Fill power is a measurement that shows how much volume down takes under pressure. In practice it says something many people recognise at once: a duvet that feels airy and gives warmth without needless 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 volume the filling keeps is measured. The result is given in millimetres per 30 grams. Fill power can also be expressed in other units, like cuin. So compare best within the same standard.
Why air makes the difference
Down does not warm by itself, but by creating an insulation layer of air. Down clusters trap air between their branches. That trapped air holds body warmth, while the duvet stays light.
Spring back
An important feature of down is that it opens again after pressure and regains volume. The better the quality, the stronger that spring back.
Never the whole story
A higher fill power is often a quality indicator, but does not say everything on its own. Filling weight, composition, casing, finishing, distribution, your warmth feel and your bedroom climate together decide comfort. So a higher fill power with less filling can be as warm as a lower fill power with more filling, but the feel can differ strongly.
Why comparing must be careful
Fill power is useful, but only within the same measuring method. A figure without context can mislead. It says something about air and spring back, not about the full feel of the duvet.
The whole decides
Filling weight, casing, finishing and your warmth feel together decide whether it is right. So the choice does not end at the number. Take the duvet in your hand, feel the weight and suppleness. What counts is how it lies when you sleep beneath it.