Fill power is the measurement for the amount of space one ounce of down takes up. It is the best quality indicator for down, so you can typically find fill power listed on higher end comforters. It’s important to know that down fill power does not tell us how warm a comforter will be because there are too many other factors affecting warmth, including how much down fill is in the comforter. Formulas below help us better understand the relationship between fill power and the weight of down fill.
For a given weight of down:
As is well known, down is warm but doesn’t do well in wet and humid conditions. Synthetic fills are designed to replicate the qualities of down, but retain them even when wet. Synthetic fills have no equivalent test to down fill power, and different synthetic fills have different characteristics. So it’s impossible to measure any synthetic' insulation efficiency using down fill power.
Although fill power is an important factor you need to take into consideration when purchasing a comforter, there are varied sides you also need to refer to. Its size, weight, thread count, construction, design and so on. All these make a difference on your way to a great night's sleep.