Fill Power or “loft” is a volumetric measurement which is used to determine the approximate insulation value of down and feather fillings (cubic inches per ounce or cubic cm per gram).  Some standards report the volume as a height measurement taken from the fill power device (mm or cm).  The greater the volume, the better the down will be.  

Fill Power is one of the best performance metrics used to distinguish different qualities of down and feathers.  Many products use fill power in making efforts.

IDFL Test Summary - Fill Power (EN,CN).pdf