Successful replacement depends on good measurements of the existing opening to find out how big your new window needs to be. Start by measuring between the left and right jambs at the top, middle, and bottom of the window; the shortest of these three measurements is the window width. Then measure the distance between the head jamb and sill at the far left side, middle, and far right side of the window; the shortest of these three measurements is the window height.

Note that window dimensions are always presented as width by height. Do not reverse these measurements!

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