Here is an easy guide to help you measure for Levolor Century Vertical Blinds.

Inside Mount Installation To find width: Measure the opening across the top where the headrail is to be installed from inside of jamb on left to inside of jamb on right. Round the measurement down to the nearest 1/8". To find length: Measure from the inside top of the opening on the left, at the center, and on the right to the sill; round the measurement down to the nearest 1/8" and note the smallest dimension. Do not make any allowances: the factory will make all adjustments so the headrail will fit the opening properly and the vanes will clear the sill by 1/4" to 1/2".
Outside Mount Installation To find width: Measure the width of the area to be covered at the place where the headrail will be installed to the nearest 1/8". Blind should overlap opening at least 4" on each side to ensure privacy. To find length: Measure the length of the area to be covered and round down to the nearest 1/8". Blind should extend at least 3" above and 3" below opening or window moulding. For ceiling-to-floor installations, make adequate allowance for carpeting. Specifying OB measurement means the factory makes no deduction allowances.
Patio Door Installation To find width: Measure the width of the area to be covered at the place where the headrail will be installed to the nearest 1/8". Blind should overlap opening at least 4" on each side to ensure privacy. To find length: Measure the length of the area to be covered and round down to the nearest 1/8". Blind should extend at least 3" above opening or window moulding. For ceiling-to-floor installations, deduct 1/2" for proper clearance. Specifying OB measurement means the factory makes no deductions allowances. To ensure easy access through the door and allow adequate overlap, consider the stack width. For questions concerning the calculation of stack width, give us a call.