For inside dimensions, simply measure from the inside of the left/top frame corner to the inside of the right/bottom frame corner both horizontally and vertically, and then add 3 to 6 inches vertically," says DeLucia. The additional vertical slack will ensure you don't have any cracks of light on the bottom. "For an outside mount, measure horizontally from the left edge of the window frame to the edge of the right-side frame and then add 1.5 to 3 inches to each side so that it overlaps.