Skip to main content

What is fill power? Understand the insulating capacity of your down gear

Jottnar Fenrir
Understanding what fill power means will help you choose the best down jacket and other insulated gear for staying warm on your adventures (Image credit: Jottnar)

What is fill power? If you’ve ever shopped for a new down jacket or sleeping bag, you’ll have noticed a term on the label that reads something 650FP and may have wondered what it means. Is higher fill power better and do light, packable down jackets have less fill power? Understanding what fill power means will help you choose the best down jacket and other insulated gear for staying warm on your adventures and we’re here to help. 

What is fill power? 

A woman admiring half dome from inside her tent

Fill power describes down’s ability to fluff back up after being compressed – the more it can fluff up, the more air it’s able to trap which means more heat provided for your body (Image credit: Jordan Siemens)

First off, the term “down” refers to the fine feathers found on a young bird, or under the tougher, exterior feathers of an adult bird. Down makes a great insulator for birds and it does the same job when it’s inside your favorite down jacket or gilet.

However, not all down is created equal and fill power, also known as “loft,” refers to the fluffiness of down and is a way to measure its insulating capabilities, or essentially its quality. Down insulates by trapping air, and the more air a certain weight of down can trap, the better it is at keeping you warm.

Fill power is measured in a lab, where an ounce of down is slowly compressed for one minute then measured for volume. The volume of that ounce of down in cubic inches equals the fill power of that down, so 300 cubic inches equals a fill power of 300, which is the lowest fill power you’ll find in gear, while the highest fill power is around 900. In short, fill power describes down’s ability to fluff back up after being compressed – the more it can fluff up, the more air it’s able to trap which means more heat provided for your body.

Best gilets

When it comes to the label on your down gear, fill power doesn't necessarily indicate how warm it will be, (Image credit: Getty Images)

The caveat is that when it comes to the label on your down gear, fill power doesn't necessarily indicate how warm it will be, as that also depends on how much of that down is inside your gear. For that, you’ll want to look at the fill in terms of grams. A 400 fill jacket containing 300 grams of down will keep you as warm as an 800 fill jacket with only 150 grams of down – it will just weigh more and be bulkier. The amount of down in a jacket isn’t usually disclosed, but sleeping bags will tell you both the fill power and amount of down in weight.

Finally, there are other factors at play too, such as the material the shell of your jacket or bag is made from and the lining.

What is a good fill power? 

The North Face Thermoball Eco Jacket

The best quality down is goose down, which compresses less than other types of down and therefore traps more air, providing more warmth (Image credit: The North Face)

As we said, fill power ranges from 300 to 900. Anything below about 450 is considered low quality down and anything above 550 is considered high quality. The best quality down is goose down, which compresses less than other types of down and therefore traps more air, providing more warmth. So objectively speaking, a down item with a fill power of 900 is the best. That said, it really depends what you need it for.

If you’re going winter camping and anticipating sub zero temperatures, then a down sleeping bag with a fill power of 800 or more would probably keep you toasty. But a down jacket with the same fill power would be far too warm if you’re planning on snowshoeing in temperatures of 30 °F or higher. 

Is higher fill power better? 

A woman curled up in a sleeping bag on a mountain

Higher fill power means higher quality and also higher price (Image credit: Getty)

Again, the answer to this question lies a little in what you need the gear for. Higher fill power means higher quality and also higher price. Assuming you want a jacket that will keep you really warm and packs down small, you’d be better off looking for one that contains a smaller amount of higher fill power down. 

Even if you don’t know how much down it contains, a light, sleek down jacket with a fill power of 700 or higher is likely to keep you very warm and not take up too much space in your backpack. This type of jacket is going to cost you more, but if you’re carrying all your gear on an overnight trip, you may be willing to fork over more cash for less weight and more packability.

down gear

The amount of down in a jacket isn’t usually disclosed, but sleeping bags will tell you both the fill power and amount of down in weight (Image credit: Rab)

A bulkier down jacket could contain twice as much down with a fill power of 350, half that of the jacket described above, and if it were made from the same shell material would keep you just as warm, but take up more space in your pack. It will also be cheaper, so the answer really depends on your needs. If you just want an affordable insulating layer for throwing on over your fleece jacket while hanging out at the crag between climbs, you might skip the expense and stay just as warm.

Ethical considerations 

Föhn

Increasingly, there are ethical options when it comes to down insulation (Image credit: Föhn)

Finally, it’s impossible to have a conversation about down without raising the ethical considerations. Increasingly, there are ethical options when it comes to down insulation and it’s worth knowing more about ethical down products before you buy. If you’re in the market for insulated gear, also take a look at our article on down vs synthetic insulation to decide which you need and what you feel most comfortable with. 

Julia Clarke

Julia Clarke is a staff writer for Adventure.com. She is an author, mountain enthusiast and yoga teacher who loves heading uphill on foot, ski, bike and belay. She recently returned to her hometown of Glasgow, Scotland after 20 years living in the USA, 11 of which were spent in the rocky mountains of Vail, Colorado where she owned a boutique yoga studio and explored the west's famous peaks and rivers. She is a champion for enjoying the outdoors sustainably as well as maintaining balance through rest and meditation, which she explores in her book Restorative Yoga for Beginners, a beginner's path to healing with deep relaxation. She enjoys writing about the outdoors, yoga, wellness and travel. In her previous lives, she has also been a radio presenter, music promoter, university teacher and winemaker.