The number of cubic inches one ounce of down insulation occupies; also, a measurement of the down’s loft (fluffiness). The higher the fill power, the warmer it is. Choose the type of fill and use the fill power as a guideline to how warm or lofty you like your bedding. Lightweight warmth has a 500-550 fill power. 550-600 fill power offers medium warmth and 600+ fill power is very warm.