Sleeping bag ratings look precise, but they are the most misunderstood numbers in outdoor gear. A bag rated to 0°C is not a bag you will be warm in at 0°C. This guide explains what the ratings actually measure, how the shapes differ, and how to choose a bag you will not regret on the second night.
What do the temperature ratings actually mean?
Most bags carry three numbers: a comfort rating, a limit rating and an extreme rating. The comfort rating is the temperature at which an average person sleeps comfortably. The limit rating is the lowest temperature at which an average person stays warm without shivering — usually around five to ten degrees colder. The extreme rating is a survival figure, not a sleeping figure. Buy to the comfort rating, not the one printed largest on the bag.
Should I choose down or synthetic fill?
Down is lighter for the same warmth and packs smaller, which matters if you are carrying it. It also lasts many years if cared for. Its weakness is moisture: a wet down bag loses most of its insulation and takes a long time to dry. Synthetic fill keeps some warmth when damp, dries faster, costs less, and is the better choice for wet climates or for occasional use.
What is the difference between mummy and rectangular shapes?
A mummy bag tapers towards the feet and hugs the body, so there is less air to heat and it is warmer for its weight. A rectangular bag gives more room to move and can be unzipped flat, which suits car camping and warm weather. If you sleep on your side or move a lot, a mummy bag will feel restrictive; if you are backpacking, the rectangular bag’s extra volume costs you weight and warmth.
How do I choose the right size?
Length matters more than width. A bag that is too short compresses the insulation at the foot and leaves your shoulders exposed when you move. Most manufacturers publish a user height range — choose by your height, not by the bag’s total length. If you are near the top of a range, take the longer size; the extra length costs very little weight.
What is “fill power” and does it matter?
Fill power describes how much space a given weight of down occupies — higher numbers mean better insulation for the same weight. A 700-fill bag and an 850-fill bag can be equally warm, but the 850 one is lighter and packs smaller. It matters most if you are carrying the bag. For car camping, a lower fill power at a lower price is entirely reasonable.
| Down | Synthetic | |
|---|---|---|
| Warmth for weight | Best | Lower |
| Pack size | Small | Larger |
| When wet | Loses insulation | Keeps some warmth |
| Drying time | Slow | Fast |
| Lifespan | Long with care | Shorter |
| Price | Higher | Lower |
| Best for | Dry cold, backpacking | Wet climates, occasional use |
Do I need a sleeping pad as well?
Yes, and it is not optional. A sleeping bag insulates above you; the ground takes heat away from below far faster than air does. A bag rated for freezing conditions on a good mat will feel cold on bare ground. The mat’s R-value — its resistance to heat flow — matters as much as the bag’s rating, and the two are designed to work together.
How do I care for a sleeping bag so it lasts?
Store it loose, never compressed — long-term compression permanently reduces loft. Wash it rarely, with a dedicated down or synthetic cleaner rather than detergent, and dry it thoroughly before storing. For down, use a tumble dryer on low with a couple of clean tennis balls to break up clumps; for synthetic, air dry and turn it regularly.
What else matters before I order?
Check the zip side — left and right zips let two bags be joined, which matters if you sleep as a pair. Check whether a stuff sack and storage sack are included; they are often sold separately. And check the packed weight and packed size, because those decide whether you will actually carry it. See our outdoor range or ask us about a specific bag.
How do I check a sleeping bag before buying?
Look for three things on the listing: the comfort rating rather than the extreme one, the fill type and fill power, and the user height range. If a listing only gives one temperature figure, treat it as the limit rating and add margin. If any of it is unclear, ask before you order rather than after.