Pick the conduit type from the links at the top, find your wire size in the left column and read across to the trade size. The number is the most wires of that one size and insulation the conduit holds: 16 of 12 AWG THHN in 3/4 in EMT, for example, or 16 of 10 AWG THHN in 1 in EMT. When a run carries more than one size, say 10 AWG for one circuit and 12 AWG for another, the chart can't add them up; the conduit fill calculator works out the total.
Most cells come from the 40% limit for three or more wires. Where three won't fit, the count drops to the smaller limits for two wires and for one, so a 2 or a 1 in a table is set by those, and a 0 means not even one wire of that size fits. 6 AWG THHN is 2 in 1/2 in EMT but 1 in 1/2 in PVC Schedule 40, and a 4/0 THHN doesn't go in 1/2 in EMT at all.
The same trade size holds different counts from one conduit type to the next because each type has its own inside area in Chapter 9 Table 4, listed at 40% in the areas table below. At 1/2 in, IMC has the most room, 0.137 sq in, and PVC Schedule 80 the least, 0.087, against 0.122 for EMT; 12 AWG THHN runs from 10 in IMC down to 6 in Schedule 80. Flexible metal conduit keeps pace with EMT in the small sizes and falls behind in the larger ones: both hold 16 of 12 AWG THHN at 3/4 in, while at 1-1/4 in EMT holds 45 and FMC 38. ENT and LFNC-B are listed only up to 2 in, so their tables stop there.
THHN tables cover all nine types, and XHHW tables cover EMT and PVC Schedule 40. The difference between the two is largest in small wire. Table 5 lists 14 AWG XHHW at about 43% more area than 14 AWG THHN, so 3/4 in EMT holds 22 THHN but 15 XHHW. At 10 AWG the gap is about 15%, and 1 in EMT holds 16 THHN against 14 XHHW. From 4 AWG up, Table 5 lists XHHW as slightly smaller than THHN, and its count is the same or one higher: 4 in EMT takes 71 of 4 AWG THHN or 72 of 4 AWG XHHW.
Every count here applies the NEC's rounding for wires of one size: when the division leaves 0.8 of a wire or more, Chapter 9 Note 7 lets that last wire count. The area of 1 in EMT works out to 6.82 wires of 6 AWG THHN, so the table shows 7, and 1/2 in PVC Schedule 80 works out to 8.97 of 14 AWG THHN, shown as 9. Checked against the NEC's Annex C tables, the same method reproduces 52 of 53 counts. The one that differs is 6 AWG THHN in 1-1/4 in EMT: Annex C gives 12, while the printed Table 4 area gives 11.79 wires, just short of the round-up, so this chart shows 11. If the job depends on that twelfth wire, ask the inspector which figure they accept.
A count in the chart is a limit on space, not on current. When more than three current-carrying conductors share a raceway, NEC 310.15(C)(1) reduces their ampacity: to 80% for 4 to 6 conductors and 70% for 7 to 9. Sixteen 12 AWG THHN fit in 3/4 in EMT, but sixteen current-carrying conductors are more than the 7 to 9 that take 70%, so look up the adjustment for that number before settling on the wire size.
Count the ground before you read the chart. Equipment grounding and bonding conductors take up space under Chapter 9 Note 3, the same as the circuit wires. In 3/4 in EMT, fifteen 12 AWG THHN circuit wires and a 12 AWG THHN ground make sixteen, the most the table allows. A bare ground has a different area from an insulated wire of the same gauge, so a run with a bare ground is a mixed run: use the calculator.