Fill Power Explained: What those numbers mean for your jacket

600. 700. 800. Here's what they're really telling you and what they're not. 

You've found a down jacket you like. Then you hit the spec sheet. Fill power: 600. Fill weight: 227g. RDS certified. Pertex™ Quantum outer. 

And suddenly buying a jacket feels like sitting a science exam. 

Here's the thing, fill power is a simple idea dressed up in technical language. Once you understand what the number means and what it doesn't mean, choosing the right jacket gets a lot easier. Let's break it down. 

What Is Fill Power? 

Fill power measures the quality and loft of down, essentially how fluffy it is. 

The number comes from a standardised lab test. One ounce of down is placed in a cylinder with a small weight resting on top. The number tells you how many cubic inches that ounce of down occupies. So, 600 fill power means one ounce of down takes up 600 cubic inches of space. 

The fluffier the down, the more air it traps. The more air it traps, the warmer it keeps you. That's it. That's the whole idea. 

Higher fill power = better quality down = more warmth for less weight. 

600 vs 700 vs 800 Fill Power: What's the difference? 

Here's the scale most everyday jackets sit on: 

400 - 500 fill power: Entry level. More down needed to achieve warmth. Heavier and bulkier as a result. Fine for mild conditions but not where you want to be for a serious NZ winter. 

550 - 650 fill power: The everyday sweet spot. This is where the Kathmandu Epiq sits. 600-fill RDS-certified down  that delivers serious warmth without unnecessary bulk. The right balance of performance, weight and value for commutes, weekends and everyday adventures. 

700 - 800 fill power: Higher quality, lighter weight, more compressible. The down clusters are larger and trap air more efficiently. Great for technical outdoor pursuits where weight and packability really matter. 

850 - 900 fill power: Premium expedition territory. Exceptionally lightweight and packable. Used in performance gear where every gram counts.  

The important thing to remember: a higher number doesn't automatically mean a warmer jacket. And that brings us to the part most people miss entirely. 

Fill Power vs Fill Weight: The bit that actually matters

This is where it gets interesting and where most people get confused. 

Fill power tells you the quality of the down. Fill weight tells you how much of it is in the jacket. You need both numbers to understand how warm a jacket will actually be. 

Think of it this way. Say you have two jackets: 

Jacket A: 227g of 800 fill power down  

Jacket B: 227g of 700 fill power down 

Same amount of down. Jacket A is warmer because the quality is higher. 

Now try this: 

Jacket C: 113g of 800 fill power down  

Jacket D: 227g of 600 fill power down 

Different fill powers, different amounts, but similar warmth. Jacket C would be lighter and more packable. Jacket D would be slightly bulkier but could feel equally warm because there's simply more down in it. 

In short: fill power rates quality. Fill weight signals quantity. You need both to get the full picture. 

So, is 600 fill power good? 

For everyday use, absolutely yes. 

600 fill power is where serious warmth meets practical wearability. The Kathmandu Epiq uses 600-fill RDS-certified duck down because it delivers real, reliable warmth for real, everyday conditions. Cold commutes. Weekend tramps. NZ winters that can't make up their mind. 

Paired with a Pertex™ Quantum outer fabric, which works with the down to maximise loft and trap heat efficiently, the result is a jacket that feels lighter than it looks and warmer than you'd expect. 

The numbers back it up. The warmth proves it. 

Other things that affect warmth 

Fill power and fill weight are the biggest factors but not the only ones. A few other things worth knowing: 

Hood or no hood. A hooded down jacket is measurably warmer because your head and neck are two of the biggest heat-loss points on your body. Read our full hooded vs non-hooded guide here.

Jacket length. Longer jackets cover more of your body and retain more heat. Parka styles will always feel warmer than cropped cuts at the same fill power. 

Outer fabric. A lightweight technical outer like Pertex™ Quantum helps the down fully loft inside the jacket, maximising the fill power you're paying for. 

Baffle construction. How the down is distributed inside the jacket matters. Well-designed baffles prevent cold spots and keep the down where it's needed most. 

The quick answer if you're shopping right now 

If you're looking for an everyday down jacket that handles NZ winter commutes, weekends, cold snaps, 600 fill power with a good fill weight is exactly where you want to be. It's the balance of warmth, weight, packability and value that most people actually need. 

Go higher if weight and packability are critical long tramps, travel, technical outdoor use. 

Stay at 600 if you want reliable, everyday warmth without paying for performance you won't use. 

The numbers make sense now. The warmth is waiting. 

Shop the full Epiq down jacket range at Kathmandu.co.nz