I have several little "card" guides that tell me how much I need for a back, however: I have found them to say different amounts. The one in front of me at the moment says : Twin 5 1/4 yds, Double 5 1/4 yds, queen 7 1/2 yards and King 8 yards.
I buy fabrics for my backs when I can get a great deal. I have a shop that recently closed ( :cry: :cry: ) that would have a sale on New Years day and a selection of quilting fabrics would be on sale for $1.69 a yard. I would buy the rest of the bolt on one's I like. 8 - 10 yards on others if possible. I have enough backing fabric laying around here right now to back, oh ummm about 75 quilts. These pieces are from 5 to 17 yards. I can usually find one out of my stash that will work for whatever I'm doing. I keep my eyes open for great sales and I make sure that it is good quality fabrics. Just got 2 more large pieces at a garage sale Saturday for $2 per yard. A 10 yard piece and a 5 yard piece and several others for a total of 32 yards.