Fill power (or loftiness) refers to the number of cubic inches that one ounce of down will displace. The higher the down's fill power, the less down is needed to achieve a given temperature rating. For example, a 10 degree sleeping bag using 800-fill-power will weigh less and use less down than a 10 degree sleeping bag that uses 600-fill-power. Higher fill power also corresponds with higher compressibility because less down is required to fill space and achieve a certain temperature rating. Less down means that there is more air in the sleeping bag. That air can be squeezed out of the sleeping bag when it is compressed, resulting in lower packed size.
What is fill power in relations to sleeping bags?
Fill power (or loftiness) refers to the number of cubic inches that one ounce of down will displace. The higher the down's fill power, the less down is needed to achieve a given temperature rating. For example, a 10 degree sleeping bag using 800-fill-power will weigh less and use less down than a 10 degree sleeping bag that uses 600-fill-power. Higher fill power also corresponds with higher compressibility because less down is required to fill space and achieve a certain temperature rating. Less down means that there is more air in the sleeping bag. That air can be squeezed out of the sleeping bag when it is compressed, resulting in lower packed size.