I'm looking at down jackets. Some labels say 650-fill, some say 700-fill–what does it mean? Is it a unit of density of the down material or is it the quality of the down material or is it something else? How does it relate to the degree of insulation of a jacket or a sleeping bag, for that matter?
Picture this: You take an ounce of down and dump it into a big, old beaker from science class with cubic inch marks on the side. If it takes up 650 cubic inches, it's called 650-fill down. If it takes up 700 cubic inches, it's, you guessed it-700-fill. You get the point. But what the heck does it mean?
When it comes to down, fluffier is better. The higher the fill power, the fluffier the jacket. The fluffier the jacket, the warmer you'll be. (The same goes for sleeping bags.) You'll pay a premium for higher fill powers because the pure feathers are scarcer. Nowadays, the highest fill power number you'll see is 900. It's supremely cloudlike stuff, the cream of the crop. On the lower end, you'll find 550-fill, which often has little twig-like feather pieces mixed in-you can actually feel them when you touch the down. Bottom line-go with the highest fill power you can afford.