How to check a second-hand sleeping bag

A low temperature printed on a sleeping bag isn’t enough to judge a second-hand buy. You need to know which rating it is, whether the bag fits and what condition the insulation is in now.

Comfort, limit and extreme: which number should you use?

Use the comfort rating as your starting point for a comfortable night, especially if you feel the cold. Compare it with the lowest overnight temperature expected at your campsite and allow some warmth in reserve.

• Comfort: The lower temperature for sleeping relaxed under the test conditions.
• Lower limit: A colder threshold based on a curled-up sleeping position. This is the edge of the bag’s sleeping range, rather than the best target for a restful night.
• Extreme: An emergency threshold where serious cold injury is possible. Do not use it to choose a normal camping temperature.

Illustrative label: comfort +2°C, limit −4°C, extreme −20°C. Treat this as a bag for nights around +2°C or warmer, with extra margin if you sleep cold—not as a comfortable −20°C bag.

ISO 23537 and the older EN 13537 are laboratory test methods using a heated mannequin. A single temperature in a product name may refer to comfort or limit, so check the full label. Your mat, clothing and the condition of the bag also affect warmth.

Fill power, fill weight and loft

• Fill power (FP): How much the down expands for its weight. Higher FP gives better warmth for the weight; 800 FP is not an 800-gram filling or a temperature rating.
• Fill weight: The amount of insulation inside the bag, usually in grams. Consider it alongside fill power, bag size and construction.
• Loft: The puffiness that traps insulating air. Check a used bag for flat areas or clumps after it has had time to expand.

Check the label on the actual bag

Ask for the model, size and a clear photo of the full temperature label. Compare the same rating type across bags, then check the used bag’s insulation and fit.

Check the insulation and shell

Ask to see the bag unpacked and given time to loft, including both sides and the footbox. Look for uneven fill, persistent clumps, thin areas, tears and poorly finished repairs.

• Ask how it was stored and whether it has been damp.
• Ask about cleaning, repairs and any persistent smell.
• Inspect the zip, hood, drawcords and seams.
• Confirm the fill type from the label rather than appearance.

Uncertain insulation condition is a reason to ask more questions, not to assume a wash will restore it.

Make sure you fit inside it

Check the bag’s stated user-length range and dimensions for that version. If possible, try it without crushing the footbox or feeling restricted around the shoulders. Confirm the zip side and whether a matching sack is included.

Think about packing space as well as sleeping space. A cheap bag that will not fit your pack can create another purchase.

Judge the whole sleep system

The mat, shelter, clothing and conditions matter alongside the bag. Our camping bed and mat guide helps with the next part of that decision.

For cleaning, follow the care label and manufacturer’s instructions for the actual fill and construction. Price in any specialist cleaning before buying. If the model or condition cannot be established, don’t assign it a reassuring temperature rating yourself.

Sources and current details

Buying guide. Sources checked 26 September 2026.

• Official source 1
• Official source 2

References for these explanations

Checked 26 September 2026. Rab: temperature ratings; Rab: down fill power.

Updated 26 September 2026: added plain-language explanations of ratings, terms and buying choices.