Measure your window width from the outside of the trim to the outside of the trim. Then determine how far beyond the trim you want the shade to go on each side, add that to your measurement. This is your width.

Measure the top of the trim to the bottom of the sill. Determine how far above the window you want to mount the shade and how far below the sill you want the shade to fall, add this to your measurement. (Be sure to add at least two inches at the top to allow for installation hardware.) This is your length*.

*If you want to maximize privacy and light blocking properties, you should extend the shade at least 2 inches beyond the trim in all directions.