What size header do I need over a door or window?

For a load-bearing wall, size the header from the IRC header span tables (R602.7). You read the table by what the header supports, the building width, and the ground snow load; the table gives the doubled lumber size and how many jack studs each end needs.

When the opening is too wide for dimensional lumber or carries heavy load — big sliders, garage openings, two floors above — use an LVL or other engineered beam. It carries more over longer spans in a slimmer, dead-straight member.

For a quick first guess on a typical single-story wall, match header depth in inches to span in feet: about a 2x8 for a 4 ft opening, 2x10 for 6 ft, 2x12 for 8 ft. Use it to buy lumber and sketch, then confirm against the span table.