Great answers have already been provided. You may take a photo of your windows and any of the valancces you plan to have made to assist in your explanation of what you want.
I agree with Vanessa's answer. But i would add, there are different types of valances so looking at patterns will be helpful to you. Also, if you take your measurements into the store with you, one of the employees will be able to help you figure the yardage.
Measure the width of the windows and then either double that width or multiply it by 1.5, depending on how much gathering you want. For a valance the length can vary to your preference. Then once you choose fabric you must consider the fabric width and the pattern repeat. A valance pattern will have these details. Or you tube. Good luck!
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