What Do All Those Numbers Mean?

A little bit of knowledge goes a long way in the world of down sleeping bags. Some of the numbers may at first seem a bit confusing but really they are quite straightforward. The three that we need to concern ourselves with are the values of fill power, fill weight and down/feather ratio. There are others but lets keep things as simple as possible.

Fill Power

One way to describe the quality of down is by its fill power, which is the volume in cubic inches (cuin) which an ounce of lofted (fluffed up) down will fill. The higher the fill power, the higher the quality of the down and the higher the thermal rating of the sleeping bag. Typical values range from 600+ for good quality down to 900+ for down of exceptionally high quality that is available in limited quantities globally. The '+' symbol simply means minimum ie. 600+ fill power down has a minimum measured fill power of 600 cubic inches.

Fill Weight

Quite simply the weight of fill in the sleeping bag. Of course, a larger quanity of lower quality (cheaper) down can be used to achieve the same warmth of as a sleeping bag containing a smaller amount of higher quality (more expensive) down. However, the cheaper bag will be heavier and bulkier. Our best-selling good quality down sleeping bags for UK use generally contain around 400-700 grams of down, depending on the user's exact requirements.

Down : Feather Ratio

What we commonly call 'down' in sleeping bags is in fact a mixture of down and feather. Down is responsible for the insulating qualities of the fill, while feathers give support to the loft (or fluffing up!) necessary to retain heat in the sleeping bag. This mixture is represented numerically as a ratio - for example 85:15 or 85/15 which means 85% pure down and 15% feathers. The greater the pure down % the better the insulation value, as long as you compare like-for-like qualities. In practice, above a 90/10 ratio it is differences in the down quality which are of more significance in determining the effectiveness of insulation. Down is more expensive than feather so this will be reflected in the price of the sleeping bag.

These three values will all affect the performance of the sleeping bag. In summary:

• Fill Power = down quality
• Fill weight = how much down
• Down : feather ratio = the percentage blend of down and feather

In essence, as these numbers rise so does the warmth (and cost) of the sleeping bag.