Fill power is a measurement that indicates how much volume down takes up and how well it springs back after pressure. In practice, it says something many people recognise immediately: a duvet that feels airy and provides warmth without unnecessary weight.
How is fill power measured?
To determine fill power, a test is used with a cylinder filled with 30 grams of down. A fixed pressure is then applied, and the remaining volume of the filling is measured. The result is expressed in millimetres per 30 grams.
Please note: fill power can also be expressed in other units (such as cuin), and the outcome depends on the standard and test method used. That’s why it’s best to compare fill power only within the same standard.
- The higher the number, the more air remains trapped between the down clusters.
- And that air is exactly what provides insulation.
Why air makes the difference
Down doesn’t keep you warm “by itself,” but by creating an insulating layer of air. Down clusters hold air within their fine structure. That held air retains body warmth, while the duvet can remain light.
When fill power is higher, it generally means that the down clusters:
- are less flattened under pressure
- can hold more air
- therefore insulate more efficiently per gram of fill
Resilience: what happens when the pressure is removed?
A key feature of down is that it “opens up” again and regains volume once the pressure is removed. The better the quality, the stronger this recovery. That’s also why down is so often used in lightweight applications such as sleeping bags: warmth with minimal weight.
Fill power matters, but it’s never the whole story
A higher fill power is often a quality indicator, but on its own it doesn’t say everything about your duvet. Comfort is also shaped by:
- fill weight (how much down is inside)
- composition (pure down or a blend with feathers)
- the casing (a fabric that breathes—or restricts)
- finishing and distribution (how evenly the down stays in place)
- your personal warmth needs and bedroom climate
That’s why a higher fill power with less fill can be just as warm as a lower fill power with more fill. But the feel can differ greatly: light and supple versus heavier and fuller.