How you measure will depend on where you plan to mount the shades.
Most shades are mounted inside the frame. For an inside mount, you’ll want to measure the width of space within the window frame. Make sure to measure at the top where the brackets will be mounted—not all windows are square! The height is less crucial, since the shades will roll up and down, but measure within the frame.
French doors, sliding doors, and some windows need an outside mount. In this case, choose a width slightly wider than the glass so that light won’t leak through the sides. Please make sure to tell us that you’re mounting outside so that we can cut appropriately. When measuring height, please go a little above the glass to leave room for the fixture.