What Is Fill Power?

T. L. Childree

Fill power is a measurement of the fluffiness of down and is also related to its insulating ability. Down is a layer of soft, inner feathers that is most frequently harvested from geese. Small, standardized samples of down are compressed in a cylinder to determine the material’s loft ability. Fill power is commonly used to determine the weight and insulating ability of down-filled outerwear, comforters, and sleeping bags. Fill power ratings for pillows and mattresses refer only to their comfort.

Fill power measures the insulating ability of things such as sleeping bags.
Fill power measures the insulating ability of things such as sleeping bags.

Down is typically rated according to its insulating strength and fluffiness. Both of these measurements affect the quality and usefulness of down-filled articles. Fill power is a measurement of the material’s ability to remain fluffy after compression. Greater fluffiness allows more air to become trapped in the down for better heat retention. The down’s fluffiness typically determines the overall weight and insulating ability of an article. Higher ratings generally indicate a well-insulated, lighter weight item while lower numbers are usually assigned to heavy, less-insulated variants.

Down feathers are frequently gathered from geese.
Down feathers are frequently gathered from geese.

Down is a material consisting of fine, soft feathers located beneath a bird’s exterior covering. It is a very good type of thermal insulation and has long been used in clothing and comforters. Down is most often harvested from geese, although other bird species such as ducks are also used. Geese raised for consumption are usually stripped of their down before being processed. This commercially harvested down typically has a lower fill power rating than feathers collected by hand.

A small standardized sample of down is tested to determine its fill power. This test is conducted by placing the sample into a transparent cylinder and compressing it with a weight. The weight is then removed, and the down is allowed to return to its normal height in the cylinder. The return height is then measured to determine the fill rating. Although the rating ia also an indication of the down’s insulating ability, it is only used to determine its lofting power.

Fill power ratings are typically assigned to down in insulating bedding and clothing. Down-filled winter outerwear such as jackets and parkas must be thick enough to retain body heat, but light enough to be worn. Comforters filled with down must remain fluffy to provide adequate insulation, but lightweight for comfortable use. Down-filled sleeping bags must also have high fill ratings to provide adequate insulation against cold temperatures. The fill rating of down-filled pillows and mattresses is typically a gauge of comfort rather than insulating ability.

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