What do those down numbers mean?

What do those down numbers mean?

November 24, 2015

Have you ever looked at a down jacket that looks as big as a house and saw a 500 fill power rating next to a thin down sweater that said 900 fill and wondered how that could be possible? Well we're here to explain that for you. That number you're seeing is something called fill power.

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. Fill power ranges from about 300 (in³/oz or 175 cm³/g or Lorch 75 mm/30g) for feathers to around 900 (in³/oz or 520 cm³/g or Lorch 225 mm/30g) for the highest quality down. Higher fill powers are associated with a larger percentage of down clusters and a larger average down cluster size.


Fill power is the most frequently used measure of down quality. It involves measurements taken of a one-ounce sample of down in a plexiglas cylinder with a weighted piston compressing the down. The test requires controlled temperature, humidity, and preparation of the sample. All other things being equal a parka (duvet or sleeping bag) made with high fill power is lighter and more compressible than an equally warm one made with lower quality down. Fill power is expressed as cubic inches per ounce (in³/oz)—a lofting power of 400–450 is considered medium quality, 500-550 is considered good, 550–750 is considered very good, and 750+ is considered excellent. So that being said, 750+ fill is quite different from 400 fills.

Almost all down commercially available is a secondary product of geese or ducks raised for consumption. It would be prohibitively expensive to raise birds for down alone.  The 500+ down fill comes from a small number of birds kept for breeding purposes throughout the year. These geese molt naturally in the spring. While their down is loose it is collected by hand. It is very rare and, of course, expensive. The larger individual plumules are what gives the greater loft. The only way to get down of this quality is by careful hand selection which is the major factor in its scarcity.No live plucking here!

Depending on the intended application, the highest available fill power may not be necessary. A high fill power will provide more insulating ability for a given weight of down. The warmth of a down jacket depends not only on the fill power, but also on the quantity of fill. So the difference between a jacket with 550 fill power down and a jacket with 700 fill power down is that the 700 fill power down comforter will be lighter for the same warmth rating. High fill powers are most important for weight sensitive applications such as backpacking etc. Down (or any insulation) provides warmth by trapping a layer of air that separates the cold side from the warm side. A thicker layer of trapped air gives more insulation. This thickness is often called "loft." A jacket that uses 550 fill power down, for example, would have to use approximately 40%-50% greater weight of down than a similar item that uses 800 fill power down to provide the same loft.

We hope this helps a little bit and sheds some light on what it means to see a number on your coat!

Remember, 500 fill can be warmer than 900 fill if there is more volume of down in it!

Until Next time!

Greg from Great Escape Outfitters

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