Down fill power

Fill power is commonly used in product descriptions for down sleeping bags, quilts, and jackets. Under standard testing, a fixed mass of down is placed in a cylinder and allowed to loft under a defined weight. The resulting volume is recorded as the fill power. Higher numbers indicate that the down clusters are larger and can trap more air per unit weight.

From a practical standpoint, higher fill power down allows manufacturers to achieve a target temperature rating with less down mass, potentially reducing the overall weight and bulk of the item. However, total warmth is still influenced by how much down is used overall, the construction of baffles, and the design of the garment or bag.

Fill power is not a direct measure of quality in all respects. Durability, consistency, and how the down behaves in real-world conditions also matter. For many hikers, there is a balance between cost, performance, and the marginal weight savings associated with higher fill powers.

In gear comparison, fill power is best considered alongside other specifications, such as fill weight, cut, fabric type, and intended temperature range.