What Is Down Fill Power?

What Does Down Fill Power Tell You?

Fill power is a number that indicates the relative quality of down. The number comes from a lab test that measures how many cubic inches of loft one ounce of that down fill produces. Higher fill power numbers indicate greater loft and insulating efficiency. (It's important to note that down fill power does not tell you how warm a jacket or a sleeping bag will be because too many other factors affect warmth, including how much of that down fill is in the jacket.)

Down Jacket Fill Power Ratings:

Lofty Down Is Warm Down
Down's reputation as the ultimate insulating material for outdoor adventures derives from its ability to compress into a small, packable shape and then puff back up later. The key to down's warmth is the formation of small air spaces within the down "plumules" (superfine featherlike plumage). Higher quality down fluffs back up to create more loft after compression, providing more heat-trapping air space within the down fill.

The Down Fill-Power Test
The test is typically done by filling a large cylinder with one ounce of down. A lightly weighted disk is placed on top, and begins to slowly sink and compress the down. After exactly one minute, the position of the disk is recorded and the volume of down compressed at that point is calculated.

A result of 450 cubic inches of down equates to 450-fill-power down. More resilient down compresses less and will loft higher. It also yields a higher test result-900 fill power is about as good as down gets.

Where High Fill Power Makes Sense
Premium fill-power down commands a premium price. It's used to keep jackets ultralight and ultra packable. So if you're trying to save weight and space in a pack or you want a super sleek winter jacket, a 900-fill-power jacket is worth considering. If overall weight and bulk aren't important to you, get a super puffy jacket and don't worry if it only has 500-fill-power down.