Down Fill Power and Weight Numbers Explained

If you’re in the market for a down-filled sleeping bag or jacket, chances are pretty high that at some point, you’ve run into the terms down fill power and fill weight.

But, what are down fill power and fill weight and what do they mean? 

Are fill power and weight numbers something you need to worry about when you shop for outdoor gear?

In this article, we’ll demystify the world of down insulation and clue you into what you need to know about down before you purchase a jacket or summer sleeping bag for your upcoming adventures.

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What does down fill power mean?

Nearly every piece of outdoor gear made with down insulation is marketed as having a certain fill power of down, but unless you already happen to be a textile engineer, it might not be clear what this actually means for your camping experience.

It turns out that fill power, also known as massic volume, is a very important metric for down gear. That’s because it is a numerical representation of the loft of the down used in a jacket, sleeping bag, or other pieces of equipment.

In many ways, we can use the fill power of down as a partial measure of its quality. While fill power doesn’t tell the whole story of a down garment’s warmth (more on that later), there’s a lot we can learn about the quality and performance of a sleeping bag or jacket by checking its fill power.

Here’s what you need to know.

The importance of loft in down

At this point, we’ve already determined that the fill power of down is a measure of its loft, but, what is loft, you might ask?

Basically, loft refers to the overall fluffiness of down, or any other fibrous insulation, such as the synthetic insulation you might also find in summer and winter sleeping bags.

The important thing to remember is that down is an insulator – not a heat source.

This means that it helps keep you warm by trapping air next to your body. Your body then heats up this air, which is used to keep you warm, even in cold temperatures.

So, the more loft that down has, the less dense it is and the more air that it can trap. The more air that down can trap, the better insulated you can be without the need for more down fill.

Ultimately, all of this means that a sleeping bag that uses loftier down can get away with using less down without affecting the temperature rating of the bag. This results in a lighter, more compact sleeping bag just by using loftier down.

What does this all have to with down fill power?

Remember : Down fill power is a numerical representation of loft. So, using a higher fill power down, such as 950 fill down means more loft, more warmth, and a lighter, less dense, and more compact sleeping bag or jacket than what you’d find in a model with 600 fill down.

How is down fill power determined?

As you can imagine, measuring down fill power isn’t as straightforward as measuring something like weight. There’s no magic scale out there that can instantaneously determine the fill power of your down jacket.

Instead, fill power is generally measured by placing 1oz (28g) of down inside a large graduated cylinder. Then, researchers will place a weighted disk on top of the down to compress it for approximately 1 minute.

After the time is up, the weighted disk is removed and the researchers record the volume of the down in cubic inches. This volume then becomes the fill power.

That means that a 500 fill power down has a total volume of 500ci (8.2L) while a 900 fill power down has a total volume of 900ci (14.7L).

As you can see, a higher fill power down has a much larger volume than a lower fill power alternative of the same weight, or, in other terms, higher fill power downs are less dense. As a result, high fill power down has more loft, which means they can trap a whole lot of warmth at just a fraction of the weight.

It’s worth mentioning, though, that there’s no universally required standard for measuring down fill power, so manufacturers can come up with their own tests if they’d like. However, most companies follow the BS EN 12130:1998 guidelines set out by the European Standard and British Standards Institution (BSI).

Does a higher down fill power mean more warmth?

It’s a common misconception that you need to have a high fill power down to stay warm at night. In fact, this couldn’t be further from the truth.

Since fill power is simply a measure of how much loft down has, it’s more of a representation of the warmth-to-weight ratio that you can expect in a sleeping bag or jacket. 

As a result, high fill power (think 900+ fill) down sleeping bags are generally the lightest and most compact on the market. They’re also some of the most expensive because that high-quality down doesn’t come cheap.

But, this doesn’t mean that they’re the warmest.

In fact, you can get very warm sleeping bags rated to -20ºF (-29ºC) that have a fill power of 650. These bags are plenty warm, but they’re just not very light or compressible.

How much down fill power do I need?

There’s no hard and fast rule about how much down fill power you need because down fill power doesn’t determine warmth in a sleeping bag. In reality, sleeping bag warmth is the result of a whole host of different factors that we’ll discuss in a bit.

When deciding how much down fill power you need for a jacket or sleeping bag, you need to ask yourself the following questions :

  • Is my primary concern weight savings and packability in my gear?
  • Am I willing to pay a substantial premium to achieve these weight savings and portability?

If your answer was yes to both of these questions, then you’re probably in the market for a premium down with a fill power of 800+.

Anything in 800+ fill down range is going to be quite compressible and have a great warmth-to-weight ratio. But, you should expect to shell out a pretty penny for this type of gear, especially if you opt for products with super-premium 1000 fill down.

For folks who answered no to these 2 questions, you’ll probably be more than satisfied with a down fill that has a fill power of less than 800.

While 550 fill down is considered the lower limit of acceptability in outdoor gear (you may find lower values in duvets and pillows), anything in the 550 to 800 fill range is just fine for outdoor pursuits where weight and bulk aren’t a key issue.

What is down fill weight?

Perhaps the most often overlooked, but incredibly important metrics that gear manufacturers use when creating down gear is the down fill weight.

Unlike down fill power, though, down fill weight has nothing to do with the quality or properties of the down itself.

Rather, down fill weight is simply the amount of down inside a jacket or sleeping bag.

Down fill weight is expressed by, well, weight, so you’ll see jackets labeled with 1oz (30g) or 2oz (60g) of down fill.

Is down fill weight important?

As we’ve mentioned, down fill weight is important in a jacket or sleeping bag. That’s because, unlike fill power, down fill weight tells you precisely how much down is actually in your gear.

Although neither fill weight nor fill power on their own tell you the whole story, when you use them together, you can learn a whole lot more about the quality and construction of a down garment.

For example, it’s pretty hard to compare 2 sleeping bags if you only know their down fill power. While a 900 fill down sleeping bag might sound more impressive than a 450 fill down sleeping bag, unless you know the fill weight of these bags, there’s really no way to compare them in terms of warmth.

Indeed, a 900 fill power down sleeping bag that has a fill weight of 12oz (340g) will be much warmer than a 600 fill power bag with a fill weight of 12oz (340g).


In both sleeping bags, you have the same total amount of down. But, in the 900 fill power sleeping bag, you have substantially loftier down, which means it can trap more air and keep you better insulated than its 600 fill power counterpart.

Therefore, knowing both the fill power and the fill weight of a sleeping bag lets you compare apples to apples to ensure that you get the right gear for your needs.

That being said, fill weight is one of those specs that’s fairly difficult to find for jackets, though you’ll likely see it listed for high-end sleeping bags. If you really want to know what the fill weight of a jacket is, you may have to contact the company directly.

Read More : 6 Down Fill Facts and Myths You Didn’t Know

Other factors that affect down quality & warmth

While fill power and fill weight are two of the most commonly used metrics to describe down sleeping bags and jackets, it’s important to note that there are many other factors that affect the overall warmth and quality of your gear.

These are some of the other key factors to keep in mind.

Down composition

The down used for jackets and sleeping bags is sourced from waterfowl, traditionally geese or eider duck. Most of the insulation put into these products isn’t 100% pure down, rather it’s a combination of down clusters, feathers, and quills, all of which get combined into the fill in your sleeping bag.

Down clusters are the primary source of insulation, both for the birds themselves and for us in our down sleeping bags and jackets. Feathers and quills, however, provide no insulating value, but you’ll often find them inside other down products, like sofas and pillows, which need to be strong enough to support human weight.

That being said, the down fill used in sleeping bags and jackets is available in a wide range of different compositions, or ratios of down clusters to feathers and quills. Since down clusters provide all the insulation, compositions that have more clusters and fewer feathers and quills are better quality and they provide better warmth to weight ratio.

As you can imagine, such compositions are quite pricey, so you’ll normally find them in premium products that are designed for ultralight hiking, climbing, and other similar pursuits.

Waterproofing treatments

One of down’s biggest drawbacks is that it offers no insulating value when wet, which can be quite a problem in damp environments like you’d find in the Adirondacks or Olympic National Park.

In an attempt to solve this problem, many manufacturers use what’s called treated down. This is essentially down fill that’s treated with a durable water repellent (DWR), such as what you might find in tent waterproofing sprays.

However, DWR-treated down tends to be more prone to clumping, which can reduce its overall insulative value. It also tends to wear out faster because the DWR coating strips the down of its natural oils. 

Therefore, water-resistant down, while sometimes beneficial, does have some consequences for your overall warmth in the backcountry.

Jacket and sleeping bag construction

Finally, a critically important factor that determines warmth in the mountains is the physical construction of your jacket or sleeping bag.

The majority of down jackets and sleeping bags use baffles, or sewn chambers within the fabric, to prevent the down from clumping up in one area, leaving behind large holes that lack insulation. That being said, the type of baffles that your garment or bag uses, also makes quite a big difference in your overall warmth.

There are 2 primary types of baffles :

  • Sewn-through
  • Boxed

Sewn-through baffles are often cheaper and lighter because they’re created simply by sewing compartments into the inner and outer layers of your sleeping bag or jacket to separate the down. This process results in quite a few seams in your gear, which allow for heat to escape, thereby reducing your overall warmth.

Alternatively, boxed baffles are a much more complex type of baffle that essentially hides the stitching within the sleeping bag or jacket. As a result, heat is less likely to escape, making for a better-insulated piece of gear, albeit at a higher price point.

Gaby Pilson

Gaby is a professional mountain guide with a master’s degree in outdoor education. She works primarily in the polar regions as an expedition guide, though she can be found hiking, climbing, skiing, sailing, or paddling in some of the world’s most amazing places when not at work.