What Is Down Fill Power (2026): Fill Weight, Synthetics

Updated February 2026: We’ve added a new section on caring for down and some notes on Outdoor Vital’s Zero Stitch fabric.

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, how well it can expand within the baffles or down chambers, how well does the fabric stop the wind, and so on.

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?

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 ability of a puffer jacket to retain heat 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 able to trap about the same amount of heat 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.

In jackets, the weight difference isn’t huge. This is why some of our favorite puffer jackets are 600 fill power. When it comes to sleeping bags, though, things are different. Since there is a lot more down in a sleeping bag, the weight difference between equivalent amounts of fill power is more significant. Unless your budget is unlimited, you’ll want to pay attention to the warmth-to-weight ratio. How much warmth do you need, and how much weight do you mind carrying?

The one downside to down fill weight is that some manufacturers don’t list this. It sounds great to say your puffer jacket as 900 fill power down, but when you have to list that it only has 2 ounces of it, it sounds less impressive. Less reputable companies often don’t advertise the fill weight. We list fill weight of all the jackets we test.

Other Factors to Consider

Courtesy of REI

While down fill power and down fill weight together give us a way to compare items, there are other things to consider to get an idea of overall warmth. The third major factor is the baffles, the compartments that are built into the product. If you just sewed up a single piece of nylon as a shell and shoved some down inside, gravity and movement would push it all down near the hem in a matter of minutes. To avoid this, garment makers add baffles to keep the down in place. Baffle type and shape play a big part in how warm your jacket, sleeping bag, or comforter ends up being.