Here is an easy guide to help you measure for Levolor Metal and Wood Blinds.

Inside Mount Installation To find width: Measure exact distance to the nearest 1/8" between inside surfaces of window frame at the top, middle and bottom, and use the narrowest width. Do not make any width deductions. To find length: Measure exact distance to the nearest 1/8" from inside surface at top of window to sill at the left, middle and right and use the longest length. If you prefer blind not to touch sill, deduct 1/4". If there is no sill, measure length to point where blind is to reach.

Outside Mount Installation To find width: Measure 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 (or 1-1/2" for the entire width). To find length: Measure exact distance to the nearest 1/8" from where top of blind or shade will be located to top of sill. If there is no sill or if blind is to overlap window frame, measure to point blind is to reach.