Conduit Fill Calculator (Max Conductors & Fill Percent)

Conduit fill is an area problem. The NEC limits how much of a conduit's inside cross-section the conductors may occupy so the wires can be pulled without damage and can shed heat. This calculator compares the total area of your conductors against the allowable area of the conduit and tells you whether the pull passes, plus how many conductors of that size the conduit can hold. It reproduces published NEC tables and is meant for planning — a licensed electrician and your local code have the final word.

The fill percentage comes from NEC Chapter 9, Table 1: one conductor may fill up to 53%, exactly two are limited to 31%, and three or more may fill up to 40%. The calculator picks the right percentage automatically from your conductor count. Conduit areas come from NEC Chapter 9, Table 4 — the tool covers EMT (electrical metallic tubing) from 1/2 inch through 2 inch. Individual conductor areas come from NEC Chapter 9, Table 5 for THHN/THWN, the most common building wire.

To find capacity, the calculator multiplies the conduit's internal area by the 40% limit and divides by the area of one conductor, rounding down to a whole wire. To check a specific pull, it adds up the conductor areas and compares them with the allowable area. If you are over, step up a trade size or use a smaller conductor. Note that a bare or insulated equipment ground still counts as a conductor for fill.