The same experiment is used over and over to determine the fill power of a duvet. A cylinder is filled with 30 grams of duvet filling. This can consist of feathers and down, or pure down. That 30 grams of filling is subjected to a fixed amount of pressure. The amount of pressure is always the same, otherwise the fill power wouldn’t be a representative number. The result is expressed in millimeters per 30 grams, and the greater the amount of millimeters, the more air is trapped between the down particles. ​

The combination of air and down clusters creates an insulating layer that retains body heat. When the fill power has a higher number, it means the down clusters can trap more air, which makes the duvet of higher quality .

In their natural state, down fluffs are fluffy because they are surrounded by air to which they can cling. If the fluffs are still fluffy in the cylinder, it means they are good at holding onto air and they can form a better insulation layer. A good duvet is all about the ability of the down fluffs to create heat-retaining air spaces in the duvet, which retain body heat during the night.

Another amazing feature of down is that it will become fluffy, or lofty, again once the pressure is removed. The higher the quality of the down, the greater its ability to fluff back up to its original shape. The ability to compress may not be of great importance in the world of duvets, unless you like to travel with your very own duvet, but it is incredibly important in the sports and outdoor world, as it makes for some very warm but lightweight gear, such as for example our​ sleeping bags! 

So, a 900 fill power duvet, that has 400 grams of down will be just as warm as a 450 fill power duvet with 800 grams of down. The difference is that the latter will be much heavier which can be uncomfortable as you may wake up sweating.