A quick, loose-math way to estimate is by the total yardage of the desired quilt size. For example, a standard king size is 120" x 120". That would be 3.33 yards long (or wide); it would take 3 lengths of 40" wide fabric (3 x 3.33) for a total of 10 yards---without allowing for seams, squaring of fabric, etc. As has already been stated, the more seams in the pattern, the more fabric required. I would be uncomfortable with less than 11 yards total and would probably start with 12 for a king size just because lots of 'fudging' room is nice!

I am holding in my hand a quilters' yardage chart prepared by Quilter's Newsletter. I keep one in my wallet also. They recommend for a king, 100 by 92, 8 yards of fabric and 1 yard for binding. You'll need to add more yardage if your quilt is larger than above.