[QUOTE=Prism99;6341343]First I figure out if the backing fabric is sufficiently wider than my quilt. For example, if my quilt is 90" wide and the backing fabric is 108" wide, I know I don't have to worry about width.
Hopefully this is the case with your quilt. If so, all you have to do is measure the length of your quilt. If you don't intend to pre-wash your backing, then you can order the length of your quilt plus 10 inches or 15 inches (depending on how much extra you want). If you are going to pre-wash, I would order at least 1/2yd extra in case the fabric shrinks.
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I agree! Measure your top and see if backing WOF will fit either length or width of quilt with extra fabric fabric.
If it fits either way then get as much fabric as it takes for the fit the the other way plus several inches. for exmple if your quilts measures 90 x110 then the 108" fabric will fit the width of 90" so you would get 3.5 yds /4yds (110 devided by 36") I have learned the hard way to always get extra. The quilt does not care which way the backing goes on. the idea of the wide backing is not to have any seams. But that may not always be posiible. If you quilt top measures 110 x110 then there is not way you can fit wide backing with out seams which as Prism99 says is a different problem!