Measure the length, width and height of the sofa cushions. A square cushion will have two large flat sides and four small sides. For instance, assume a cushion measures two feet by two feet and it is six inches tall.
Calculate the surface area of the cushion. For example, the two large flat sides are two feet by two feet for a surface area of four square feet. Since there are two sides, the total area is eight square feet. The smaller sides are six inches by two feet long, meaning each side is one square foot for a total of four square feet. Thus, the total surface area of one cushion is 12 square feet.
Determine the amount of fabric needed for three cushions. Each cushion needs 12 square feet so three cushions require 36 square feet. A yard of fabric is three feet. Therefore, at a minimum, the fabric yardage should be 12 yards. However, due to cutting and sewing, it is advisable to buy more than the minimum required yardage. For this example, a purchase of 15 yards should allow plenty of fabric.