Have you run across the term “down fill power” while shopping for sleeping bags, puffer jackets, or a duvet? These numbers range from 450 to 900, and sometimes above. Generally—but not always—the higher that number, the better the warmth-to-weight ratio. But what does fill power actually measure?

To measure fill power, companies perform a lab test to measure the fluffiness of down. The technical test is how many cubic inches of loft come from one ounce of down. A container is filled with one ounce of down insulation, then a weight is placed on top. After one minute of being compressed by the weight, the volume of down under the weight is measured. The important thing to keep in mind is that fill power is a measure of the quality of the down, not the quantity. Higher fill power means larger down clusters, and larger down clusters fluff up more than smaller down clusters. The graphic below illustrates the difference in loft between various fill powers.

Outdoor gear generally ranges from 500 fill power at the low end to over 900 fill power for high-end ultralight jackets and sleeping bags intended for cold-weather backcountry adventures. The loftiness (that is, the higher fill power) also means that the parka or sleeping bag will compress down much smaller, making them more packable for travel or backpacking.

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 and how well it can expand within the cut or design of the jacket or sleeping bag.

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?

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 warmth of a puffer jacket 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 the same warmth 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

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.

The cheapest and most common form of baffle is a sewn-through baffle. As the name suggests, the chambers are stitched in place, with thread passing through the outer shell to the lining fabric, trapping the down between. This is the lightest-weight baffle, but the down side (sorry) is that the large chambers mean down can settle more, leaving potential cold spots.

The box baffle technique still stitches through but makes little boxes, which do a better job of keeping the down in place. Box baffles are slightly warmer. The third major method of baffles is what's called welded baffles (also sometimes heat-seamed). This eliminates the seam, which makes a jacket more wind resistant (since there's no stitching holes for the air to pass through).

Another things to consider is how much room there is for the down to expand. If you have a sleeping bag with 1,000 fill power but not enough room for that down to fully expand, then it won't be as warm as it could be. This is the toughest thing to measure. Few manufacturers list stats like loft height or garment thickness. But if you're looking at two sleeping bags with equal fill power and equal fill weight, but one is noticeably fluffier than the other, chances are the fluffier one will be warmer.

Different Kinds of Down

Most down is goose down, but manufacturers have started using duck down to keep prices lower. In my experience testing puffer jackets and sleeping bags, there isn't a huge difference between the two, but right now most of the highest-quality down products on the market use goose down.

Another thing to look for is whether the down comes from sources approved by the Responsible Down Standard. You'll often see that abbreviated “RDS approved.” The RDS is a voluntary standard that is trying to get the down industry to treat ducks and geese more humanely. It's not perfect, but if you're concerned about the ethics of down, looking for the RDS seal of approval is good place to start.