We wrote a new blog called How to Measure Curtains that contains more information on this topic.

To ensure that curtain panels look ample when closed, they should have a combined width that is 1.5 to 2.5 times the width of the window. That means if you are ordering grommet style or rod pocket style, the total drapery width should be at least doubled window width. 

French pleated style panels have fullness built in, so if you are ordering French pleated heading style, you should order the panels that are the same width with your window. They will cover your window with a nice fullness. 

Exceptions: If you’re hanging curtains just to frame a window and don’t intend to shut them, you can round down to 1.2 times the width.