Fill power

Fill power is a number that tells you how fluffy and warm the down in a jacket is. The higher the fill power, the more air the down can trap and the more insulating the jacket will be for its weight. For example, a jacket with 800 fill power down will be lighter and more compressible than a jacket with 600 fill power down, but both will have the same warmth. Fill power is measured by putting a small amount of down in a cylinder and seeing how much space it takes up after being compressed by a weight.