How to use the conduit fill calculator
- Choose the conduit or tubing type. EMT, IMC, rigid metal (RMC), PVC Schedule 40 or 80, ENT, flexible metal (FMC) or liquidtight (LFMC). Each type has a different inside diameter, so this matters more than most people expect.
- Select the trade size, from 3/8" (flex only) up to 6".
- Add every conductor that will be in the run: hots, neutrals, travelers and equipment grounds. For each row pick the insulation type, the AWG or kcmil size and the quantity.
- Read the result. The calculator totals the wire area, applies the correct Table 1 limit (53%, 31% or 40%), shows pass or fail and tells you the smallest trade size that would work.
- Need a quick count instead? Open the Max wires of one size tab and pick a single wire type to get the Annex C-style maximum for every trade size.
Conduit fill formula
Conduit fill is a simple ratio of the space taken by the wires to the space inside the raceway:
The conductor areas come from NEC Chapter 9, Table 5 (insulated building wire) or Table 5A (compact conductors), and include the insulation. The internal area of each raceway comes from Table 4. The result must not exceed the percentage allowed by Table 1:
| Number of conductors | Maximum fill | Why |
|---|---|---|
| 1 | 53% | A single wire can't jam against others |
| 2 | 31% | Two round wires can wedge side by side across the pipe |
| Over 2 | 40% | Standard limit for most branch circuits and feeders |
| Nipple ≤ 24 in. | 60% | Chapter 9 Note 4 — short sections are easy to pull |
To find the maximum number of identical wires, flip the formula around: max wires = (conduit area × 40%) ÷ area of one wire, rounded down — or rounded up when the decimal is 0.8 or more (Note 7).
Worked example: 12 AWG THHN in EMT
Say you're running six 12 AWG THHN circuit conductors and one 10 AWG THHN ground in 3/4" EMT, and you want to know whether it fits.
- Wire area: 6 × 0.0133 in² + 1 × 0.0211 in² = 0.1009 in².
- 3/4" EMT internal area (Table 4): 0.533 in².
- Fill: 0.1009 ÷ 0.533 = 18.9%. Seven conductors means the 40% limit applies, so this passes with room to spare.
How far could you go? 1" EMT has 0.864 in² of space and 40% of that is 0.3456 in². Divide by 0.0133 in² per wire and you get 25.98 — the decimal is above 0.8, so Note 7 lets you round up to 26 conductors. That matches the published figure in NEC Annex C, Table C.1.
Conduit fill chart: THHN in EMT
Maximum number of THHN/THWN-2 conductors (all the same size) in EMT, calculated with the same rules as NEC Annex C, Table C.1.
| Wire | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" | 3-1/2" | 4" |
|---|---|---|---|---|---|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 | 84 | 138 | 241 | 364 | 476 | 608 |
| 12 AWG | 9 | 16 | 26 | 45 | 61 | 101 | 176 | 266 | 347 | 443 |
| 10 AWG | 5 | 10 | 16 | 28 | 38 | 63 | 111 | 167 | 219 | 279 |
| 8 AWG | 3 | 6 | 9 | 16 | 22 | 36 | 64 | 96 | 126 | 161 |
| 6 AWG | 2 | 4 | 7 | 12 | 16 | 26 | 46 | 69 | 91 | 116 |
| 4 AWG | 1 | 2 | 4 | 7 | 10 | 16 | 28 | 43 | 56 | 71 |
| 3 AWG | 1 | 1 | 3 | 6 | 8 | 13 | 24 | 36 | 47 | 60 |
| 2 AWG | 1 | 1 | 3 | 5 | 7 | 11 | 20 | 30 | 40 | 51 |
| 1 AWG | 1 | 1 | 1 | 4 | 5 | 8 | 15 | 22 | 29 | 37 |
| 1/0 AWG | 1 | 1 | 1 | 3 | 4 | 7 | 12 | 19 | 25 | 32 |
| 2/0 AWG | 0 | 1 | 1 | 2 | 3 | 6 | 10 | 16 | 20 | 26 |
| 3/0 AWG | 0 | 1 | 1 | 1 | 3 | 5 | 8 | 13 | 17 | 22 |
| 4/0 AWG | 0 | 1 | 1 | 1 | 2 | 4 | 7 | 11 | 14 | 18 |
Conduit internal areas (NEC Table 4)
Total internal area in square inches. The 40% column you'll use most often is simply these values × 0.40.
| Trade size | EMT | PVC Schedule 40 | PVC Schedule 80 | RMC | IMC |
|---|---|---|---|---|---|
| 1/2" | 0.304 | 0.285 | 0.217 | 0.314 | 0.342 |
| 3/4" | 0.533 | 0.508 | 0.409 | 0.549 | 0.586 |
| 1" | 0.864 | 0.832 | 0.688 | 0.887 | 0.959 |
| 1-1/4" | 1.496 | 1.453 | 1.237 | 1.526 | 1.647 |
| 1-1/2" | 2.036 | 1.986 | 1.711 | 2.071 | 2.225 |
| 2" | 3.356 | 3.291 | 2.874 | 3.408 | 3.630 |
| 2-1/2" | 5.858 | 4.695 | 4.119 | 4.866 | 5.135 |
| 3" | 8.846 | 7.268 | 6.442 | 7.499 | 7.922 |
| 3-1/2" | 11.545 | 9.737 | 8.688 | 10.010 | 10.584 |
| 4" | 14.753 | 12.554 | 11.258 | 12.882 | 13.631 |
| 5" | — | 19.761 | 17.855 | 20.212 | — |
| 6" | — | 28.567 | 25.598 | 29.158 | — |
Wire areas by insulation type (NEC Table 5)
Insulation thickness changes the answer a lot. A 12 AWG RHW-2 with outer covering takes more than 2.6 times the space of a 12 AWG THHN.
| Size | THHN / THWN-2 | XHHW-2 | TW / THW | RHW-2 (covered) |
|---|---|---|---|---|
| 14 AWG | 0.0097 | 0.0139 | 0.0139 | 0.0293 |
| 12 AWG | 0.0133 | 0.0181 | 0.0181 | 0.0353 |
| 10 AWG | 0.0211 | 0.0243 | 0.0243 | 0.0437 |
| 8 AWG | 0.0366 | 0.0437 | 0.0437 | 0.0835 |
| 6 AWG | 0.0507 | 0.0590 | 0.0726 | 0.1041 |
| 4 AWG | 0.0824 | 0.0814 | 0.0973 | 0.1333 |
| 2 AWG | 0.1158 | 0.1146 | 0.1333 | 0.1750 |
| 1/0 AWG | 0.1855 | 0.1825 | 0.2223 | 0.3039 |
| 4/0 AWG | 0.3237 | 0.3197 | 0.3718 | 0.4754 |
| 250 kcmil | 0.3970 | 0.3904 | 0.4596 | 0.6291 |
| 500 kcmil | 0.7073 | 0.6984 | 0.7901 | 1.0082 |
Conduit fill rules that trip people up
- Grounds count. Equipment grounding conductors, insulated or bare, are part of the fill (Note 3).
- Mixed sizes can't use the 0.8 rule. Rounding up under Note 7 is only allowed when every conductor is the same size.
- Jam ratio. When pulling exactly three conductors, check that the conduit ID ÷ conductor OD is not between 2.8 and 3.2 — wires can line up and jam (Informational Note to Table 1).
- Fill isn't ampacity. A legal fill can still require derating once you have four or more current-carrying conductors. Run the numbers in the ampacity calculator.
- Boxes have their own rules. Junction and device boxes are sized by volume under NEC 314.16 — use the box fill calculator.
- Code edition. Table 4 and Table 5 values used here match NEC 2017, 2020 and 2023. ENT dimensions changed in 2017; if your area still enforces an older edition, check the book.
From fill to a complete design
Conduit fill is usually the last step. First pick the conductor with the wire size calculator, confirm that long runs stay within about 3% using the voltage drop calculator, then come back here to choose the smallest raceway that holds everything.