Backing 108" ?
 

When using the 108" backing fabric how many yards does it take to do a Queen - King Quilt?

Probably only 3 yds. then it would be 108 x 108 " square

A yard is 36 inches so 3 yards should be enough for the king unless you're making a really large one.

it depends on the length of the quilt as well as the width. If your quilt is 105" or wider, you'll need two widths (or one width of 108" and one of 45"). Then, it also depends on the length of your quilt. If you have a quilt that is longer than 108" you'll need more than three yards. Say your quilt is 100"x120". Then you need one width (108") and at least 3-1/2 yards (126").

Laurie