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800 fill sleeping bags

Have a question about sleeping bags.  My current bag is the Marmot Helium (older version that has 850 fill) and show rated to 15 degrees.

I’m going on a trip and was looking for a -40 bag, but the one I was looking at also has an 850 fill (Mountain Hardware Phantom Gortex -40). 

How do I know that with the same fill rating the other bag is actually going to work to -40?  The trip recommended bags from 0 to -15

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@Erin Thanks for reaching out!

This is a great question! The fill-power of down does not measure it's warmth, rather it is a measurement of how many cubic inches an ounce of down will fill. As such, an 850 fill sleeping bag has down that one ounce lofts to 850 cubic inches. So, a 40 degree sleeping bag and a -40 degree sleeping bag can both have 850 fill down. The difference between the two will be how much down is used, with the -40 degree sleeping bag being filled with significantly more down than a 40 degree sleeping bag.

The higher the down fill (i.e. 850 vs 600) the higher the loft of the down, which makes it warmer for its weight. There is a lot of good additional info found about temperature ratings and fill in this Expert Advice article about How to Choose a Sleeping Bag for Backpacking. Feel free to check it out!

Hope this helps, thanks!

At REI, we believe time outside is fundamental to a life well lived.
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The fill rating is to do with the quality of the down.  A higher rating means you need less down for the same insulation resulting in a lighter more compressible bag. 

The amount of insulation and the bag construction determines the temperature rating.  Generally you have to go by the temperature rating the manufacture gives and read reviews that indicate how true their rating is.  The Mountain Hardware Phantom Gortex -40 is a fairly specialized bag so if you are going somewhere where it is actually called for I would get advice from other people that do the trip you are planning.

I am not an REI employee.

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