Decorator Rules for Window Valance Length
If your single panel window treatment is looking a little bland, a valance can add depth and detail to your window decor. A simple style of drapery that runs horizontally across the top of a window, a valance can give a bare window a little color or add a charming layer under or on top of plain panels.
If a traditional look is what you want, make your window valances at least 1/5 to 1/6 the height of the window, according to Deb Barrett of Window Dressings in Kaneville, Illinois. For window treatments with longer floor-length panels, you can play with valance length a little more. Some designers, like Matt and Shari from HGTV's "Room by Room," recommend a different formula of 1/4 the window height plus 1 inch for traditional valance treatments.
Old School Style
If you're creating an old-school, "Mad Men"-inspired room reminiscent of the 1950s and 60s, the rules for modern valance length are kicked to the curb. A short 6-inch crenellated valance--one with a wavy or repetitive block edge--creates a stage curtain effect when placed over a long, free-flowing panel. You can add some detail and the appearance of length by using a balloon valance all by itself in a window.
If you're installing a swag style valance, the length of the valance is decided by a formula, but the final product is created by sight. To create a swag valance, measure the distance between each end of your curtain or drapery rod, then add 36 inches to get the full length of the material for your swag. Once all the edges on your material have been hemmed, find the center of your swag material and create the basic swag on the floor before putting it up. Like traditional valances, most swags are typically equal to 1/4 to 1/6 of the window in length at their lowest points. However, unlike traditional valances, swag valances are not exact, and final placement is typically done by sight, so start with the formula and adjust as necessary.