Conduit Fill Calculator

Enter your wire count, wire diameter, and conduit inner diameter to get the fill percentage and whether it meets the NEC max.

How to use this calculator ↓

What your result means

Fill is the wire cross-section as a percent of the conduit's inside area; the NEC max depends on the number of wires (53% for one, 31% for two, 40% for three or more). This uses the wire OD you enter, counts every wire including the ground, and doesn't handle ampacity derating from bundling. Verify against the current NEC and your local inspector before you pull.

How to use this calculator

  1. Count wires going in.
  2. Look up wire diameter (THHN #12: 0.13 in, #10: 0.16 in).
  3. Look up conduit ID (1/2 in EMT: 0.622 in, 3/4 in: 0.824 in).
  4. Read fill % and NEC max fill.
  5. Calculator says YES/NO for code compliance.

The formula

wireArea = PI * (wireDia/2)^2 fillPct = numWires * wireArea / (PI * (conduitID/2)^2) * 100 maxFill = 53% (1 wire), 31% (2), 40% (3+)

Add up the wire areas, divide by the conduit's inside area, and multiply by 100 for the fill percent. The NEC max is 53% for one wire, 31% for two, and 40% for three or more.

Worked example

Say you've got 3 wires of #12 THHN (0.13 in OD) in 1/2 inch EMT (0.622 in ID). That's 13.1% fill against a 40% max, so it passes the NEC with lots of room.

NEC Chapter 9 max fill

WiresMax fill
1 wire53%
2 wires31%
3 or more40%

Tips & gotchas

  • NEC Chapter 9 Table 1 max fill: 53% for one wire, 31% for two, 40% for three or more.
  • Count every wire; ground, hot, and neutral all count equally.
  • Use the actual wire OD with insulation, not the bare conductor.
  • Near max fill, you'll want pulling lubricant.
  • Derate ampacity when you bundle conductors.

Frequently asked questions

What is conduit fill?

The percentage of the conduit's cross-section taken up by wires.

What's the NEC max conduit fill?

53% for one wire, 31% for two, 40% for three or more.

Why does conduit fill matter?

Too many wires trap heat and can damage the insulation.

Do I count the ground wire?

Yes. Every wire counts toward fill.

Where do I find wire diameter?

NEC Chapter 9 Table 5, or the manufacturer's spec sheet.

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Maintained by the CalcWright Team. Formulas verified against industry-standard sources. Last updated: May 25, 2026

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