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.
Fitting the sized cable into real conduit
By the time you reach this step, the conductor cross-section is already fixed: the wire gauge calculator chose the minimum mm² for ampacity and voltage drop, the voltage drop calculator confirmed the run stays within the IEC 3% limit, and the fuse calculator sized the overcurrent protection to match that conductor. You are here: fit the cable in conduit, which is the last physical check before the circuit is complete. This calculator tells you whether your chosen conduit trade size can legally accept all the conductors you plan to pull through it. Once the conduit is confirmed, the supply side of the circuit is still open: the power supply calculator sizes the source rating, and on reactive or motor loads the power factor calculator converts watts to the VA figure the supply must actually deliver. After the circuit is energised and feeding appliances, the appliance running cost calculator and the electricity cost calculator turn load into a monthly figure. One thing worth keeping in mind at this stage: bundled cables in a packed conduit run hotter than they do in free air, and that extra heat derates their ampacity, which means a gauge that passed the ampacity check on its own can still fail once it shares a crowded conduit. The conduit fill check and the ampacity derate check belong in the same conversation.
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.
If the fill check fails, should I upsize the conduit or switch to a smaller conductor?
Almost always upsize the conduit, not the conductor. Your conductor size is set by ampacity and voltage drop: shrinking it to fit the conduit puts the circuit below code. The practical fix is to go up one trade size on the conduit, which roughly doubles the internal area and often drops fill from above 40% to well below it. If you are constrained on conduit size (a buried duct that is already in the ground, or a sleeve through a structural element), the only legal path is to split the conductors into two separate conduits, not to use a smaller gauge wire. The exception is when you are still in the planning stage and the conductor size was already one standard size above the minimum: then switching down to the true minimum may bring fill within range, though this leaves no headroom for future loads.
Methodology and sources
This tool computes raceway fill as the ratio of total conductor cross-sectional area to the internal cross-sectional area of the conduit, then reports the smallest standard conduit that stays at or below the allowable fill for your conductor count.
- Method: Fill% = n × (wire OD / 2)² / (conduit ID / 2)² × 100, where n is the conductor count. Because both areas use the same π factor, it cancels in the ratio, so the result depends only on conductor outer diameter and conduit internal diameter.
- Standards and sources: NEC (NFPA 70) Chapter 9, Table 1 fill limits (53% for 1 conductor, 31% for 2, 40% for 3 or more) and Table 4 EMT internal diameters; AWG conductor outer diameters taken from NEC Chapter 9, Table 5 (THWN-2). The IEC metric presets are typical rigid-PVC internal diameters that vary by manufacturer.
- Assumptions and limits: Conductors are treated as circles using the outer diameter you enter (from your cable datasheet). The built-in calculator assumes all conductors share one OD; for mixed sizes, sum each conductor's area separately. EMT internal diameters are fixed NEC values; metric conduit IDs are approximate and must be confirmed against your manufacturer's datasheet before ordering.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This is a planning aid, not a substitute for a qualified electrician or your local wiring and building code; verify conduit fill, conductor selection and installation against the edition of NEC or IEC 60364 that applies where you work before doing any wiring.
Next step in this workflow
Conduit sized: now verify the wire gauge handles your load and voltage drop.