What Is Down Fill Power (2026): Fill Weight, Synthetics
Wired
by Scott GilbertsonFebruary 22, 2026
AI-Generated Deep Dive Summary
**Understanding Down Fill Power and Its Importance**
When shopping for items like jackets or sleeping bags, you've likely come across terms such as 450, 600, or 900, which refer to down fill power. This metric indicates the insulation ability of the down by measuring its loft in a lab test where one ounce of down is compressed under weight and volume measured post-compression.
While higher fill power generally suggests better insulation due to increased fluffiness, it's not the sole factor to consider. Fill weight, which quantifies the actual amount of down used, plays a crucial role too. For instance, a jacket with 900 fill power but only 2 ounces of down might be just as warm as one with 600 fill power and 3 ounces, depending on other factors.
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Originally published on Wired on 2/22/2026