Fill Power is the universal rating system for goose and duck down, and whilst it's not the easiest measure to understand straight away, once you get to grips with it, choosing your ideal down sleeping bag or jacket will get an awful lot easier.
Fill power is diplayed in the form of a number, which you'll normally find stitched somewhere on the outside of a down sleeping bag, or on the sleeve of a down jacket. Down clothing and equipment at Webtogs ranges in fill power from 550 (medium fill power) to 900 (very high fill power). This number represents the volume in cubic centimetres of a single gram of down, when fully lofted - i.e. when fully 'fluffed up'. Higher quality down (e.g. 900 fill) traps more insulating air pockets between it's fine filaments than down of a lower quality (e.g. 550 fill), and that's why it expands to take up more space per gram. The more insulating air pockets down traps, the warmer it will keep you! So in simple terms, the higher the fill power of a down jacket or sleeping bag, the warmer it will be for its weight
. The image below illustrates the difference in volume between a gram of 600, a gram of 700, and a gram of 800 fill power down.
Just because one garment contains a higher fill power than another doesn't necessarily mean it's going to be warmer - the quantity
of down also has an effect. A jacket with 300g of 550 fill down, for example, will provide more warmth than one with 120g of 800 fill down. However, it will obviously weigh a lot more! It will also be far less compressible due to it having far less air to be squeezed out. To sum up this example - it should now be clear that you need a lot of low fill power down to do the same job as a small amount of high fill power down! As a result, lower fill power down garments (i.e. in the 550-650 fill power range) will usually be too heavy, bulky and cumbersome to wear for high intensity activities like climbing, skiing or hiking.
It is in this sort of highly active, weight-sensitive pursuits that high fill power down comes into its own. High fill power garments provide all the warmth that the wearer could need, whilst being light and compressible enough to allow sufficient freedom of movement for fast, vigorous activity. They also provide the user with the option of packing them down small into a backpack or stuff sack when not in use. It's for these reasons that high fill power down is a wise investment for the active outdoor user.
Slightly lower fill power down (i.e. 650-700 fill power) is sometimes used in high-contact zones of some active garments. Examples include the forearm, shoulder and leg regions of climbing and expedition down jackets/suits - where it is able to resist compression better than higher fills. In the majority of cases though, for climbing, hiking and any active outdoor pursuit you'll want the highest fill power you can get your hands on!
How Fill Power Affects Pricing
Each duck or goose will only yield a small quantity of down clusters; 100g for a typical goose, and of those only a small proportion will be of sufficient quality to go into a high fill power down garment or sleeping bag. For this reason higher fill power down garments are almost always more expensive than those with a lower fill power. However, as you've seen, the gains in weight and compressability mean they're well worth the investment for active and extreme users.
But is high fill power better for everyone? Not necessarily! If you’re looking for a down jacket to keep you warm around a ski resort or in the urban environment, lower fill-power down will do you just fine. The bulk and weight won't matter as much, and you'll save yourself cash. There's loads of urban and lifestyle products out there made with down in the 550 - 600 fill power region, which still provide great warmth as well as great looks.