Conduit Fill Calculator
Check NEC conduit fill — pick the conduit type and trade size and add your conductors (THHN/THWN by AWG) to see the fill percentage against the allowed limit.
Informational estimate only — not a substitute for a licensed electrician or NEC compliance. Based on NEC Chapter 9 (Table 1 fill limits, Table 4 conduit areas, Table 5 conductor dimensions) for the common conductor types shown; your jurisdiction, conductor insulation and fittings may change the result. Verify against the current NEC.
How to calculate conduit fill
Conduit fill is the share of a conduit’s internal area taken up by the conductors inside it. You add up the cross-sectional area of every conductor (NEC Chapter 9, Table 5), divide by the conduit’s internal area (Table 4), and multiply by 100. Then you compare the result to the maximum fill allowed by Table 1. The calculator looks up the table values for your conduit type and trade size and your THHN/THWN conductors automatically.
fill % = total conductor area ÷ conduit internal area × 100
For example, three 12 AWG THHN conductors (0.0133 in² each = 0.0399 in²) in 3/4-inch EMT (0.533 in²) is 0.0399 ÷ 0.533 = 7.5% fill — well under the limit. The tool checks your fill against the right limit and tells you if you need a bigger conduit.
Maximum conduit fill (NEC Chapter 9, Table 1)
| Number of conductors | Maximum fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
This is an informational estimate based on the common THHN/THWN conductor sizes — your jurisdiction, insulation type and fittings can change it, and more than three current-carrying conductors may require ampacity derating. It is not a substitute for a licensed electrician or NEC compliance. Sizing the wire for distance too? See the voltage drop calculator.
Frequently asked questions
What is the maximum conduit fill?
Under NEC Chapter 9, Table 1, the maximum fill is 53% for one conductor, 31% for two conductors, and 40% for three or more conductors of the cross-sectional area of the conduit. The calculator compares your conductors’ total area to the 40% limit (or the right limit for one or two).
How do I calculate conduit fill?
Add up the cross-sectional area of all the conductors (NEC Table 5), then divide by the internal area of the conduit (NEC Table 4) and multiply by 100. Compare the result to the allowed fill percentage. The calculator looks up the table values for your THHN/THWN sizes and conduit automatically.
Can you go over 40% conduit fill?
No — exceeding the allowed fill makes pulling wire hard, can damage insulation and fails inspection. If you are over, move up a conduit size, use a larger conduit type, or split the run. Derating for more than three current-carrying conductors may also apply.