Conduit Fill Calculator
Enter the outer diameter of your conductors and the number of wires to find the minimum conduit size. The calculator checks your fill against NEC Chapter 9 limits (53% for 1 wire, 31% for 2 wires, 40% for 3 or more) and shows pass or fail for every standard conduit size. Works for any wire type: enter the OD from your cable datasheet or pick a preset.
Last updated: May 2026
Enter conductor OD and count to find the minimum conduit size.
Fill% = n × (wire OD / 2)² / (conduit ID / 2)² × 100. NEC limits: 1 wire 53%, 2 wires 31%, 3+ wires 40%.
How conduit fill is calculated
Conduit fill is the ratio of the total cross-sectional area of all conductors to the internal cross-sectional area of the conduit. Because conductors are circular, the area of each conductor is pi times the square of its radius: A = pi × (OD/2)². The fill percentage is (n × wire area / conduit area) × 100. No correction factor, no packing coefficient: NEC and IEC both use this straightforward geometry.
The limits reflect how circles pack at different quantities. Two circles pack less efficiently than three or more (two circles in a tube leave more corner space wasted than three circles can fill with rotation), which is why the 2-conductor limit (31%) is lower than the 3-conductor limit (40%). The 1-conductor limit (53%) reflects that a single cable can be pulled into a much tighter fit.
NEC fill limits (Chapter 9, Table 1)
| Number of conductors | Maximum fill | Basis |
|---|---|---|
| 1 | 53% | Single cable, easiest to pull and seat |
| 2 | 31% | Two circles pack less efficiently than three or more |
| 3 or more | 40% | Multiple cables, standard installation |
EMT internal diameters (NEC Chapter 9, Table 4)
| Trade size | Internal diameter (mm) | Internal area (mm²) | Max conductors (40% limit) |
|---|---|---|---|
| 1/2" EMT | 15.80 | 196 | 3 × AWG 12 THWN-2 (35.6%) |
| 3/4" EMT | 20.93 | 344 | 3 × AWG 10 THWN-2 (30.6%) or 5 × AWG 12 |
| 1" EMT | 26.64 | 557 | 3 × AWG 8 or 7 × AWG 10 THWN-2 |
| 1-1/4" EMT | 35.05 | 965 | 3 × AWG 6 or 5 × AWG 8 THWN-2 |
| 1-1/2" EMT | 40.89 | 1314 | 3 × AWG 4 or 7 × AWG 8 THWN-2 |
| 2" EMT | 52.50 | 2165 | 3 × AWG 2 or 5 × AWG 4 THWN-2 |
Internal diameters from NEC Chapter 9, Table 4. AWG ODs derived from insulated conductor areas in Table 5 (THWN-2). "Max conductors" examples assume 3-plus fill limit of 40%.
About the IEC metric conduit presets
IEC metric conduit internal diameters vary by manufacturer and installation method (corrugated, rigid PVC, HDPE). The presets used in the calculator above (M20 = 16.0 mm, M25 = 21.0 mm, M32 = 27.5 mm, M40 = 34.5 mm, M50 = 43.0 mm, M63 = 54.5 mm) are typical for rigid PVC conduit. Always verify the internal diameter from your conduit manufacturer's datasheet before ordering materials.
Frequently Asked Questions
Why is the 2-conductor limit (31%) lower than the 3-conductor limit (40%)?
When two circles sit inside a larger circle, they leave large crescent-shaped voids at the sides that cannot be filled with more circles of the same size. With three or more circles the remaining voids shrink relative to the total area. NEC Chapter 9 Table 1 reflects this packing geometry: two conductors at 31% and three-plus at 40% is not a mistake or a conservative rounding: it is the correct order. Do not change 31% to 40% for a 2-conductor pull.
Does fill percentage include the equipment grounding conductor?
Yes. Every current-carrying conductor and the equipment grounding conductor (EGC) must be counted in the fill calculation. A standard 20A branch circuit (one hot, one neutral, one ground) is a 3-conductor run and must meet the 40% limit. The ground conductor may be smaller than the phase conductors, so use its actual outer diameter in the calculation, not the phase conductor OD.
What OD should I use if I have different wire sizes in the same conduit?
Calculate each wire's area separately (pi × (OD/2)²), sum the areas, and divide by the conduit area. The calculator above assumes all conductors are the same OD. For mixed sizes, use the formula directly: total fill area = sum of (pi × (OD_i / 2)²) for each conductor i. Then fill% = total fill area / conduit area × 100, compared against the limit for the total conductor count.
Can I use a conduit that is exactly at the 40% fill limit?
Technically yes: NEC states the fill must not exceed the limit, so exactly 40% is code-compliant. In practice, leave a small margin. Conductors at exactly 40% fill are very difficult to pull without damaging insulation, especially with bends in the run. Most electricians target 30 to 35% to keep pulling tension manageable. If you are at 38 to 40%, go up one conduit trade size.
Does the fill calculation change for flexible conduit?
The percentage limits are the same (53/31/40%) but the internal diameters differ from EMT. Flexible Metal Conduit (FMC) and Liquidtight Flexible Metal Conduit (LFMC) have their own internal diameter tables in NEC Chapter 9. The calculator lets you type any conduit ID, so enter the ID from the FMC/LFMC table for your trade size and it will calculate correctly.
Next step in this workflow
Conduit sized: now verify the wire gauge handles your load and voltage drop.