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 a certain weight of the down can trap, and thus the more insulating ability the down will have.
Fill power is commonly given as a specific volume (the inverse of density), expressed in cubic inches per ounce.
Common fill power values range from about 300 cubic inches per ounce (170 cm3/g) for feathers to around 900 in3/oz (520 cm3/g) for the highest quality goose down.
The rare and relatively expensive down of certain wild waterfowl species such as the Muscovy duck or Common eider can have higher fill powers than goose down.
It involves measurements taken of a 30g[1] sample of down in a plexiglas cylinder with a weighted piston compressing the down.
The higher the fill number, the warmer the product is, given that the total mass remains the same.
High fill powers are most important for weight sensitive applications such as backpacking.
The warmth of sleeping bags may be more accurately compared using European Norm (EN 13537) temperature ratings that are based on realistic independent laboratory tests.
In the United States, sleeping bags are sold by temperature rating (in degrees Fahrenheit).