Conduit Fill Calculator

Pick the conduit and the wire — get the legal maximum conductor count.

Conduit Trade size Conductor (THHN/THWN-2)

How fill is figured

NEC Chapter 9 limits conduit fill to 53% of internal area for one conductor, 31% for two, and 40% for three or more. The calculator divides the internal area of your EMT or PVC Schedule 40 size by the per-conductor area of THHN/THWN-2 and floors the result at the applicable limit — the same arithmetic an inspector runs.

Handy references

At 40% fill, 1/2″ EMT takes up to 9 × 12 AWG THHN, 3/4″ EMT up to 16, and 1″ EMT up to 25. PVC Schedule 40 runs slightly lower than EMT at the same trade size because its walls are thicker. Mixed wire sizes need an area sum — v1 covers single-size pulls; use the per-size counts as bounds.

Counts are for THHN/THWN-2. Other insulations (XHHW, NM cable) have different areas. Equipment grounding conductors count toward fill; pull complexity may argue for a bigger conduit even when the math passes.