calculator for diagonal pieced backs
 

http://www.multi-patch.com/html/fabric_calc.php I usually do the algebra myself but couldn't find my formula so I tried to find it again and make more copies. I found that John Flynn has a calculator to do it for you! Backs pieced on the diagonal are so much easier for frame quilting because you don't get a big lump from the seam rolling on top of itself when you have a vertical seam.
in case the link doesn't work: I just googled Flynn formula for wide quilt backs and I think this was the first choice.

I did the Flynn formula for a lap quilt and it really turned out nice. I think the idea works better with an all over print fabric...the diagonal seam is less noticeable than one would see with a solid fabric.

Thank you Kathy! I could have used this a while back.

I use the Flynn method for lap quilts and twins -- and a coupe of wall hangings that I was short on fabric. I have done one fabric and different fabrics -- whatever works. I love that method.

I just figured out yardage for a 60x80 quilt with a 4" extra all the way around and it said I needed over 6 yards. This seems like a LOT of fabric. I could use 4 yards and do a horizontal seam.

Uh, there seems to be a bug in their software. I put in a 72 x 80 quilt and this is what I got:

Backing Width: 81.00
Backing Length: 88.00
Fabric Length Inches: -3,520.00
Fabric Length Yards: -97.78

I'm not a math whiz, but I'm pretty sure that is not the right answer! :-)