Choose the conduit type and trade size, then enter each group of wires: how many, the size and the insulation. Count every wire in the run, the ground included. Chapter 9 Note 3 counts equipment grounding and bonding conductors in the fill at their actual size, bare or insulated. The result gives the percent full, the limit that applies, the smallest trade size that holds the same wires, and a row for every trade size to show the margin. Take three 12 AWG THHN and one 10 AWG bare stranded ground in 1/2 in EMT: they total 0.0505 sq in, which is 16.6% of the tube's 0.304 sq in, inside the 40% limit of 0.122 sq in.
The limit depends on how many wires share the conduit. NEC Chapter 9 Table 1 allows 53% of the inside area for one conductor, 31% for two and 40% for three or more. The limits leave room to pull wire around bends without scraping the insulation, and air around the conductors to carry off the heat they make under load. Two wires get the lowest share of the three. A nipple, a piece 24 in or shorter between enclosures, is allowed 60% under Note 4, whatever the number of wires.
Insulation changes the count as much as gauge does. Table 5 gives 12 AWG THHN an area of 0.0133 sq in and 12 AWG XHHW 0.0181 sq in, more than a third larger. At 40% of 1/2 in EMT, that is the difference between 9 THHN and 6 XHHW. THW and TW have the same 12 AWG area as XHHW. The insulation type is printed along the wire's jacket.
Trade size is a name rather than a measurement, and the inside area varies with the conduit type. 1/2 in EMT has 0.304 sq in inside; PVC Schedule 40 has 0.285, and Schedule 80, with its thicker wall, 0.217. That takes the 12 AWG THHN count from 9 in EMT to 8 in Schedule 40 and 6 in Schedule 80. The tables below the calculator give THHN counts for every size of EMT and of PVC Schedule 40, which Table 4 lists together with HDPE. The calculator covers the other types: rigid and intermediate metal conduit, flexible and liquidtight conduit, and ENT.
When every wire is the same size, Note 7 lets a count that comes out at .8 or more round up to the next whole wire. With 8 AWG THHN in 3/4 in EMT, the 40% area holds 5.82 wires, so the count is 6. Mixed sizes don't round: their areas are added, and the total has to be within the limit. The NEC's Annex C tables list same-size counts, and this calculator matches them in 52 of the 53 counts checked. The exception is 6 AWG THHN in 1-1/4 in EMT. Table C.1 shows 12, while the printed Table 4 figure, 0.598 sq in at 40%, divided by 0.0507 sq in a wire comes to 11.79, so the calculator says 11. It keeps the printed area, the conservative choice; whether the Annex C count is accepted is the inspector's call.
Passing fill doesn't settle the wire size. With more than three current-carrying conductors in one raceway, NEC 310.15(C)(1) lowers their ampacity to 80% for 4 to 6 and 70% for 7 to 9. Nine 12 AWG THHN fit in 1/2 in EMT, but if all nine carry current, each is held to 70% of its table ampacity. Grounding conductors don't count toward that. The calculator flags the adjustment when more than three insulated wires are entered, without working it out.
Cables are a separate case. Note 9 counts a multiconductor cable as one conductor, and an oval cable as a circle the width of its widest dimension. Cable sizes aren't in the NEC tables, so this calculator handles individual wires only. Bends, pull length and local amendments matter too; an electrician sizes the conduit for the run, and the inspector decides whether it passes.