Down Fill Power

Fill power measures how much space an ounce of down lofts to fill — higher numbers (e.g., 800+) mean more warmth per unit weight, so a jacket can be lighter and more packable for the same insulation. It is a quality-of-down measure, not a total-warmth measure: a low-fill-power jacket stuffed with lots of down can be warm but heavy and bulky. Fill weight (how much down) matters alongside fill power. Watch also for down-versus-synthetic and the fill ratio. On a spec sheet, fill power plus fill weight together predict warmth-to-weight far better than either alone.