Measure with a tape measure the width of the window opening. Record this number.
Divide this number by 6 to calculate the length of the rod extensions or the stack back on either side of the window. Work with a calculator, if necessary. Round up to the nearest inch. For example, a 48-inch-wide window opening divided by 6 equals 8 inches for the rod extensions on each side of the window. If your curtain is a single panel that pulls back to one side or the other, divide the window width by 3 to calculate the stack back.
Add the window width plus the rod extensions for each side of the window in inches to calculate the rod length. In this example, 48 inches plus 8 inches for each side equals 64 inches. The face of the rod should measure approximately 64 inches to draw the pair of curtains off the window.