NEC 2026

Conduit Fill Calculator

Check whether your wires fit in EMT, PVC, RMC or flex conduit. The result updates as you type.

Updated

Conductors

Add at least one conductor to see the fill.

Building a quote? Save this result and send a professional estimate from one place.

Add to quote

For reference only. Always verify with the NEC edition adopted by your AHJ.

How conduit fill works

Conduit fill compares the combined cross-section of your wires against the inside area of the conduit. The NEC 2026 Chapter 9 tables supply both numbers: Table 4 for the conduit and Table 5 for each wire.

Table 1 then sets the limit: 53% of the area for one wire, 31% for two, and 40% for three or more. A nipple of 24 inches or less gets 60% under Note 4.

Note 7 covers the borderline case. When every wire has the same insulation and size, the maximum wire count can be rounded up to the next whole number if its decimal is 0.8 or higher. Mixed sizes do not get that rounding, and the calculator only applies it to three or more wires.

The result shows PASS or FAIL, the fill percentage, and the limit that applied. The line under it names the rule used.

Conduit fill questions

How full can a conduit be under the NEC?
Chapter 9, Table 1 sets the limit by wire count: 53% of the conduit area for one conductor, 31% for two, and 40% for three or more.
Does a short nipple get a higher limit?
Yes. Chapter 9, Note 4 allows 60% fill for a conduit or tubing nipple 24 inches or shorter. Tick the nipple box above to use it.
What is the Note 7 rounding rule?
When you work out the maximum number of conductors that fit and the decimal is 0.8 or higher, Chapter 9 Note 7 lets you round that number up to the next whole conductor. For example, 15.85 allowed wires becomes 16. It only applies when every conductor has the same insulation and size. The calculator applies it to three or more conductors.
Which wire area does the calculator use?
Chapter 9, Table 5 gives the area of each insulated conductor by type and size. THHN, THWN and THWN-2 share one set of values, as do XHHW and XHHW-2.
Why does 3/8 flex conduit show an error?
Table 4 lists 3/8 FMC, but NEC 348.20(A) only permits it for limited uses, and its fill is set by Table 348.22 rather than the Table 1 percentages. The calculator does not compute it. Check those sections for your installation.
Can I rely on this for an inspection?
It is a quick reference. Local amendments and the NEC edition your AHJ has adopted decide what passes, so confirm with them.

You can read the NEC 2026 for free at NFPA free access.