Sleeping bag temperature ratings explained
Most sleeping bags sold in the UK carry three temperatures on the label: comfort, limit and extreme. Shop by the comfort rating. Pick a bag whose comfort rating is at or below the lowest temperature you expect overnight, and a few degrees lower if you feel the cold.
The short answer
Find the forecast overnight low for where you're camping. Look for a bag with a comfort rating at that temperature or lower. If you tend to sleep cold, go a few degrees lower again.
How the ratings work
The ratings come from a standard test, EN ISO 23537, which uses a heated model to measure how well a bag keeps warmth in. The result is turned into three figures:
- Comfort. The lowest temperature at which someone who tends to feel the cold should sleep comfortably, lying relaxed. This is the one to use.
- Limit. The lowest temperature at which someone who sleeps warm should get through the night curled up. It's a lower number than comfort, which is why some shops lead with it.
- Extreme. A survival figure, not a sleeping one. Never plan a trip around it.
Not every bag goes through the test. Some cheaper bags show a "season" rating or a single temperature without saying how it was measured. Treat those with caution and compare bags by their comfort rating where you can.
What to look for
- The comfort rating, in °C. Ignore the big number on the front of the bag until you know which rating it is.
- Shape. Mummy bags hold warmth better; rectangular bags give more room to move. Families often find rectangular bags easier for children.
- Fill. Down packs smaller for the same warmth. Synthetic fill copes better when it gets damp, which matters in a British tent.
- Length. A bag that's too long leaves cold air at your feet; too short and you'll squash the fill.
The mat matters as much as the bag
A sleeping bag only works if what's under you stops the cold ground taking your heat. The fill underneath you is squashed flat and does very little. A thin mat, or an airbed full of cold air, can make a warm bag feel cold.
That's why we pair the bag with a mat. Mats are rated by R-value: the higher the number, the better it insulates. Our guide to sleeping mat R-values explains how to choose one.
Layering and tips
- Wear something warm to bed. A clean base layer and a hat make a real difference.
- Add a liner or a fleece blanket if the night turns colder than your bag.
- Eat something before bed and go to the loo before you settle down.
- Keep the bag dry. Air it each morning if you can.
- Check the forecast for the site itself. A clear, still night in a valley can be much colder than the nearest town.
This is comfort guidance only. Everyone feels the cold differently, and children may need more layers than adults.
FAQs
What rating do I need for summer camping in the UK?
It depends on where and when. Clear summer nights can still get cold, especially in the hills and in Scotland. Check the forecast low for your site and use a bag whose comfort rating is at or below it.
Why is my bag rated colder than it feels?
Usually because the figure on the front is the limit rating, not the comfort rating, or because the mat underneath is letting the cold through.
Are children's bags rated the same way?
Not always. Check with the maker how a child's bag was rated, and pack an extra layer for them.
Is a two-season bag enough for autumn?
Season ratings vary between makers. Compare bags by their comfort rating instead, and see our guide to camping in autumn.