Fill power refers exclusively to down outerwear. More specifically, it is how “full” of down the garment is. When purchasing a down jacket, somewhere on the jacket will be a number that indicates the fill power. The fill power is a measure of down density; it is how much space an ounce of that density down will fill, in terms of square inches. For example, a jacket that has 700-fill down means that an ounce of that down will expand to fill 700 cubic inches. Therefore, a jacket that has a higher fill power will usually be warmer. Fill power numbers usually go from about 500, all the way up to 1000, in increments of 50.