Where do you want to mount your rods--from the ceiling, above the window molding, or do you have to mount on the molding itself? (This should be avoided if at all possible, as mounting rods on the molding can split the molding and it’s difficult to... moreWhere do you want to mount your rods--from the ceiling, above the window molding, or do you have to mount on the molding itself? (This should be avoided if at all possible, as mounting rods on the molding can split the molding and it’s difficult to repair the holes.) Curtain pole manufacturer would like to teach you how to measure for a bay window curtain rod.
Once you have determined where to mount the rods, measure the wall (or ceiling) from angle-to-angle across the back window, then measure each side--they can vary!--from edge of wall to angle, or angle-to-angle if you have a bay with more than three sections. Even if all the windows and/or wall sections appear to be the same, measure each one! There may be differences, and you need to take them into consideration.
If you have existing blinds or other window treatments you will be leaving in place, measure how far out they project from the window or wall. You will need brackets deep enough to clear them.