Fill power refers to the size of a down cluster, and is a reliable predictor of how well the down will insulate relative to its weight. Larger down cluster (higher fill power) will trap more warm air than an equal weight of smaller down cluster, which makes high fill power down a prime material for ultralight sleeping bags and airy comforters. The method for assessing down fill power involves compressing one ounce of down at a set temperature and humidity in a standardized testing cylinder with a piston set at a standard pressure. The volume (in cubic inches) occupied by the semi-compressed down determines the fill power measurement for that batch. Fill power values range from 500 (usually duck or young goose down) to 950 (mature goose down), and typically rise in price exponentially along that scale. Reliable measurements of down fill power are conducted by certified testing labs and reported by manufacturers. For more information, check out the page all about our down.