First, measure the width of your fabric. Most home dec fabric is 54", but measure for accuracy. Let's assume you are working with a solid or an all-over design that does not need to be matched up on the sides. I will write a tutorial on how to match up fabric designs on the sides - in the near future.
13 . 5 yards of fabric. Then multiply this by how many windows you have to reach your final yardage amount.