Inside Mount Installation To find the Width Measure to the exact distance to the nearest 1/8" between the inside surfaces of the window frame at the top, middle and bottom. Do not make any width adjustments as we will make the proper adjustments from the width measurement you submit. To find the length (Height) Measure the exact distance to the nearest 1/8" from the inside surface at the top of the window to the sill at the left, center and right and use the longest length. Outside Mount Installation To find the Width Measure the exact distance to the nearest 1/8" between the outermost points where the blind is to be placed. For optimum light control this measurement should extend 3/4" beyond the window opening on each side for outermost sides (or 1-1/2" for the entire width. To find the length (Height) Measure the exact distance to the nearest 1/8" from where the top of the blind will be located to the top of the sill. If the blind is to overlap the window frame, measure to the point the BlindGraphicsTM is to reach. Note: Outside Mounts are usually considered when covering odd shaped windows, windows with widely varying measurements and when no recess is available. If you need further assistance measuring, please give us a call at 800.216.8986 Ext. 1, Monday thru Friday, 8AM to 5PM, PST.