Field note
How to read a sleeping bag temperature rating
Updated October 5, 2026
Quick answer: which number to plan with
| Number on the label | What it means | Plan with it? |
|---|---|---|
| Comfort | A standard sleeper has an okay night | Yes. Match it to the coldest night you expect. |
| Limit | Lower than comfort, a cooler sleeper's survival class | Only with margin. Labels often print this one. |
| Extreme | A survival figure | No. |
| One number, no standard named | Treat it as optimistic | Treat it as a limit and add margin. |
If you sleep cold, buy about ten degrees warmer than the coldest air you expect. For most three-season trips, a 15°F or 20°F down mummy is the default.
Comfort versus limit
Comfort is the number for a standard sleeper having an okay night. Limit is lower. Extreme is a survival figure, not a planning figure. If a brand prints one number and will not say which, treat it as optimistic.
You are the variable
Sleeping cold at home predicts sleeping cold outside. A pad with too little R-value makes a good bag feel like a bad bag. Eat before bed. A dry hat helps.
Gear that shows the point
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Best default down bag
REI Co-op Magma 15
The Magma 15 is the practical down mummy: light enough for a weekend, warm enough for a surprise frost, and sold where you can lie in it and return it. Treat 15°F as a limit unless the brand states a comfort rating. Cold sleepers should buy warmer than the forecast, or pair this bag with a high-R pad such as the XTherm, not with a three-season XLite alone. The cut is a slim mummy. If you thrash or sleep with your knees out, a roomier bag is the fix, not a different fill power in the same shape. A quilt such as the Enlightened Equipment Revelation wins only if you sleep warm and will strap it to the pad. The bag is not the pad. Down also fails when it is soaked, so a wet trip is a different choice.
Who should buy it. Backpackers building a first real sleep system for spring through fall.
Pros
- Usable warmth for the weight
- Easy to try on
- Hood that cinches
Cons
- Slim cut
- Not expedition fill
Not sold on Amazon US.
Closest on Amazon: NEMO Disco 15 Down Sleeping Bag. A 650-fill down backpacking bag from NEMO in a 15°F version, the same rating as the Magma 15.
Check current price on the listing.
30°F down mummy, widely available
REI Co-op Magma 30
Treat the 30°F number as a limit, not a comfort promise. Cold sleepers should think of this as a summer bag and add a warmer pad before they add a liner. The draft collar on warmer Magma models matters more than a logo.
Who should buy it. Hikers who sleep in the 40s and want down that packs small.
Pros
- Packs small
- Responsible down in current production
- Hood
Cons
- Too cold for many 30°F nights
- Down fails when soaked
High-end narrow mummy
Western Mountaineering Alpinlite
Western Mountaineering bags are sewn as tools. The Alpinlite is warm for its rating class and tight if you sleep in a starfish. Try the girth, not the stuff sack. It is a poor value if you only camp twice a summer.
Who should buy it. People who sleep cold, know they like a mummy, and will use it for years.
Pros
- Warm for the weight
- Durable shell
- Hood that actually closes
Cons
- Expensive
- Narrow cut
Check current price on the listing.
FAQ
Are quilt ratings the same?
They should name the standard. Many small makers use their own class. Read their definition.
Does a liner add ten degrees?
No. It adds a little and keeps the bag cleaner.
