Down is rated according to fill power. This specifies the number of cubic inches one ounce of down will occupy. For example, if one ounce of down takes up a volume of 700 cubic inches, it is given a 700 fill power rating. Loft refers to the thickness of the insulating material in a sleeping bag or garment. The quality of the down is directly related to its fill power rating. In other words, down warmth is a function of both fill power and the amount of fill in a sleeping bag or garment. High-quality down has a high fill power and is loftier than down of lower quality, requiring fewer ounces of down to create the same insulation.
Most companies use only goose down, which has a minimum fill power of 500 to 550. The higher the fill power, the better the down will insulate, because there is less of a chance of cold spots. Today’s higher-end manufacturers offer 600 to 750 fill power down as their standard fill. If a label doesn’t specify fill power, it is usually because the down falls in a range below 400 cubic inches or the item contains less than 75% down.
Down also comes in a number of different grades (or qualities). For example, a 90% goose down garment will consist of 90% down and 10% feathers. The higher the percentage, the purer the down. High-percentage grade insulation will also be very low in weight and bulk, but higher in price.