Volume 248: What is Down Fill Power?

There are many variables to look for when choosing your down products. In jackets for example, I look for packability (does it have a stow-n-go pocket), sizing (is it long enough to keep me warm but also harness compatible), weight (I have to carry this thing after all), and WARMTH! There is no point in hauling around a jacket that won't keep you warm in those unexpected moments. For warmth, most people turn to Down Fill Power, but do those numbers really tell you how warm a product is? I think the answer will surprise you.

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Down fill power is an oft-stated number. We explain what this spec does and does not tell you about quality and warmth in a down jacket or bag.

Down fill power is a number you encounter even when you're not delving into technical specs or shopping for outdoor gear. If you've ever pondered the virtues of an 800-fill-power down duvet with a 650-thread-count Egyptian cotton cover, you'll know what I mean.

What Does Down Fill Power Tell You?

Fill power is a number that indicates the relative quality of down. The number comes from a lab test that measures how many cubic inches of loft one ounce of that down fill produces. Higher fill power numbers indicate greater loft and insulating efficiency. (It's important to note that down fill power does not tell you how warm a jacket or a sleeping bag will be because too many other factors affect warmth, including how much of that down fill is in the jacket or bag.)

Lofty Down is Warm Down

Down's reputation as the ultimate insulating material for outdoor adventures derives from its ability to compress into a small, packable shape and then puff back up later. The key to down's warmth is the formation of small air spaces within the down "plumules" (superfine featherlike plumage). Higher quality down fluffs back up to create more loft after compression, providing more heat-trapping air space within the down fill.

The Down Fill-Power Test

The test is typically done by filling a large cylinder with one ounce of down. A lightly weighted disk is placed on top, and begins to slowly sink and compress the down. After exactly one minute, the position of the disk is recorded and the volume of down compressed at that point is calculated.

A result of 450 cubic inches of down equates to 450-fill-power down. More resilient down compresses less and will loft higher. It also yields a higher test result—900 fill power is about as good as down gets.

Where High Fill Power Makes Sense

Premium fill-power down commands a premium price. It's used to keep jackets ultralight and ultra packable. So if you're trying to save weight and space in a pack or you want a super sleek winter jacket, a 900-fill-power jacket is worth considering. If overall weight and bulk aren't important to you, get a super puffy jacket and don't worry if it only has 500-fill-power down.

Fill Weight: An Important Spec You Rarely Find

A 900-fill-power down jacket that has only 30 g of down fill won't be warmer than a 450-fill-power jacket with 100 g of down fill. In theory, 50 g of 900-fill-power down would be the same warmth as 100 g of 450-fill-power down; it would also compress to about half the space. Very few jacket makers report fill weights, though. And any simple formula for warmth calculation won't account for differences in how the down fill is distributed within baffles and around the jacket.

The Rest of the Warmth Story

Your other clothing layers, the jacket's shell material and many other design details also figure into how warm you will feel. Couple that with differing metabolisms of people and widely varying weather they might encounter, and you can understand why a temperature rating on a jacket wouldn't be meaningful.

Down Fill Power and Synthetic Fills

Synthetic fills have no equivalent test to down fill power, and each brand of synthetic insulation has unique characteristics. So, it's not possible to use down fill power to compare insulation efficiency to any synthetic fill, and there's no way to compare the efficiency of two different synthetic fills.

Sleeping Bags Do Have Warmth Ratings

Unlike jackets, backpacking sleeping bags have ISO (International Organization for Standardization) ratings, which provide an apples-to-apples comparison of bag warmth. Much like EPA gas mileage ratings, actual experience varies, but the spec does allow you to reliably compare relative warmth between any two ISO-rated bags. A higher fill-power spec can then be used to explain why one bag weighs less yet has the same temperature rating. That lighter bag will also pack down smaller. The ISO spec has a lot of nuance to it.

Written by Ken Knapp for REI Expert Advice

N.A.C. NEWS

(Sunday, January 4, 2026)

Good Morning,

Well, it's January 2026, a New Year!

Did you make a resolution?

Will you keep the resolution?

As always, our resolution is more adventure. Adventure is a truly relative term, but according to Webster's Dictionary Adventure is:

• 1a. an undertaking usually involving danger and unknown risks

• 1b. the encountering of risks

• 2. an exciting or remarkable experience

At NAC we carefully plan our adventures to eliminate nearly all the risk but leave just enough danger! We are planning many new adventures for 2026, and we hope you will be able to join us while we explore new experiences and take on new challenges.

This past Friday, we started our 2026 Season with our Annual Festival of Lights Walk. We had 16 participants from all over. We had participants join from Toronto, Belleville, Niagara, and even Pensylvania, USA. This was the largest turn out we have ever had for the walk and it was great group of people (and one amazingly cute Husky)! After completing half the walk, we paused at the famous Table Rock to enjoy the Niagara Parks Fireworks Display. After the fireworks, we continued our walk to enjoy the remainder of the incredible light displays. But as if that wasn't enough, at the end of our walk, we were mesmerized by a Laser Light Show at Victoria Park. It was a wonderful show and a wonderful event over all. Photos and videos will be available at our Facebook Page shortly!

We currently have 7 more events planned for 2026 and there are many more coming. There are currently 2 day hikes, 2 indoor climbing events, 2 trade shows and a 3 day multiday hike on our calendar! Visit our Events Page for full details on all these upcoming events.

If you haven't already, please consider purchasing an NAC Adventure Pass. There are three options available, an Annual Adventure Pass, a Three Season Adventure Pass, and a Trial Adventure Pass. The Annual and Three Season Adventure Pass lets you choose your start date and grants holders access to many amazing discounts from our NAC Partners.

NAC operates as a not-for-profit club, but we still have many operating costs. Your purchase of an Adventure Pass will enable us to keep providing many more adventures for the 2026 season.

That's all the news for now. As always, stay safe, and happy adventures!

Lenny Burch

Niagara Adventure Club