Fill power
Also searched as: loft, down fill power, cuin, cubic inches per ounce, FP, down loft test
A measure of down's loft: the volume a conditioned sample of down occupies per unit mass under a light standard load, quoted in cubic inches per ounce in US practice.
In detail
Down clusters are plumules in which soft barbs radiate from a small base; their barbules lack the hooklets that zip contour feathers together, so the cluster stays three-dimensional and traps still air. Fill power expresses how much space a given mass of down occupies after it has been conditioned to relax and then compressed gently in a test cylinder under a specified weight. Higher fill power means more volume per unit mass, which usually means more insulation for the weight. Results depend on the method, so values are meaningful only when the test protocol is stated.
The science and numbers
In a typical test, a sample of set mass is conditioned so that clusters recover from packing, placed in a transparent cylinder of standard diameter, and loaded with a light piston; after a set time, piston height is read and converted to volume per mass. Loft depends on cluster size and maturity, the proportion of feathers, fibers and broken pieces, residual moisture and oils after washing, and compression history. Wet clusters collapse and lose loft, which is why hydrophobic down treatments exist. Laboratories usually report fill power together with a composition analysis, in which a weighed sample is hand-sorted into down clusters, down fibers, feathers and residue, because a high loft figure means little if the blend is mislabeled.
A practical example
Two jackets each contain 150 grams of down, one rated 600 fill power and one rated 800 under the same test method. Because 800 divided by 600 is about 1.33, the 800 down occupies about a third more volume and can loft more in the same baffles. A student should still check the down to feather ratio, fill weight and baffle design before assuming the higher-rated jacket is warmer.
What to distinguish
Fill power describes loft of the fill, not the warmth of a garment, which also depends on fill weight, construction, shell fabric and fit. Protocols differ in conditioning and apparatus, so results from different methods are not directly interchangeable. Fill power also says nothing about species, feather content, sourcing or animal-welfare practices such as live-plucking.
Origins and history
Fill power arose as bedding and outdoor-equipment makers needed a single number to compare down quality, and the inch-per-ounce unit reflects its adoption in the US market. Composition testing developed alongside it, since loft depends on the share of true down clusters versus feathers and fragments. Industry testing regulations and national standards were written to reduce disagreement between laboratories, but they have not produced one universal protocol.
Related terms
Sources & further reading
- Progress in Histochemistry and Cytochemistry: Cornification during feather morphogenesis (Alibardi and Toni, 2008)
- EFSA Journal: Scientific Opinion on the practice of harvesting (collecting) feathers from live geese for down production (2010)
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.