Fill Power or “loft” is a volumetric measurement which is used to determine the approximate insulation value of down and feather fillings. It measures the volume that 30 grams of material will occupy under a specified weight. If the product is labeled 650 fill power, that means 30 grams of material will fill 650 cubic inches. A 900 fill power product is better than a 650 fill power product because more loft is provided at the same weight making a lighter, warmer product.
Each labeling standard/region uses a different measurement system. The most common system used internationally is in cubic inches. The higher the number the better the insulation value.