Fill Power is a measure of how well the down will expand after being compressed. This correlates to warmth for weight and packability. It is NOT a measure of how much down is in a product. There are 2 fill power tests. They are Lorch (EU) and US. The US test returns a higher score for the same down sample. For US fill powers 800 and above then subtract 50 to get the Lorch result. For US Fill Powers below 800, subtract 25 to get the Lorch result. For fill powers below 600 no adjustment is needed. Warning - much detail has been glossed over in this summary, please read below.
Our down pages offer filtering by fill power. This is done using Lorch results or US results adjusted as explained above. This is a basic comparison only. Wherever a fill power is quoted it will have either EU (Lorch) or US after it.
The Fill Power Rating of down measures the strength of the down or its ability to resist compression. Essentially this is a measure of how much your bag or jacket will expand or 'loft' after coming out of your sack. The warmth of down bags and clothing comes from the large volume of air trapped inside the shell by the down feathers. The more space taken up by the uncompressed feathers means more air trapped and more insulation. A Fill Power Rating of 750+ indicates that a 30g sample of down will occupy at least 750 cubic inches when subjected to a standard compression force.
The Lorch Fill Power meter is the recommended standard of the International Down and Feather Laboratory (IDFL) and the standard generally used by European sleeping bag and clothing manufacturers. The US Fill Power rating is different. Comparisons of the US standard to the IDFL/Lorch machine have been carried out by the IDFL, and findings indicate the US Fill Power test gives a higher reading. The difference is not linear and the difference increases as the quality of the down increases. Generally for the highest qualities of down, adding 50 on will take you from the Lorch result to the US one. The difference is reduced for less lofting down and negligible below fill power 500. In Spring 2009 Marmot decided to rate their down for sale in Europe under the Lorch power machine to remove this confusion, but then in 2010 Rab changed to US Fill Power rating because they were selling so much down in the US. Montane also use the US system.
The Fill Power Ratings are best used to compare the fillings of different bags or jackets. A sleeping bag with a higher fill power rating (if everything else is the same i.e shell material, total fill weight and lining) will pack up smaller, be lighter and be warmer than a bag with a lower fill power rating. However the type of down (goose or duck), the mix content (ratio of stalk to feather) and most importantly the total weight of actual down fill also need to be taken into consideration for assessing overall performance and so a single number like fill power only gives limited information. Make sure that when you compare Fill Power Ratings you are comparing like for like. If the information doesn't specify which test protocol was used then double check.