What is down fill power and why does it matter?

When you need to choose a down jacket to keep your core body insulated during winter, how do you decide which one? And what exactly do the sleeve numbers mean?


Understanding down

Down is the light, fluffy undercoating that’s clustered beneath the feathers of waterfowl. It helps protect them from the cold.

Many of Kathmandu’s insulation jackets have down fill. The down:

  • Provides an exceptional amount of warmth for minimal weight.
  • Offers a comfortable feeling.
  • Compresses easily.

Down jackets are worn for warmth and to protect you against the wind. They aren’t waterproof but can be water repellent to some degree.

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.

What exactly 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
  • Providing a better warmth to weight ratio

Fill power numbers help you decide which down jacket to choose for your outdoor plans.

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, our 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.

Which one is right for you? Which one is right for you?
Which one is right for you?

Benefits of wearing a down jacket with a higher fill power number

If the down in your jacket has a higher fill power number, the:

  • Larger the down clusters will be – which are generally stronger and able to loft more fully
  • Warmer it will be for its weight

Your jacket will also be lighter than one with a lower fill power number that has similar warmth, fabrics and design features.

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.

Different fill power, same weight Different fill power, same weight
Different fill power, same weight

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.

Different fill power, different weight Different fill power, different weight
Different fill power, different weight

Higher fill power doesn’t necessarily mean warmer

A higher fill power number doesn’t automatically mean a jacket will be warmer.

For example, our Heli Lightweight Down Jackets with 550 fill power and our XT Ultralight Down Jackets with 800 fill power have similar warmth.

The advantage of choosing the XT Ultralight is that it has less fill because it uses a higher fill power, so it’s:

  • Lighter
  • More compressible

Busting a few urban myths

There are still a few urban myths around the down traps. Some that we can categorically dismiss are that:

  • 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 always 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.

Man walking on Great Wall of China wearing down
When temperatures drop, you can trust down to keep you warm

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.

Finding the best down jacket for you

Base your choice of down jacket on the main activity you want to use it for. How warm and compressible do you want it to be?

Consider whether you need a jacket filled with water repellent down – one which will take longer to absorb:

  • Your moisture as you perspire
  • Water when you get caught in light rain.

Also take into account the construction and fit of the jacket you want to buy and how you want to layer it – as a midlayer under your rain shell or as an outer layer over your thermals.

At Kathmandu, we label each of our down jackets on a ‘warm’, ‘warmer’ or ‘warmest’ scale along with weight and packability levels – so you can make a well-informed choice on which down jacket is best for you.

Explore our range of down jackets

  1. Member Offer
    Heli Men's Lightweight Down Vest
    Heli Men's Lightweight Down Vest
    $199.98
    Members Price $99.98
  2. Selected Clearance
    Epiq Women's Down Vest
    Epiq Women's Down Vest
    $135.00 - $299.98
  3. Federate Men’s Down Bomber Jacket
    Federate Men’s Down Bomber Jacket
    $449.98
    Members Price $359.98
  4. Federate Women’s Down Bomber Jacket
    Federate Women’s Down Bomber Jacket
    $449.98
    Members Price $359.98
  5. Federate Women’s Stretch Down Hooded Jacket
    Federate Women’s Stretch Down Hooded Jacket
    $499.98
    Members Price $399.98
  6. Federate Men’s Stretch Down Hooded Jacket
    Federate Men’s Stretch Down Hooded Jacket
    $499.98
    Members Price $399.98
  7. L-TRA Women’s Hybrid Down Jacket
    L-TRA Women’s Hybrid Down Jacket
    $349.98
    Members Price $279.98
  8. L-TRA Men’s Hybrid Down Jacket
    L-TRA Men’s Hybrid Down Jacket
    $349.98
    Members Price $279.98