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 » Down Fill Power Explained

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 clothing will expand after coming out of it's stuff sack or your pack. The warmth of down 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 used by European sleeping bag and clothing manufacturers. The USA Fill Power meter is different. Comparisons of the US standard to the Lorch machine have been carried out by the IDFL, and findings indicate the USA Fill Power meter gives a higher reading. So when comparing fillings between US and European bags make sure you also adjust for the different tests. Generally if you Subtract 100 from the US reading you get the equivalent Euro reading.

The Fill Power Ratings are best used to compare the fillings of different clothing. A jacket 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 jacket 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 also need to be taken into consideration for assessing overall performance and so a single number like fill power only gives limited information.

One thing you can be sure of is that both Marmot and Mountain Equipment both use very high quality down across their fill powers to produce great down insulated clothing.