Insulation
Fill power is volume per gram, not warmth. Warmth is fill power times fill weight — and a high number on a light fill can be the colder jacket.
— cm
Loft volume spread over the insulated area
Fill power is measured on conditioned down in a free cylinder; a real garment restricts it, which is what the loft factor carries. Loft that is compressed — at the shoulders under a pack strap, or anywhere the baffle is too shallow — insulates in proportion to its compressed thickness, not its nominal one.
Down Fill Power to Loft & Thermal Insulation Modeler — free, with the formula and a worked example, at Textile School.