Conduit Fill Calculator
Size conduit per NEC Chapter 9. Add THHN, XHHW, or other conductors and get the minimum trade size and fill % for EMT, IMC, RMC, PVC, ENT, FMC, and LFMC. Free, no sign-up.
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Explanation
Conduit fill is the calculation that checks how many conductors a raceway can hold under NEC Chapter 9. The rule is an area comparison: the combined cross-sectional area of the conductors must not exceed a percentage of the conduit’s internal area. This calculator adds up the conductor areas from Table 5, applies the Table 1 fill limit, and returns the smallest trade size that fits for each conduit type.
How the calculation works
The maximum fill percentage depends only on how many conductors are in the raceway, per NEC Chapter 9, Table 1:
| Conductors | Maximum fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| Over 2 conductors | 40% |
Each conductor’s area comes from Table 5 and already includes the insulation, so a 12 AWG THHN is smaller than a 12 AWG XHHW-2 or RHH. To look up the bare conductor size, ampacity, or to size the conductor for a load first, use the wire size & ampacity calculator and the AWG wire size chart.
Reference tables
The conduit areas are the total internal area (100% column) from NEC Table 4, and the conductor areas are the approximate areas from Table 5.
Notes and limitations
This tool applies the Chapter 9 fill percentages for new work with conductors of the same or mixed sizes. For raceways that are entirely one conductor size, NEC Annex C gives exact conductor counts and may be more convenient. Conduit fill does not address ampacity adjustment for bundled conductors (310.15(C)(1)), conduit nipples 24 in or shorter (60% fill), or derating — confirm those separately and verify the final design against the current NEC and the authority having jurisdiction.