Conduit Fill Calculator (NEC Chapter 9)
Check whether your conductors fit a conduit per NEC Chapter 9, and find the smallest conduit size that passes. Supports EMT, IMC, RMC, PVC Schedule 40/80, ENT, FMC and LFMC with THHN/THWN-2 and XHHW conductors.
How to use it
- Select the conduit type and trade size.
- Add each conductor's insulation, size and quantity.
- Read the fill percentage and pass/fail; the tool also recommends the smallest conduit that fits.
Conduit fill compares the total conductor cross-sectional area (NEC Chapter 9, Table 5) against the conduit's allowable area (Table 4 area × the Table 1 percentage). The allowable fill is 53% for one conductor, 31% for two, and 40% for three or more.
Frequently asked questions
How do you calculate conduit fill?
Add up the cross-sectional area of all conductors (NEC Chapter 9, Table 5), then divide by the conduit's total interior area (Table 4). Compare that to the Table 1 limit for the number of conductors.
What is the maximum conduit fill percentage?
Per NEC Chapter 9, Table 1: 53% for a single conductor, 31% for two conductors, and 40% for three or more conductors.
Does conduit fill depend on wire insulation type?
Yes. Different insulations (e.g. THHN/THWN-2 vs XHHW) have different diameters for the same wire size, so they take up different amounts of space. Codewire accounts for the insulation type you choose.
All Codewire calculators · Free for electricians. Verify against the NEC edition adopted by your local AHJ.