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Fill power is a unit of measurement for loft, which is the fluffiness of down in insulation. You may encounter the term fill power, which can be understood as "filling strength," when purchasing a winter jacket, sleeping bag, or pillow. It is a parameter that indicates the quality of thermal insulation of down products, designed to protect against adverse environmental conditions.

This parameter measures the volume occupied by one ounce of down (approximately 28 grams in the metric system). The fill power value is given in cubic inches (in³) and ranges from about 400 to 900 in³ (6554 to 14,748 cm³). The higher the fill power value, the higher the loft of the down, and thus the better the quality. The down is simply fluffier and occupies a larger volume. It can trap more air and thus better protect against low temperatures. Additionally, the higher the fill power value, the lighter the comparable size product can be.

The fill power value of down used for a specific product is measured in laboratory conditions. A predetermined amount of down is placed in a cylinder and then compressed with a standardized weight. The space in cubic inches occupied by one ounce of down is calculated, and this number is reported as the fill power value.

Fill power is a key factor in assessing the thermal insulation properties of a down product. Higher fill power means the down has a better ability to trap air, which improves insulation. This means that less material is needed to achieve the same level of thermal comfort, resulting in a lighter and more compact product.

For example, if we have two identical jackets, and one has a fill power of 500 and the other 800, the second jacket will be much lighter than the first and will also provide better insulation to the wearer (you'll simply be warmer in it at the same outdoor temperature). It is worth paying attention to the fill power, especially for outdoor clothing and equipment intended for use in extreme conditions, including challenging expeditions.

For professional expedition jackets that you won't hesitate to wear in Antarctica or Siberia, the fill power should be 800 or higher. The corresponding price tag will be accordingly high, which may not be completely accessible for the average person. If you are looking for a down jacket for everyday use in a Central European winter, a fill power value of approximately 500 to 600 may suffice, or even lower.

However, fill power is not the only factor affecting the quality of a down-insulated product. It also depends on the thickness of the insulation used and even the type of down used. The highest quality goose down can naturally be more expensive due to its processing difficulty and lower availability. The construction of the product itself is also important and should not have thermal bridges.