Some ways to measure for window valances include measuring the window's width from the frame's inner sides or measuring the same including the window frame. In either case, multiply the width measured by a factor of 1.5 to three. Measure the valance length and if required, the rod projection.
Measuring the window's width from the frame's inner sides is appropriate for valances that are to be mounted inside using tension rods, for pleated valances and for flat valances. For gathered valances, multiply the window width by 1.5 or 2.5, depending on the fullness desired.
In this case, measure the valance length from the inner side of the window frame's top edge till that spot on the pane where the valance's hemline is to fall.
If the valance is to be mounted outside, include the window frame when measuring its width and add an extra 2 inches. Also, add an allowance of 2 inches if the valance is to be hung over existing drapes. Multiply by 1.5 to three for full valances.
Additionally, measure the projection of the valance rod in such valances. To do this, measure from the rod's front till the wall or the rear bracket edge. Double this measurement, and add it to the window width. There is no need to measure rod projection if the valance is to be mounted on a board or hung on a pole rod.
In this case, measure the valance length from the valance rod or the mounting board till the valance's hemline.