Montbell Maths

My last summer bag rated to 45 degrees, which I could get to 38 without extra clothing had 11 oz of fill weight. 6 oz seems to be really, really optimistic.

All I know is that the general consensus on this forum is that MB ratings are normally spot on. All I can say is that my #2 served me well at 25 degrees earlier this year and the weight is as advertised.

I believe I am correct in stating that due to the stretch fabric used MB are able to use less fill to achieve temp ratings.

However something just doesn't look right about that #5 spec by comparison to their other rated sleeping bags in the same range.

The amount of fill will vary some depending on the cut of the bag as well.

For reference though, the 35 degree Western Mountaineering Highlite also only has 8 ounces of fill and Western bags are well known to be as rated if not conservative. The Western Ultralite is a 20 degree bag with 16 oz of fill so I would expect 14 oz for 25 degrees to be about right in a effecient bag for smaller people.

At the beginning of this decade the #5 listed 8 oz of fill, then mid-decade it went down to 6.3 oz, and now it's down to an even 6 oz. All the while the temperature rating remained the same. My guess is that it's due to use of a higher grade of down (and I don't know the mathematics of down, sorry).

I started with a ss #3 and couldn't stay warm at the rated number, 30 for that bag. Switched to a #2, and it is easily warm enough for me to the number, 25 for that bag. Go figure. IMO, if you are bigger person, MB ratings are optimistic on the ss bags. If you stretch them, you better buy a warmer bag. I am not a big person, but for some reason the #3 didn't work, and the #2 is great and I love the bag. Not much help for you thou.