It is three tables, not one rule
Conduit fill gets talked about as "the 40% rule," which hides where the number comes from. There are three NEC Chapter 9 tables in play. Table 5 gives the cross-sectional area of each insulated conductor — a #12 THHN is 0.0133 sq in, and the same #12 in XHHW is 0.0181, about 36% fatter. Table 4 gives the interior area of each conduit at each trade size. Table 1 gives the percentage of that interior area you may fill. Multiply, compare, done.
Because the areas come from a published table rather than a formula, the honest tool reads the real numbers rather than approximating a wire as a circle of its nominal gauge. That is why insulation type is an input here and not a footnote: the difference between THHN and XHHW is large enough at small gauges to change the conduit you buy.
The two-conductor trap
The fill limit depends on how many conductors you are pulling, and not in the direction most people assume. One conductor may fill 53% of the pipe. Three or more may fill 40%. But exactly two conductors may fill only 31% — the strictest case in the table.
The practical consequence is genuinely counterintuitive: adding a third wire can let you use a smaller conduit than two wires would allow. Two 4/0 THHN come to 0.6474 sq in, which is under 40% of 1½″ EMT and would look fine on any tool that assumes 40% — but the real two-conductor limit is 0.6312 sq in, so it fails. Get this wrong and you have bought and bent the wrong pipe.
Fill can pass and the pull can still jam
Passing the fill check does not guarantee the wire goes in. Chapter 9, Note 4 describes jamming: three conductors of the same diameter can wedge in the pipe when the ratio of conduit inside diameter to conductor outside diameter lands near 3.0. It is specific to the three-same-size case, which is exactly what an ordinary three-wire circuit is, so this calculator raises a flag on that combination instead of showing a green result and leaving you to find out on site.
Bends matter too. NEC 358.26 and its siblings cap a run at 360° of total bend between pull points. Four 90° elbows and you are at the limit regardless of how much room the fill calculation says you have.
Conduit fill chart — maximum THHN/THWN-2 conductors
Maximum number of same-size THHN/THWN-2 conductors at the 40% limit, by conduit type and trade size. Every cell is derived from NEC Chapter 9 Tables 1, 4 and 5 rather than copied from anywhere, so you can reproduce any number here by hand.
These cells assume three or more conductors. A run of exactly one or two conductors uses the 53% or 31% limit instead — use the calculator above for those. The conduit fill chart guide works through why the two-conductor case is stricter than three, and lists every cell where the published charts permit one conductor more than Chapter 9 allows.
Where this differs from NEC Annex C: Annex C is NFPA's own precomputed table for runs where every conductor is the same size, and in a handful of cells it permits one more conductor than a hand calculation from Chapter 9 does — ¾″ EMT with 8 AWG THHN is the clearest example, where Annex C says 6 and the Chapter 9 arithmetic gives 5 (6 × 0.0366 = 0.2196 sq in against an allowable 0.2132). The difference comes from rounding in the published areas. This chart shows the Chapter 9 result, which is the conservative one and the method you must use anyway once the conductors are not all the same size. If you are pulling all-identical conductors and need the extra one, Annex C is the table your inspector will accept.