Add the Bare Minimum
To determine the width of a valance for a 36-inch window, the Window Treatment Expert website suggests adding 2 to 4 inches on each side, adding less for narrower windows and more for wider windows. For a 36-inch window, this measurement would provide you with a valance that has a width of 40 to 44 inches.
Add More For Fullness
If you prefer a look that is full and bunched, you will need a wider valance. For a gathered, full effect, multiply the window width by 2 to determine the valance width. The valance for a 36-inch window, therefore, should have a width of 72 inches.
The size of your valance is based on your personal preference. If you prefer a sleek, tailored look, choose a valance of 40 to 44 inches. If you prefer a romantic, full look, choose a valance with a width of up to 72 inches. Keep in mind that a fuller valance will most likely cost more, as it has much more fabric.