What is fill power?
Fill power is a technically term used to measure the amount of space that one ounce of down occupies when it blooms to its maximum.
Fill power usually ranges from 400-900. One ounce of 900 fill power down will occupy 900 cubic inches of space when it lofts to its maximum. While 400 fill power down will occupy much less space comparatively.
The higher the fill power is the fluffier and the better quality the down will be. Thus the better the insulation performance will be.