Down quality is measured by its fill-power, which in simple terms is a measure of the down cluster’s size. Larger clusters trap more air and are both warmer and lighter compared to lower quality down. The larger the down cluster, the higher the fill-power rating.
Fill-power represents how many cubic inches one ounce of down will occupy. For example, if one ounce of down takes up 700 cubic inches of space, it is rated as 700 fill-power down. This means that less down will be needed to achieve the same warmth as a 600 fill-power or lower down.
There are 3 different grades of down often used: 550 to 650 fill-power (high quality downs), 650 to 750 fill-power (performance downs), and 750 to 900 fill-power (premium downs—the lightest, warmest, and most expensive).