Have you run across the term “down fill power” while shopping for sleeping bags, puffer jackets, or a duvet? These numbers range from 450 to 900, and sometimes above. Generally—but not always—the higher that number, the better the warmth-to-weight ratio. But what does fill power actually measure?
To measure fill power, companies perform a lab test to measure the fluffiness of down. The technical test is how many cubic inches of loft come from one ounce of down. A container is filled with one ounce of down insulation, then a weight is placed on top. After one minute of being compressed by the weight, the volume of down under the weight is measured. The important thing to keep in mind is that fill power is a measure of the quality of the down, not the quantity. Higher fill power means larger down clusters, and larger down clusters fluff up more than smaller down clusters. The graphic below illustrates the difference in loft between various fill powers.
Courtesy of Down and Feather Company
Outdoor gear generally ranges from 500 fill power at the low end to over 900 fill power for high-end ultralight jackets and sleeping bags intended for cold-weather backcountry adventures. The loftiness (that is, the higher fill power) also means that the parka or sleeping bag will compress down much smaller, making them more packable for travel or backpacking.
What Does Down Fill Power Tell You?
The higher the fill power, the greater the loft. Down puffer jackets and sleeping bags keep you warm by trapping the warmth coming off your body, retaining it in air pockets between the down. A higher down fill power means the down has more loft, which means there are more air pockets, which means that more heat is retained. If everything else is equal, that means that a higher fill power garment will be warmer than one with a lower fill power.
Unfortunately, everything else is never equal. Fill power alone is not enough information to know how warm something will be. There is no direct correspondence between fill power and how warm a product will keep you, because there are many other factors to consider, like how much of that fill is in the product and how well it can expand within the cut or design of the jacket or sleeping bag.
To know how warm a down jacket, sleeping bag, or comforter will be, you need to know at least one other number: the fill weight.
What Is Down Fill Weight?
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Photograph: Tatiana/Getty Images
Down fill weight is a simple number. It’s the amount of down in the product, usually measured in ounces or grams. Using down fill weight and down fill power together can give you way to compare two items. For example, the relative warmth of a puffer jacket can be estimated by multiplying the fill power by the fill weight. This means that a 900 fill power jacket with 2 ounces of fill weight will be the same warmth as a 600 fill power jacket with 3 ounces of fill weight. The big difference between them, and the reason they are priced differently, is the weight of each and the packed size.
