To measure windows for replacement, measure the width of the window between the jambs at the top, middle and bottom, and use the smallest measurement. Measure the height at the left, right and center of the window. Be sure to measure from the surface of the jambs, not from the extensions that hold the window. Measure the depth at the shortest point, and if you can't open the window, measure the width on the inside and multiply it by two.
To measure windows for curtains, decide if they are going to be on the inside or the outside of the frame because it affects the measurement procedure. Measure the depth of the window opening. If you intend for the blinds to be on the inside of the window opening, be sure to measure in front of handles and other obstructions there may be.
Measure the inside width. If the blinds are going to be on the inside of the frame, measure at the bottom, middle and top of the window and choose the smallest result. Measure the inside height three times, and choose the largest result. Measure the outside width and height, and use the largest results. If you intend to mount the blinds above the window, measure the height from the intended mounting position to the bottom of the frame instead.