When purchasing ready-made curtains, you need to know the measurements of the drapery rod, the fullness of the gathers for the header style, and the panel width to calculate the number of panels to update your window treatments. For example, curtains with a particular header style look their best with gathers, such as classic pinch-pleat curtains with a fullness factor of at least 2.5 times the length of the rod. This fullness ratio is not exact, but only a guide to help you estimate enough fabric for an attractive window treatment. Bringing a small calculator, a pencil and a notepad to the store will help you calculate the number of panels you need.
1 Measure the length of the installed drapery rod from left to right. If this rod includes two returns that project out from the wall and support the rod, add these two measurements to the rod. For example, if the rod measures 48 inches across, and the two returns are 4 inches each, add 48 plus 4 plus 4 for a total of 56 inches. Record this length with in your notepad.
2 Multiply the rod or track length from Step 1 by the fullness factor for the curtain style. For example, a pencil-pleat panel needs a fullness of at least 2 times. Pinch-pleat panels need at least 2.5 times for heavy gathers. In this example, use a calculator and refer to your notes to multiply 56 inches for the rod length by a fullness factor of 2 for a pencil-pleat panel: 56 inches multiplied by 2 equals 112 inches. In this example, 112 inches is the width of fabric that will cover your 48-inch rod from left to right plus the two returns for a pencil-pleated window treatment.
3 Divide this width in Step 2 by the panel’s finished width. The finished width is usually printed in inches on the curtain package or displayed for online ordering. Otherwise, just measure this width if the curtain is out of its package. In this example, if the panel measures 50 inches wide, divide 112 inches by 50 inches, which equals 2.24. If there is a remainder in your calculations, round this figure to a whole number to calculate the number of panels for your window treatment. In this example, if your calculation results in 2.24, purchase three panels to be safe.