It is the unit of measurement used to test the quality of the quilt, to clearly differentiate it from the piumette. The quilt is in fact a real three-dimensional bow, without rigid core in the form of a central rod named rachis, typical instead of piumette. The bow is made of a central core from which thin elastic filaments branch off, needful to guarantee its unequaled insulating power.
In nature there isn’t a "synthetic" product that can replicate its isothermal and hygroscopic properties. The Fill Power measures the capacity of the quilt to take up volume, added to the elastic power to return swollen after compression. The greater the inflating power is, the better the performance of the padding will be in terms of lightness, warmth and comfort. The longer the filament of the staple is the greater will be its inflation capacity *.
The padding of a down jacket ensures guarantees warmth, trapping a layer of air, that insulates the body from the outside, the greater the amount of air, the higher its thermal resistance will be. The thickness of air is generally named "Loft". In order to create the same size as Loft, the Fill Power Down Jacket 800 employs 40% more feather less than a 550 Fill Power, resulting however much lighter and more comfortable.