Two tables and one percentage rule
Conduit fill comes from three numbers in NEC (NFPA 70) Chapter 9. Table 4, "Dimensions and Percent Area of Conduit and Tubing," lists each conduit type's internal cross-sectional area at 100% fill, by trade size. Table 5, "Dimensions of Insulated Conductors and Fixture Wires," lists each conductor's cross-sectional area, insulation included, by wire gauge and insulation type. Chapter 9, Table 1 then sets how much of that 100% area conductors are actually allowed to take up: 53% for a single conductor, 31% for exactly two, and 40% for three or more. The two-conductor number sits lower than the three-conductor one on purpose. With only two wires there is nothing to hold them apart inside the tube, so they can twist around each other and jam during a pull; three or more wires tend to lie against the conduit wall instead.
A note to Table 1 carves out one exception: a nipple, a conduit run of 24 in. or less between two enclosures, is allowed a flat 60% fill no matter how many conductors are inside. Short runs do not get pulled the way long ones do, so the jamming concern that drives the 31%/40% split does not apply.
Worked example: four 12 AWG circuits sharing a run
Four 20-amp branch circuits, each a hot and neutral in 12 AWG THHN plus one shared 12 AWG ground, run together through a single EMT nipple: 9 conductors total. Each 12 AWG THHN conductor has an area of 0.0133 sq in. (Table 5), so 9 of them use 9 × 0.0133 = 0.1197 sq in. A 1/2 in. EMT has a 100%-fill area of 0.304 sq in. (Table 4). As a nipple, the allowance is 60% of that: 0.304 × 0.60 = 0.1824 sq in. 0.1197 sq in. fits inside 0.1824 sq in., about 39.4% fill, so the run passes in a 1/2 in. nipple.
Run the same 9 conductors through a full-length branch (not a nipple, so the three-or-more-conductors rule applies) instead of a short nipple, and the allowance drops to 40% of 0.304 = 0.1216 sq in. 0.1197 still just fits, at 98.4% of that lower allowance, close enough that adding one more 12 AWG conductor (0.1330 sq in. total) would push it over. That is the kind of margin worth re-checking with the trade size one step up before pulling wire.
Conduit type changes the available area, not the fill rule
The 53%/31%/40% percentages are the same across every raceway type; what changes is the internal area behind them. A 1 in. trade size is not the same physical opening in every material, because each conduit type has its own outside diameter and wall thickness standard for that label. At 1 in., NEC Table 4 lists 0.864 sq in. of 100%-fill area for EMT, 0.887 sq in. for RMC, 0.959 sq in. for IMC, and 0.832 sq in. for PVC Schedule 40. IMC has the most room and PVC Schedule 40 the least: PVC Schedule 40 holds roughly 13% less area than IMC at that trade size. The gap rarely changes a pass/fail result by itself, but it can on a fill that is already close to the limit, which is why switching conduit type mid-project is worth re-running the numbers for rather than assuming the trade size alone carries over.