How fill power is actually calculated goes like this: One ounce of down is placed in a container that has markings for 500 cubic inches, 600 cubic inches, and so forth. The down is allowed to expand, and the volume in cubic inches that it fills is the fill power. That's italas, no industrial fans or people running around a big room with charts and tape measures.
Why does it matter? Two reasons. One, the more fill power per ounce of down, the less down you need in a sleeping bag, and therefore the lighter the bag. That doesn't make a huge difference in a 20-degree bagmaybe an ounce or twobut in a cold-weather bag rated to zero or below it can become a factor. More importantly, perhaps, down with high fill power is typically higher quality down, usually plucked from more mature geese and with a better-developed plumule structure. So it lasts longer and can withstand more stuffings into a compression sack. In fact, a well-made down sleeping bag, with good-quality down, can nearly be considered a lifetime investment. It should last for many, many years.
Get more advice on the anatomy of a sleeping bag, plus reviews of the year's best bags, in Outside's 2004 Buyer's Guide.
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