The warmth of a down product comes from the large volume of air trapped inside the shell by the down feathers; Fill Power is the measure of loft of a product that is related to the insulating value of the down or to put it simply, a measure of how well the down will expand after being compressed. This is a key factor that should be taken into account when purchasing a down product. The higher the fill power number the greater the loft and the greater the performance of the down insulation, resulting in a lighter product.
The number of the fill power e.g. 700 refers to the quality of the down and this is a volume - the amount of cubic inches one ounce of down occupies. There are two different standard measuring methods; one used in Europe and the other in the US. For simplicity we try to stick to one standard, which is the US one.
The U.S. standard utilizes the fill power system of the International Down and Feather Bureau (IDFB) which uses 'steam conditioning' as its official method. In the IDFB method, precisely one ounce (28.4 grams) of conditioned down is placed in a cylinder (290 mm in diameter and 500 mm in height). The down is then compressed from above with a flat object, or plate mass, weighing 94.25 grams. The down volume is then measured and assigned a corresponding fill power.
Fill weight is defined by the total amount of down placed in a product, which is measured in grams. Generally speaking, the higher the fill weight of a product the warmer it will be - because fill weight is just the amount of down used, although it isn't quite that simple. The warmth of a down sleeping bag or jacket is determined by the quality of the down (Fill Power), the amount of down present in the product (Fill Weight) and the construction of the product.
Construction of the product is important because down needs to 'loft' to be effective, i.e. it needs empty space to expand into freely. A badly constructed product will have baffles that restrict the down and make it less effective, the construction of the baffles in a product is a mixture of 'art and science' and is a subject much too big to cover here. The Fill Weight can also vary across the garment or sleeping bag; because the down is contained in baffles manufacturers have the choice of how much down they put in each baffle, some manufacturers put more down in the top of a sleeping bag because the bottom of the bag may be insulated by your sleeping mat and they find that overall this is more efficient. A few manufacturers have open baffles so you can shake the down and move it round the bag - so that you can vary the amount of down on the top and therefore control the performance of the sleeping bag. Some manufacturers produce women's specific sleeping bags with the down 'distribution' being different to a man's sleeping bag.
Fill weight is an exact measurement in grams. To find the weight of the sleeping bag shell, the fill weight can be subtracted from the total weight of the sleeping bag. The total fill weight is distributed throughout the chamber baffles and the down is strategically placed to maximise functionality and performance.