Konboy, linage footage is the total lenght of something such as a board, Sq footage is total area of something, in your case a wall. For linage footage add up the total lenghts of each wall, to figure the sq. footage muiltply lenght x width of each wall and add them together, this gives your total sq footage of your space, Now measusre your windows lenght x width to get sq. footage and take that away from total sq. footage.