What Is Down Fill Power (2025): Fill Weight, Synthetics

Goose down has never been hotter. This natural insulation material is popping up inside a growing number of jackets and parkas these days, even in pants and bed comforters. It's the material of choice for most high-end sleeping bags. If you've ever shopped for these items, you've likely come across the term “down fill power,” but what does it mean? Don't fret. Here's what you need to know.

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What Is Fill Power?

Fill power is a way to measure the loft, aka fluffiness, of down. What is down? It's a layer of fluff underneath duck and goose feathers that keeps the birds (and you) warm.

Most companies use goose down, but the jury's still out on whether it's warmer than duck down. We've heard the argument that since goose down clusters are typically larger—and therefore loftier—they should be warmer, given the same quantity and fill power. But there's no study that conclusively proves it. Since goose down has historically been the down of choice and duck down has only recently become more prominent, due to cost, this issue isn't settled yet.

Obtaining a fill-power measurement involves filling a receptacle with one ounce of down insulation and then placing a weight on top. The volume of down under weight is measured, and that's the fill power. A measurement of 500 cubic inches means the fill power of that down is 500. In simpler terms, the higher the down fill-power number, the warmer it is.