What Is Down Fill Power? The Complete Guide
RevolutionRace
1min
Updated: 31 October 2026
Published 31 October 2026
Higher fill power means a warmer down jacket, right? Not necessarily.
Down fill power measures the quality and efficiency of down insulation, not the total warmth of a jacket. How warm you’ll actually be also depends on how much down the jacket contains and how it is constructed.
Here’s what the numbers mean – and how to use them when choosing a down jacket.
What Does Down Fill Power Mean?
Down keeps you warm by trapping air. Fill power measures how much the down expands, or lofts, and therefore how efficiently it can trap insulating air.
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Simply put:
| Fill power | Generally best suited for |
|---|---|
| 400–550 | Everyday use where weight and packability matter less |
| 550–700 | Everyday outdoor use and hiking |
| 700–800 | Activities where lower weight and packability matter |
| 800–900+ | Specialist use where maximum efficiency is a priority |
Keep in mind: these ranges are a guide - not a warmth rating.
Fill Power vs. Fill Weight
This is the most important distinction to understand.
Fill power tells you how efficient the down is.
Fill weight tells you how much down is inside the jacket.
That means a 600-fill-power jacket can be warmer than an 800-fill-power jacket if it contains more down.
A useful way to think about it:
Lower fill power + more down: warm, but generally heavier and bulkier.
Higher fill power + less down: lighter and more packable, but not necessarily warmer.
Higher fill power + more down: high warmth with excellent weight efficiency.
Construction, fit and shell material also influence how warm the finished jacket feels.
What Is a Good Fill Power for a Down Jacket?
There’s no single best fill power. Choose based on how and where you’ll use your jacket.
| Activity | Fill power to consider |
|---|---|
| Everyday wear and dog walking | 550–650 |
| Hiking and active outdoor use | 650–750 |
| Alpine and cold-weather activities | 750–850 |
| Specialist ultralight or extreme cold use | 850+ |
Is 600 fill power enough? For many everyday and outdoor activities, yes. The amount of down matters just as much.
What about 800 vs. 900 fill power? 900-fill-power down is more efficient, meaning it can provide insulation with less weight and bulk. It isn’t automatically warmer.
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What About Synthetic Insulation?
Fill power only applies to natural down. Synthetic insulation uses man-made fibres and is measured differently, so the two cannot be directly compared by fill power.
Down is particularly good at providing warmth with low weight and bulk. Synthetic insulation generally handles moisture better, making it a practical choice for wet or changeable conditions.
Goose vs. Duck Down – Does It Matter?
Goose down can reach very high fill powers, but it isn’t automatically better than duck down. At the same fill power, the quality and loft of the down matter more than the bird species alone.
You may also see a ratio such as 90/10, meaning the filling consists of 90% down and 10% feathers by weight. This is separate from fill power: the ratio tells you what the filling contains, while fill power tells you how efficiently the down lofts.
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Does Down Fill Power Decrease Over Time?
Down needs to stay lofty to insulate effectively. Dirt, moisture, repeated compression and improper washing or storage can cause it to clump or stay compressed, reducing its ability to trap warm air.
Proper care helps maintain that loft. Follow the care instructions, use a detergent suitable for down, dry the jacket thoroughly and avoid storing it tightly compressed for long periods.
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How to Choose the Right Fill Power
Instead of choosing the highest number, ask yourself:
How cold will it be? More insulation is important in colder conditions.
How active will I be? High-output activities often call for lighter insulation.
Do weight and packability matter? This is where higher fill power really earns its value.
Will it be wet? Synthetic insulation may be more practical in consistently damp conditions.
The Key Takeaway
Fill power tells you how efficiently down insulates for its weight – not how warm a jacket is on its own.
Look at fill power together with fill weight, construction, conditions and intended activity to find the right balance of warmth, weight and packability.