Conduit Fill Calculator (NEC)

How to use this calculator

1. Pick the conduit type (EMT, PVC, RMC, IMC).
2. Choose the wire insulation (THHN or XHHW) and the conductor size.
3. Enter the number of conductors going in the conduit.
4. Click Calculate to get the smallest trade size and the resulting fill %.

How conduit fill is calculated

NEC Chapter 9 limits how much of a conduit's internal cross-section the conductors may occupy, to leave room for pulling and heat dissipation. The maximum fill depends only on the number of conductors: 53% for one, 31% for two, and 40% for three or more. The calculator multiplies the cross-sectional area of one conductor (Table 5) by the number of conductors, then finds the smallest trade size whose allowable fill area (Table 4) is at least that total.

Quick reference: max THHN conductors in EMT

Maximum number of same-size THHN conductors (NEC Annex C.1):

Notes & limits

• This tool assumes all conductors are the same size and insulation. For mixed sizes, add the individual Table 5 areas together.
• Equipment grounding and bonding conductors count toward the total.
• Values use round conductors and standard trade sizes ½″ through 4″.

Always confirm against the current code edition and local amendments.

Frequently asked questions

How many wires fit in a ½″ conduit?

It depends on wire size and type — for example a ½″ EMT holds up to 12 × 14 AWG or 9 × 12 AWG THHN conductors (NEC Annex C.1).

What is the 40% fill rule?

For three or more conductors, the total conductor area may not exceed 40% of the conduit's internal area. It is 53% for a single conductor and 31% for exactly two.

Do ground wires count in the fill?

Yes. Equipment grounding and bonding conductors count toward the fill, even if they don't carry normal current.