Technically speaking fill power is a measurement of the amount of space one (1) ounce of down (as shown in the cylinders below) will occupy in cubic inches when allowed to reach its maximum loft. For example, one (1) ounce of 800 fill power goose down will loft to 800 cubic inches.
Correspondingly, what does 700 fill power mean?
Fill power is a measure of the loft or "fluffiness" of a down product that is loosely related to the insulating value of the down. The higher the fill power the more air an ounce of the down can trap, and thus the more insulating ability an ounce of the down will have.
What is a good fill power for a down comforter?
Down fill power is the measure for the amount of space one ounce of down takes up. You can typically find fill power listed on higher end comforters. The higher the fill power, the better the insulating ability and loft (fluffiness) a down comforter will have. Fill power is a good indicator of warmth and quality.