Brad is correct but what I have found is that with the lower fill power down you are getting more feathers. I think the down lasts longer than the feathers. Somewhere around 600-650 and lower you are getting a feather mix and less loft. Initial warmth as per Brads equation will be the same. With your numbers you would have 3,000 and 3,360 ci. respectively so not equal. I have been happy with 6-650 fill items but 500 or less is better for pillows or duvets where the less compaction is an advantage.

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