Fiberglass cloth that weighs 4 oz., 6 oz., and 10 oz. per square yard comes in either 38" or 50" widths. The 38" wide fabric is 9 sq. ft. per linear yard, and the 50" wide fabric is 12 sq. ft. per linear yard.
To figure out how much fiberglass cloth you need, measure the area to be fiberglassed and order enough cloth to cover that area. Double the amount if you are applying two layers.
If you are laminating an area 8 ft. wide by 12 ft. long (8 x 12 = 96 sq. ft.). You would need two pieces of 50" wide cloth that are 4 yards long each and would need to overlap the seam by 1-4 inches. If using 38" wide cloth, you would need three pieces that are each 4 yards long.