The first step in determining valance size is to carefully measure the window that will be treated with a valance. The valance size will always need to be at least somewhat wider than the window. Depending on where the hardware is located, the valance may also need to be wider than the window frame. A valance is often used to hide the rods, brackets, or other hardware so you will want one large enough to cover these and allow plenty of space between the piece and the hardware. For gathers, you will need to add a good deal more width when configuring valance size.
Once the curtain rods have been put in place, you will be able to measure the width of the rod as well as the return. The return is the length of space between the wall and the face of the curtain rod. You will need to measure one side then multiply it by two and add it to the width of the rod face to determine the appropriate valance size.
For a wooden valance or one made from other solid material, you will need to make sure its return extends far enough to leave plenty of room between it and the curtain hardware. For a fabric valance, you will need enough material to cover the total width including both return measurements and leave room for a seam. If you prefer a gathered look, more fabric will be needed. You will need to at least double the valance size, and perhaps add an additional 50% depending on how much gathering you would like.
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Once the width is calculated, it’s time to move on to the length. The length of the valance is determined by a number of different elements, including its height and the height of other items in the space. If you want to go with a standard size, a typical valance is about 14 inches (or 36 centimeters) long from the lower edge of the rod to the bottom of the hem.
If you plan to make your own treatment, you need to start calculating the valance size with at least 14” (or 36 centimeters) adding about one inch (or 2.54 centimeters) for the hem. You will also need to add the appropriate amount of fabric for the rod pocket. Measure the rod and allow enough additional room to let the rod slide smoothly and to include the seams. Add this number to your total. If you want to add a ruffle at the top, you will need to add twice the height you want the ruffle to be. A one inch (or 2.54 centimeter) ruffle will require two inches (or 5.08 centimeters) of fabric.