Inside Mount
Width: Measure the inside width of the window opening at the top, middle, and bottom. Use the smallest measurement.
Height: Measure the window height on the left, center, and right. Use the longest measurement.
Outside Mount
Width: Measure the full area you want the shade to cover. Adding about 3–6 inches to the total window width can help reduce light gaps and improve privacy.
Height: Measure from the point where you plan to install the top of the roller blinds down to where you want the bottom rail to stop, usually at the windowsill or below the window trim.