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A backpacking sleeping bag has to fit both the night and the pack. Begin with the temperatures expected on your route, then choose a bag that gives you enough usable sleeping space without crowding the rest of your gear.
Compare warmth before ounces
Read the complete temperature rating, including whether it is comfort or limit. REI recommends comparing bag weights within similar temperature ratings. A summer bag will often look lighter than a colder-weather bag because it solves a different problem.
Include the sleeping pad in the plan. Check its insulation and how its packed size fits your load. Saving space on the bag while carrying an unsuitable pad is not a useful trade.
Down or synthetic for this route?
Compare the specific models' total weights and packed volumes. If choosing down, consider how you will protect it from wet weather and condensation. If choosing synthetic, check whether its packed volume leaves room for food and other equipment. Do not choose purely by a fill-power number or a generic claim that one material is best for every climate.
Shape and access
A mummy shape may suit someone who likes a snug fit; a broader shape may be easier for a restless or side sleeper. Try the shoulder and foot space, and confirm the length for your height. Operate the zipper while lying inside the bag. A hood adjustment, a useful zip guard and access to a small pocket can matter more over several nights than an extra feature you never use.
Pack and care
Check whether the quoted weight includes the sack and whether a compressed volume is supplied. Try fitting the bag inside the backpack you will use rather than assuming a catalog photograph proves it will fit. Follow the care label and let it dry before storage. Choose by the route, fit and complete carried setup.
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