Fill power is the measurement in which an ounce of down will loft and expand to fill an empty space. Down warmth is a function of both fill power and the amount of fill in a comforter.

Down quality is rated by cubic inches of fill power during a special laboratory test. A good range for a down comforter is 475 or greater cubic inches. Fill power of 600 and higher is considered excellent.

Higher quality down has higher fill power and will loft higher, requiring fewer ounces of down to create insulating warmth. So, all other things being equal, a 650 fill power comforter will have fewer ounces of fill than a comforter of equal warmth filled with 525 fill power down.

The higher the fill power, the better the down will insulate, and there is less chance for "cold spots"- areas in a comforter where there is no down.

If a comforter label does not specify fill power, it is usually because the down falls in a range below 400 cubic inches, or the comforter contains less than 75% down, which is why feather beds will not have a Fill Power rating.

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