Fill power is the measured loft or fluffiness of a down product and it’s somewhat related to the insulating value or quality of the down. The higher the fill power, the more air pockets and thus, the more insulating the product will be. The measurements range from just 175cm3/g to an impressive 900cm3/g for the top quality down.
The measurement is done by taking a one ounce sample of down and placing it in a Plexiglas cylinder, which has a weighted piston compressing the down. All duvets, sleeping bags and pillows made with high fill power are lighter than when made with lower quality down.