I have spent over 12 years helping people research, compare, and buy outdoor gear. During that time I have found myself repeatedly discussing the three-digit number that is associated with the down fill of a sleeping bag. This rating (named fill power) is like many other numbered systems, the higher the better. However, there is much more behind that number that deserves some consideration when making your next purchase.

Not all down is created equal. Consumer products are usually filled with either duck down or goose down. Duck down is rarely used on its own as it contains barbs that allow the feathers to cling together, which doesn’t allow for easy or efficient use in sleeping bags. Goose down does not have such attachment points, is easier to use in the manufacturing process, and provides a better result versus cost of use. Therefore goose down is primarily found in sleeping bags, although it is perfectly legal to use a mixture of duck and goose down if it meets the stated fill declaration.

There are two main sources for the down used in the outdoor industry: Europe and Asia. European down is gained from taking feathers from mature birds that have been well cared for and thus provide a high quality feather cluster. This down accounts for the highest quality, and therefore is found in the most expensive down products.

Asian down is a byproduct of the food industry. The birds from which the down is harvested are younger in age and therefore have not developed the full down clusters that their European counterparts produce. Asian down is much more economical to use as it is more plentiful than European down, more cost effective to produce as the birds are not kept to maturity, and it benefits from the relatively low cost of exporting from Asia. As a result, these fills are most often found in entry-level or low-cost products.

Down is rated on a scale from 500 to 900 and usually appears in increments of 50. This is a measurement of the number of cubic inches that one ounce of down will fill. European down is a much larger feather and therefore will displace more space than a similar Asian feather. It is this ability to occupy a large amount of volume for a small amount of weight that makes European down reign supreme in the feather world. The European down is represented on the top end of the scale, usually in the 750 to 900 range.

Asian down represents the lower end of the scale (500 to 700), not only because of the smaller size of the Asian feathers, but the overall cleanliness and care of the feathers. The feathers are not thoroughly washed to remove particles that prevent full extension of the feather’s spine and subsequent branches. Often the down is compressed for long periods of time during the shipping process, damaging the feathers and losing the lofting characteristics needed to retain heat.

The spine is critical to the loft of the bag as it is the support structure of the feather. The spine can become damaged from repeated or prolonged compaction leading to a loss of loft. European down is more susceptible to this damage due to its comparatively long length. This fact can be overcome by storing your bag unbound. The best scenario will find your bag hanging in your closet or sprawled out under your bed. The worst thing you can do is leave it in a compression or stuff sack, especially if it is wet.

Downs’ insulating properties come from its ability to trap air and use your body heat to warm that air. As higher quality down can create a larger amount of space per feather, it requires fewer feathers to trap a similar amount of air.

For example, if we take two sleeping bags with identical fabric, dimensions, and temperature ratings, but use 600 fill in one bag and 800 fill in the other bag, we will achieve two different results. The bag that contains the 600 fill will require more down to reach the target temperature rating than the 800 fill bag. This fact will result in a heavier bag, when using 600 fill, when compared to the 800 fill bag.

Right now you are probably thinking to yourself, why wouldn’t I always go with the higher fill? There are two downsides to the European down – durability of the feathers and cost. It requires more time and money to bring the birds to maturity, and the down is more thoroughly cleaned and cared for during the shipping.

When shopping for your new down sleeping bag keep in mind your end goals. If you are a person that is less concerned about weight and durability, and more concerned about price, the lower end of the down spectrum is for you. If you are like me and look forward to carrying a lightweight, yet billowy cocoon of warmth, the extra care and money required is an easy hurdle to overcome.

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