What is fill power (2026): Fill weight, synthetic

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

What does Down Fill Power tell you?

The higher the fill power, the larger the cage. Down jackets and sleeping bags keep you warm by trapping the warmth coming from your body and keeping it in air pockets between the down. A higher down fill power means the down has more cage, which means there are more air pockets, which means more heat is retained. All else being equal, this means that a garment with a higher fill power will be warmer than one with a lower fill power.

Unfortunately, all else is never equal. Fill power alone is not enough information to know how hot something will be. There is no direct correlation between fill power and how warm a product will keep you, as there are many other factors to consider, such as how much of that fill is in the product, how well it can expand inside the bulkheads or down chambers, how well the fabric blocks 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 fill weight?

Photo: Tatiana/Getty Images

Fill weight is a simple number. This is the amount of fluff in the product, usually measured in ounces or grams. By using down fill weight and down fill power together, you can provide a 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 oz fill weight will be able to trap about the same amount of heat as a 600 fill power jacket with 3 oz fill weight. The major difference between them, and the reason they are priced differently, is the weight of each and the packaged size.

In jackets, the weight difference is not great. That’s why some of our favorite puffer jackets are 600 fill power. When it comes to sleeping bags, however, things are different. Since there is much more down in a sleeping bag, the weight difference between equivalent amounts of fill power is greater. Unless your budget is unlimited, you need to pay attention to the warmth-to-weight ratio. How much warmth do you need, and how much weight do you mind carrying around?

The one downside to fill weight is that some manufacturers don’t list it. It sounds great to say your puffer jacket has 900 fill power, but when you have to list that it only has 2 ounces of it, it sounds less impressive. Less reputable companies often do not advertise the fill weight. Our list fills 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 trays, the compartments built into the product. If you just sewed a single piece of nylon as a shell and pushed a little inward, gravity and movement would push it down near the hem in a few minutes. To avoid this, clothing manufacturers add baffles to keep the down in place. The type and shape of the baffle plays a big role in how warm your jacket, sleeping bag or comforter ends up being.