The general rule of thumb is to extend the curtain rod at least 6 to 12 inches beyond the window frame on each side. This will allow you adequate space to hang the curtains and let them drape gracefully. In addition to this, the wider the window is, the more projection from the wall you’ll need.

According to Bob Vila, curtains should hang at least 4 to 6 inches above the top of the windows. Height isn't the only consideration when determining where to hang curtains. Curtain rods should extend at least 3 to 6 inches past the window frame.