Once you have chosen the style of curtains that you want, you need to determine how many curtain panels are necessary to create the look you desire. You will generally use the width of the curtain rod in comparison to the width of a curtain panel to determine how many panels you need in each window. If you are making your own curtains, you can adjust the panel width accordingly to have one panel per side, but if you are purchasing curtains, make your decision based on the width the curtains come in.
Measure the width of the curtain rod if you have chosen it. If you haven't chosen one, measure the width of the window and add 1 to 3 inches for each side of the window. The amount you add depends on how far to the side of the window you want the curtains to hang.
Multiply the rod length by 1.5 for minimum fullness, 2 for medium fullness and 2.5 for an extra full look. For example, for a 36-inch rod, your measurements would be 54 inches for minimum fullness, 72 inches for medium fullness and 90 inches for an extra full look.
Divide your total inches measurement by the width of the panels you have chosen. If you want the curtains to be able to close fully, you should always use enough panels to cover the width of the window. Round up to the next whole number if you want to achieve more fullness. For example, if you have a 28-inch-wide panel and a 36-inch curtain rod, purchase two panels for minimum fullness, three panels for medium fullness and four panels for extra fullness.