If you have different wire sizes in the same conduit (e.g., two 10 AWG THHN and four 12 AWG THHN), you must calculate the area.
Here are some common conduit sizes and their typical applications: conduit sizing
Consult NEC Chapter 9, Table 5 . This table lists the approximate cross-sectional area in square inches for different wire types and sizes. If you have different wire sizes in the same conduit (e