Fill power is a number that indicates how much space is occupied by one ounce of down. Higher quality down has a higher fill power; therefore, a higher number indicates a thicker, warmer duvet. For example:
400:Lightweight for warm weather
400-600: Good range for year-round use
600-800: Extra warmth in cold weather
800 and above: Maximum warming power
Fill weight indicates the ounces of down inside the duvet. A higher fill weight equals a heavier duvet.