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A 0°F sleeping bag is a bag with a nominal zero-degrees-Fahrenheit rating, not a promise that every person will sleep comfortably at that temperature. Before choosing one, find out whether the number is a comfort rating, a lower-limit rating or the maker's estimate.
Read the rating before the insulation label
REI explains the distinction between comfort and limit ratings. If two products advertise 0°F using different rating conventions, their large headline numbers are not enough for a meaningful comparison. Ask for the complete temperature specification and the size to which it applies.
Down versus synthetic is the next question. For carrying a winter bag in a backpack, compare packed volume and total weight alongside the rating. For a damp campsite, consider how you will keep the bag dry. A water-repellent shell is not permission to expose the insulation to persistent moisture.
Check the features that stop drafts
Try operating the zipper from inside the bag. Check for an anti-snag arrangement and a draft tube behind the zip. Inspect how the hood and collar close without pressing uncomfortably against your face or neck. These details help you compare how the bag will work during a cold night.
The sleeping pad matters too. Review its insulation information for the planned cold conditions rather than pairing a winter bag with whichever mat is already in the cupboard. The bag, pad, clothing and shelter form the overnight setup.
Fit and venting
Measure length and shoulder space, then try the bag in your usual sleeping position. Avoid choosing an oversized bag solely to gain room. Check whether the zipper can vent the foot area when the night is warmer than expected. For storage at home, follow the care instructions and avoid leaving the bag tightly compressed for long periods.
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