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Conduit Fill Calculator

Find the maximum number of THHN conductors that fit in a conduit, per the NEC's 40% fill rule.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Maximum conductors

16 conductors (maximum)

Check your inputs

This uses standard EMT and THHN area tables — different conduit types (PVC, rigid) or conductor insulation types have different area values. Always verify against the current NEC Chapter 9 tables for your specific materials.

Available fill area (40% of conduit)
0.21 in²

With the figures above, the maximum conductors comes to 16 conductors (maximum). The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 16 conductors (maximum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • NEC Chapter 9, Table 1: conduit fill is limited to 40% of internal area for 3 or more conductors
  • Approximate THHN conductor areas and EMT conduit internal areas, per NEC Chapter 9 tables

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why only 40% fill instead of filling the conduit completely?
The remaining space allows conductors to be pulled through without excessive friction or insulation damage, and helps conductors dissipate heat — packing a conduit too tightly can cause overheating and make future wire pulls impossible.
Does this rule change for 1 or 2 conductors?
Yes — the NEC allows a higher fill percentage (53% for 1 conductor, 31% for 2) since there's less friction and heat interaction with fewer conductors. This calculator assumes the common 3-or-more-conductor scenario at 40%.
Does mixing different wire gauges in one conduit change this math?
Yes — for a mixed bundle, add up each individual conductor's area (not just count) and confirm the total stays under 40% of the conduit's internal area, rather than using a single gauge's area for every conductor.