Free Conduit Fill Calculator

Per NEC Chapter 9, Table 1: one conductor may fill up to 53% of the conduit's internal area, two conductors up to 31%, and three or more up to 40%. This calculator applies the right limit automatically based on how many conductors you enter.

Add up the cross-sectional area of every conductor and divide by the raceway's internal area, then compare the result to the 53% / 31% / 40% limit for your conductor count. This tool works each area out as π/4 × diameter², using conductor outside diameters from a THHN manufacturer spec sheet and raceway inside diameters from manufacturer/ANSI dimension data. The NEC's own Chapter 9 dimension tables govern — results here land close to them, but verify with your adopted edition.

It uses stranded copper THHN/THWN-2 — by far the most common building wire — with diameters from a manufacturer spec sheet. Other insulations (XHHW, RHW, USE) are thicker or thinner; for those, use the dimension table in NEC Chapter 9 of your adopted edition or the wire manufacturer's data sheet.

Yes. Every conductor in the raceway counts toward fill, including equipment grounding conductors and neutrals. Add them as rows here.

The percentages come from heat dissipation and the practical ability to pull conductors without damaging insulation. More conductors means more bundled heat and friction, so the allowable fill drops from 53% (one) to 40% (three or more).