Blinds width will 2/5 narrower for a snug internal installation. Actual Blinds Size is 26 3/5 x 64 H. Choose the size SAME or CLOSEST to your window width for INSIDE mount. For Example if your windows are 30 wide order 30 or slightly less for internal installation. For OUTSIDE mount we recommend order (2-4)inch wider than your window width for better coverage. For Example if your windows width is 30 order size 34 would be okay for outside mount.