A lot of quilters hate math so I am going to try to simplify quilt backings a little for you.
If you are making a quilt less than 40" wide, one length plus 6 inches is probably enough for your quilt backing. This would be a baby quilt. If your quilt is 40" wide by 45" long, you would need 45 + 6 = 51 inches for your quilt backing. This would be 1 1/2 yards and then you have some to square up the backing and your long arm quilter will love you!
If you quilt is between 40" to 80" wide, you need two lengths of your quilt top. If your quilt is 76 wide by 90" long, you need 90 + 90 + 12 = 5 1/2 yds and this gives you a little to spare for squaring up your backing.
If your quilt is over 80" wide, you need three lengths of your quilt top. If your quilt is 90" X 100" long, you need 100 + 100 + 100 +12 = 8 1/2 yards.
If I don't know what I am going to use the backing fabric on when I purchase it (I love sales!) I buy 9 yards, it will do any size quilt backing and I have some left over.
My above figures are based on piecing lengthwise of the quilt and using no piecing EXCEPT to get the width of the quilt.
Hope this helps.
Barb
Barb