Pick a conductor, insulation type, count and raceway — get the minimum trade size under the NEC Chapter 9 fill rules, plus the fill percentage for every size from ½″ to 4″.
NEC Chapter 9, Table 1 caps conduit fill at 53% for one conductor, 31% for two, and 40% for three or more. The limits exist for heat dissipation and to make pulling physically possible — a 40% fill is already a firm pull on a long run with bends. Conductor areas are THHN/THWN-2 and XHHW-2 from Chapter 9 Table 5; raceway areas are from Chapter 9 Table 4 for EMT, PVC Schedule 40 and rigid metal conduit (RMC).
Count every conductor occupying the raceway, including grounds — fill is about physical space, not current.
Slightly — internal areas differ by type. PVC Schedule 40 is a touch smaller than EMT at most trade sizes and RMC a touch larger, which is occasionally the difference at a size boundary. Pick your type in the selector.