How Warm is Your Jacket, Really? The Down Equation

Mother nature knows what she’s doing, at least if down fill is any indication. Decades after it was first borrowed from birds to keep us warm, down—with only a handful of improvements—is still one of the premier technologies for keeping us warm. The problem? Many of us don’t understand it or how it works, which means when it comes time to buy a jacket, we’re left in the dark. The tag on that jacket has a lot of numbers on it, many of which tie into how warm, compressible, and lightweight it will be while you’re wearing it or stuffing it in your pack. You just need to know how to make sense of them.

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A High Point in Warmth

Synthetic insulation seems to get better every year, but down still reigns supreme for many users, environments, and activities. Lightweight, packable, and warm are just a few of the characteristics that have kept down as the go-to insulation of outdoor adventurers for almost a century. On the 1922 Everest expedition, mountaineer George Finch drew many suspect looks from his team members for his departure from traditional tweed and wool outerwear in favor of an eiderdown-lined coat and pants with a shell made of a bright green hot-air balloon fabric. Finch would go on to climb to a height of 8,360 meters—an altitude record at the time—and set the stage for down to become the favored insulation of those who work and play in the cold.

Put simply, down is the fluffy group of soft feathers that sit between a bird’s (usually a goose or duck) outer feathers and skin. Under a microscope, the plumes are a network of tiny filaments, woven and and networked together to trap air between then—This is why down is so warm. Microscopic pockets between filaments trap body heat next to your skin.

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Not All Down is Created Equal

No down is the same, which is why we need a system to explain how warm one particular jacket is over another. We use two big metrics to sort this out: the quality of fill used, which is measured by down fill power, and the amount of insulation used, which is measured by fill weight.

Down Fill Power

Just like so many other materials, some down is a higher quality than other down. Generally speaking, the “fluffier” a plume of down is—meaning, how much loft it has—the warmer it will be, and the higher quality it will be. This is fill power.

Fill power is calculated by a lab test that determines how many cubic inches of loft a certain weight (one ounce) of down produces. The higher the number, the more loft and the warmer than down will be for its weight. For example, a down fill rating of 800 means that one ounce of down covers 800 cubic inches. In general, jackets ranging from 600- to 1,000-fill power are considered high quality.

Down Fill Weight

But remember one key piece about fill power: It measures how warm down is for its weight. If the weight of the down feathers in a jacket is equal, a jacket with higher fill power will be warmer than one with a lower fill power. But not all jackets have the same weight, which is why we also need to consider that piece of the puzzle.

Down fill weight indicates how much insulation is used in its construction. Sometimes listed in grams, other times in ounces, though often not at all (more on this ahead), the fill weight is simply the weight of the down that was used to make a jacket.

Consider the EMS Featherpack Hooded Down Jacket and the EMS Ryker Down Parka. The Featherpack Jacket is made with roughly 140 grams of 800-fill down, while the Ryker Parka is made with 300 grams of 650-fill down. Because it has more than twice the amount (or weight) of down inside, the Ryker looks much larger and will likely be warmer, even though the Featherpack uses much higher quality down. On the other hand, the Featherpack is far lighter and will compress much smaller thanks to its 800-fill down, making it a better option for weight- or pack space-conscious users. It’s easy to get lured into thinking that a jacket made with higher fill power is warmer than a jacket with a lower fill power, but that’s not always the case.

Long story short, how warm your jacket will be is a function of both fill power and down weight. A jacket with a high fill power but low down weight may not be as warm as a jacket with a high down weight and low fill power. But a high combination of both means a super warm and super light and compressible jacket.

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Putting it All Together

A challenge in choosing a down coat is to balance your need for a high fill power (lightweight and compressibility) with fill weight (warmth). This is often complicated because many manufacturers are quick to highlight fill power, but are less likely to divulge fill weight. If the fill weight of a jacket you’re interested in isn’t available, your next best option is to find one in person and compare its weight and compressibility against other jackets with known fills. If that isn’t an option either, the best you can do is look at the jacket’s total weight, account for features like pockets and zippers, along with the shell material, and then guess at the fill weight.

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Choosing a Down Jacket

Putting all this together, down jackets with high fill powers are a great choice for hikers, backcountry skiers, and ice and alpine climbers who want a puffy that provides maximum warmth while taking up a minimum of space and not weighing too much. They’re also great for those looking for a super-warm jacket that maintains a sleek cut. Conversely, those less interested in packability and compressibility—like, say, a sport climber with a short approach—might look to jackets that use down with slightly less fill power, but use more of it so that they can stay warm as they belay somebody on their project.