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What is down fill power and why does it matter?

Down fill power

The insulation of a down jacket is tough to beat. But what exactly do the numbers on the sleeves mean and how do you know which down jacket is best for you?

There's a huge array of down jackets on the market. And while they all offer you exceptional warmth, there are differences in the type and quality of down in each one.

Down with Daan

What is fill power?

Fill power determines the loft or ‘fluffiness’ of down. Higher fill power reflects better quality down that’s capable of trapping more air, and providing a better warmth to weight ratio.

You can measure the quality of down by taking note of the fill power – usually shown as a number on the sleeve of your jacket.

When you understand the quality and benefits of a particular down jacket, you can make an informed decision on the best jacket for your activity.

Sleeves 3
Which one is right for you?

Understanding fill power numbers

Kathmandu’s down jackets range in fill power from 550 to 800. But what does that really mean?

The fill power number on your jacket comes from a standardised lab test and indicates how many cubic inches one ounce of down occupies with a standard weight resting upon it.

A higher number means the down takes up more space and is capable of trapping more air.

As an example, Epiq Down Jackets are filled with 550 fill power down – so in the above test, one ounce of this down takes up 550 cubic inches of space.

Benefits of a higher fill power number

If you choose a down jacket with a higher fill power number, your jacket will be filled with larger, generally stronger down clusters which are able to trap more air and give you more warmth for its weight.

It'll also be lighter than a lower fill power down jacket that has similar warmth, fabrics and design features.

That said, a down jacket with a higher fill power number doesn’t always mean it will be warmer than one with a lower fill power. The respective weights of the down filled into them need to be considered.

For example, the Heli Lightweight Down Jackets with 550 fill power and the XT Ultralight Down Jackets with 800 fill power have similar warmth, although the XT Ultralight is lighter, more compressible and filled with higher quality clusters of down.

Comparing fill power in down jackets

Let’s assume fabrics and design features are the same for these examples.

If Jacket A is filled with 227 grams of 800 fill power and Jacket B is filled with 227 grams of 700 fill power, Jacket A would be warmer.

Why? Because down with a higher fill power will trap more air and create a thicker layer between your body and the environment.

Down Different Qualities
Different fill power, same weight.

But, if you had 113 grams of 800 fill power in Jacket C and 227 grams of 650 fill power in Jacket D, they could have similar warmth.

However, Jacket C would be lighter and more compressible because it contains less down of a higher quality.

Down Same Volume
Different fill power, different weight.

Busting a few urban myths

You’ll want to be aware of the urban myths still hanging around the traps, such as:

  • A higher fill power number is always warmer – this isn’t necessarily true as the warmth of a jacket also depends on the weight of the down filled into it (as Jacket D in the above example shows).

  • Goose down is better than duck down – a high quality duck down can be better than an average goose down.

  • Down jackets are filled with feathers – they’re actually filled with down clusters that grow under the feathers of ducks and geese.

It’s important to know that all down jackets will have some minimal plumage. A few feathers are valuable for giving your down jacket a little bit of robustness and structure.

Equally important is that all down should be certified by the Responsible Down Standard.

Explore our range of down jackets