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Feather Down Rating Explained

in this article we attempt to explain exactly what feather down is and how it is used as thermal insulation, and what the crucial and often overlooked difference between Fill Power and Fill Weight is, how to make sense of warmth-to-weight and down-to-feather ratios, and how to appreciate how warm your jacket is likely to be.


What is Feather Down and Why is it Used?

Down comes from the soft plumage of a bird, it's the fluffy undercoat that lies beneath the tougher feathers. The most common types of down come from ducks and geese. In either case, the more mature the bird is, the larger its plumage tends to be. This results in a greater loft and a higher Down Fill Power Rating. We'll define loft and fill power in a minute. But first, why do we use down at all?

The purpose of insulation is to trap the heat from your body and to prevent it from escaping into the cold. It's a principle of thermodynamics that heat will always move from a hotter region to a colder region to achieve equilibrium. In other words, the heat your body produces is always trying to escape out into the air around you. Insulation acts as the barrier between your body and the outside and helps retain the heat.


During production, down gets formed into clusters which are then used for items such as jackets, pillows, quilts and sleeping bags. Each cluster is made of thousands of tiny fibres criss-crossing in every direction creating air pockets between them. Just like a sponge traps water, these down clusters trap air in a way that heats it up while allowing the material itself to breathe. Down fill has a phenomenal "warmth to weight" ratio and it resists damage incurred from compression (think: stuffing it into your backpack). This is why down is widely considered to be the best insulator known to man for jackets or sleeping bags etc. Down jackets keep you comfortably warm without weighing you down.  You simply need less of it to stay warm.  It's not all about a thick puffer style blimp result!

There are three things to consider when purchasing a down insulated product: Fill Power, Down Weight, and Down Ratio.

Fill Power (CUIN)

Fill Power is a measure of volume or the loft of a Down product, which is essentially the quality of the down. Maximum loft occurs when the down clusters are fully expanded. The Fill Power rating value is calculated by measuring how many cubic inches (CUIN) an ounce of down creates at its maximum loft. A Down Fill rating of 600, for example, means that one ounce of down can cover 600 cubic inches. The process of establishing the Down Fill ratio involves putting the down into a glass cylinder and slightly compressing it with a weight.  This is then the "measure" used for the CUIN rating.

It's important to note that fill power is measured differently in the United States than it is in Europe due to the different types of cylinders that are used. In the U.S., the cylinder has a diameter of 241 millimetres and weights around 68.3 grams. In Europe, the cylinder is larger at 289 millimetres and weighs more at 94.25 grams.

The fill power rating can range from 300 to 900 and above. The most common down products have a rating of 400 to 500. These are considered to be relatively low quality, however, as they come from immature geese and ducks and therefore are made from smaller down clusters which can bunch over time and be less effective.

When purchasing a high quality down fill jacket, you should look for a fill power rating of at least 550. A jacket of this calibre will be both warmer and more comfortable.

Fill Weight

While fill power is a measure of volume, fill weight is a measure of weight. Fill Power effectively measures the quality and fill weight indicates the quantity. If a jacket has a down weight of 12 ounces, that means that 12 ounces of down was used to make it. While many outerwear brands proudly draw attention to their high fill power, they often fail to mention the fill weight. This is extremely important, however, as down weight also plays an equal part in determining the product's performanece and shouldn't be overlooked by consumers.

The fill weight is what determines how soft or firm a product is. It is also what determines how well the product compresses. Like we mentioned above, a great feature of down is its ability to be stuffed into a backpack without taking on damage. The heavier a down jacket is, the less it will be able to compress.

Down to Feather Ratio?

The down to feather ratio calculates the percentage of down to the percentage of feathers in the product. The numbers are generally 70/30, 80/20, or 90/10. The first number represents the percentage of down, and the second number represents the percentage of feathers. The feather content allows the manufacturer to bulk out their down products with a less expensive option but still qualifying as a "down" product.  Generally speaking, a higher first number means a higher quality (and more expensive) product. Whilst down costs more, it is a much better insulator than feathers and thus is preferred in good jackets. A 600 fill power down jacket with an 80/20 down to feather ratio will be warmer than a 600 fill power jacket with a 70/30 ratio.

Why is it that down makes a better insulator than feathers? Down is sourced from underneath the outer feathers. Because the feathers are on the outside, they are firmer and have quills that act almost as a stem. Getting poked by one of these hurts! Feathers are ideal for harder and more durable products like seat cushions, but down is better for providing warmth and comfort in jackets, sleeping bags and duvets and you tend not to get pricked by a feather quill!

Assessing Your Jacket's Warmth

When purchasing a down jacket, should you pay more attention to the fill power or to the down weight? If you're looking for a jacket to keep you warm, you need to consider both! Here are some examples to help illustrate how fill power and down weight work together:

  • A jacket with 500 fill power and 10 oz. of down will be warmer than a 800 fill power jacket with 5 oz. of down.
  • With the down weight and down to feather ratio being equal, an 800 fill power jacket will be warmer than a 500 fill power jacket.
  • An 800 fill power jacket will require less down than a 500 fill power jacket to provide the same warmth.

It's not easy we accept but it's easy to see why many brands put so much emphasis on their fill power. An 800 fill power jacket will weigh less than the 500 fill power jacket while providing the same level of warmth. This is why higher fill power jackets are used by performance athletes such as hikers, climbers, and skiers who seek lightweight jackets that provide mobility, performance and warmth when active or passive.

Trying to make sense of the relationship among fill power, down weight, and the down to feather ratio can be a little confusing. To make matters easier, many companies use the Clo value. Invented in 1941, Clo measures the insulation value of clothing needed to maintain comfort for a person sitting at rest in room temperature with humidity at less than 50%. That's just a fancy way of saying that Clo measures how warm an article of clothing is in a given situation to create a "measure".

The higher the Clo value, the warmer the product is. For example, a jacket with...

    ...550 fill power with 1 oz. of down has a Clo of approximately 0.7,

    ...625 fill power with 1 oz. of down has a Clo of approximately 0.92,

    ...800 fill power with 1 oz. of down has a Clo of approximately 1.68.

For reference, the naked body has a Clo value of zero, and a Clo value of 1 is what a person needs at room temperature. According to Adventure Poet, winter temperatures from -4º to -40º F require a person to wear clothing totaling a Clo value of 4 or greater.

Hopefully this guide made it easier for you to make sense of all the numbers you see when purchasing a down jacket. Remember, it's important to pay attention to all of the aspects that go into making a down product, especially the fill power, down weight, and down to feather ratio. If things get a little complicated, you can feel confident in simply looking for a higher Clo value. And lastly, the way the down is distributed through the jacket, either sewn through or in box baffles (tubes), plays a part in whether the warmth of the jacket will be consistent.

It's important to be safe while encountering the cold. Now you'll be able to informatively choose a high-quality down jacket to protect you from the elements and keep you comfortable longer. Stay warm, and keep adventuring.