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FILL POWER is a measure of the LOFT or expansion ability of down insulation. It denotes the volume (in cubic inches) to which 30 grams of down will expand when it takes in air. The greater the FILL POWER, the greater the expansion, the warmer the sleeping bag. (All other things being equal) TYPICAL PERFORMANCE FIGURES (FILL POWER) FOR DOWN USED IN SLEEPING BAGS cu in duck down 550 Chinese goose down 600 Hungarian goose down 660 Polish Standard goose down (grey) 680+ Polish Super goose down (white) 800+ Cumulus Polish goose down has been tested and certified by the International Down & Feather Testing Laboratory, Salt Lake City, Utah, USA to a MINIMUM rating of 680 cu in and 800 cu in respectively. Our newest 'diamond grade' white down scored 870 cu in, after steam conditioning.