To make certain you have enough fabric to bind your quilt, use the following equations :

(quilt length + quilt width) x 2 = quilt diameter
(quilt diameter / bias fabric width) x bias seam allowance width = inches of fabric needed

Keep a fudge factor in mind to account for shrinkages and any mishaps and buy just a bit more than calculated with the above equation. If you are using a bias-cut binding, add a generous extra amount onto your total to make sure there are enough pieces of the proper length.