How do I choose a sleeping bag?

Sleeping bag ratings are a standardised test, not a promise, and they assume you are on an adequate mat wearing a base layer. Most complaints about cold bags are about the mat or the rating being read optimistically.

Step by step

1. Understand the rating numbers.The EN or ISO standard gives comfort, limit and extreme figures. Comfort is where an average person sleeps comfortably; limit is where an average person can sleep curled up; extreme is a survival figure and should be ignored when choosing.
2. Buy for the coldest night you expect.You can always vent a bag that is too warm, and you cannot do anything about one that is too cold. A bag rated a few degrees below your expected minimum is sensible.
3. Account for sleeping cold.The standard assumes an average sleeper. Many people, particularly those who feel the cold generally, need a bag rated several degrees lower than the temperature they expect.
4. Choose down or synthetic deliberately.Down is warmer for its weight, packs much smaller and lasts longer, but loses almost all insulation when wet and dries slowly. Synthetic is heavier and bulkier, cheaper, and keeps working damp.
5. Favour synthetic for wet British trips.For general UK camping, festivals and anything where the bag might get damp, synthetic is the more forgiving choice. Down comes into its own where weight matters and you can keep it dry.
6. Check the fill power if buying down.Fill power measures the loft of the down — higher means more warmth for the same weight. Also check whether it is hydrophobic-treated, which helps but does not make it waterproof.
7. Match the shape to how you sleep.A mummy shape is warmer and lighter because there is less air to heat; a rectangular bag is roomier and colder. If you move a lot in your sleep, a tight mummy bag will annoy you every night.
8. Check the length and girth.Too long and you heat empty space at your feet; too short and the insulation is compressed. Women's and short versions are usually differently shaped rather than simply smaller.
9. Budget for the mat at the same time.A warm bag on a thin mat is cold. The two are a system, and it is common to spend as much on the mat as on the bag.
10. Look at the practical details.A draught collar around the neck, a baffle behind the zip and a hood that actually tightens make a large difference at the cold end of a bag's range.
11. Store it uncompressed.Hung up or in a large cotton sack, never in its stuff sack. Long-term compression permanently destroys loft, which is where the warmth comes from.

Tips

• Use the comfort rating. The limit rating is not a comfortable night.
• Synthetic for British damp, down for weight where you can keep it dry.
• Never store a bag compressed. It kills the loft permanently.

Common mistakes

• Choosing a bag by its extreme rating — The extreme figure is a survival number, meaning a risk of hypothermia rather than a night's sleep. Bags chosen on it are typically ten or more degrees short of what is needed.
• Buying a good bag and a cheap mat — Insulation beneath you is compressed by your weight, so the mat does that job. A high-rated bag on a summer mat leaves you cold from below regardless of what the bag cost.

If it doesn't work

Questions people ask