Calculators
Conduit Fill Calculator
Calculate conduit fill for THHN conductors and the smallest EMT trade size within the 40% fill limit.
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- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Choosing conduit size for a circuit run
- Checking an existing conduit for added wires
- Electrical estimating
- Apprentice training
About this tool
What the Conduit Fill Calculator does
Electrical codes limit how much of a conduit’s cross-section conductors may occupy: 53% for one conductor, 31% for two and 40% for three or more. Add up the area of each conductor including insulation and compare it with the conduit’s internal area. This calculator uses standard THHN and EMT dimensions.
Formula
- Total area = Σ conductor area × quantity
- Maximum fill = 53% (1 conductor), 31% (2), 40% (3 or more)
- Fill = total area ÷ conduit internal area
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Conductors | 2 entries |
| Result | Value |
|---|---|
| Smallest EMT size | 1/2 in |
| Fill in that conduit | 25.2% |
| Maximum fill allowed | 40.0% |
| Total conductor area | 0.0766 |
| Number of conductors | 4 |
How to read the result
Include equipment grounding conductors in the count and area.
More conductors in a conduit can also require ampacity adjustment for heating, which is a separate calculation.
For planning only. Electrical, gas and refrigerant work must follow current codes and be carried out or checked by a licensed professional.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
40% of the conduit's internal area for three or more conductors, 31% for two and 53% for one.
Yes. Equipment grounding and bonding conductors are included.
This uses EMT dimensions; PVC and rigid conduit have different internal areas.
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