Fill power is a way of measuring how much space goose down or duck down can fill. For example, 1 ounce of 800 fill power down will fill 800 cubic inches, whereas 1 ounce of 9oo fill power will fill 9o0 cubic inches. More down generally means more warmth. So a 900 fill power down quilt provides more warmth than an 800 fill power quilt for the same weight of insulation. It also means that you can use less 900 fill power down by weight to get the same warmth as that supplied by 800 fill power down, thereby reducing the weight of your quilt.