiction has adopted.
Last reviewed October 11, 2026. Results are estimates for planning. Always verify against the edition of the NEC adopted in your area, local amendments and the authority having jurisdiction. Found a mistake? Tell us.