What is Bottom Fill Power (2026): Fill Weight, Synthetics

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

What does the bottom fill force tell you?

The higher the fill power, the greater the loft. Puffy jackets and sleeping bags keep you warm by trapping the warmth emanating from your body, holding it in air pockets between your down. A higher fill power means the down has more height, which means more air pockets, which means more heat is retained. All else being equal, this means that clothes with a higher fill power will be warmer than clothes with a lower fill power.

Unfortunately, everything else is never equal. Fill strength alone is not enough information to know how warm something is. There is no direct correspondence between fill strength and how warm a product will keep you, because there are many other factors to take into consideration, such as how much of that filling is in the product, how well it can expand within baffles or lower chambers, how well the fabric stops the wind, and so on.

To know how warm a jacket, sleeping bag, or comforter is, you need to know at least one other number: the weight of the filling.

What is the weight of the down filling?

Photography: Tatiana/Getty Images

The weight of the down filling is a simple number. It’s the amount of fluff in a product, usually measured in ounces or grams. Using bottom fill weight and bottom fill strength together can give you a way to compare two items. For example, the relative ability of a puffy jacket to retain heat can be estimated by multiplying the fill strength by the fill weight. This means that a 900 fill power jacket with 2 ounces of fill weight will be able to trap almost 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 for their different prices, is the weight of each and the size of the package.

In jackets, the weight difference is not that great. That’s why some of our favorite puffer jackets have a fill capacity of 600. But when it comes to sleeping bags, things are different. Since there is a lot of stuff in a sleeping bag, the weight difference between equal amounts of packing power is more significant. Unless your budget is unlimited, you’ll need to pay attention to the warmth-to-weight ratio. How much warmth do you need, and how much weight do you mind carrying?

The only downside to filler weight is that some manufacturers don’t include this. It sounds great to say your puffer jacket has 900 low energy, but when you have to state it only has 2 ounces, it sounds less impressive. Less reputable companies often do not advertise packing weight. We list the packing weight of all the jackets we test.

Other factors to consider

Courtesy of REI

While down fill power and down 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 partitions, the compartments built into the product. If you sew a single piece of nylon into a shell and put some of it inside, gravity and movement will push it all down near the hem within minutes. To avoid this, clothing makers add baffles to keep the bottom in place. The type and shape of the barrier plays a big role in how warm your jacket, sleeping bag, or duvet will be.