It works best if the curtain has an even number of grommets, so also take this into consideration when determining the width.
To determine the size to cut your fabric, take the dimensions you want your curtain panel to be and add 4” to the width and 16” to the height to account for the hems. To figure out what total width your curtains should be, multiply the width of the window by 1.5 or 2, depending on how full you want the curtains to look.
[Finished Width] + 4″ by [Finished Height] + 16″
This curtain will be 3 ft wide and 4 ft tall when finished so I’m cutting the fabric 40” wide by 64” high.