Sleeping Bag Temperature Ratings: Comfort vs. Limit Explained
A '20-degree' bag doesn't mean the same thing on every label. Here's how ISO comfort and limit ratings work, why quilts are different, and how to pick the right rating for your trips.
Sleeping bags and quilts are named by temperature: a 20-degree bag, a 30-degree quilt. But that
headline number can mean different things depending on the brand, and buying on it alone is the most
common reason people sleep cold. Here's how to read the ratings, and how to choose one for the trips
you actually take.
The standardized ratings
Many sleeping bags are tested to ISO 23537, which replaced the older European
standard EN 13537. In the test, a heated, instrumented mannequin wearing a base layer lies in the bag
on a standard pad in a cold chamber. Measuring how much power it takes to keep the mannequin warm
gives three ratings:
Comfort: the temperature at which a typical cold sleeper (the standard models a
woman) stays comfortable lying in a relaxed position. This is the most useful number for most
people.
Limit (lower limit): the temperature at which a typical warm sleeper (modeled on
a man) stays warm, curled up. Many brands name the bag after this number.
Extreme: a survival figure: below it, you'd be at risk of hypothermia. Ignore it
when buying.
ISO 23537 gives three ratings. Buy on the comfort rating if you sleep cold; the limit rating is often 8 to 12°F colder, and the extreme rating is a survival figure.
The comfort and limit ratings are often 8 to 12°F apart, so a bag sold as a "20-degree bag"
may have a comfort rating near 30°F. That gap is the difference between sleeping well and
lying awake at 3 a.m.
Most quilts, and many bags from small cottage makers, aren't ISO-tested: the test is expensive,
and a quilt doesn't fit a mannequin test designed for bags. Their ratings are the maker's own
estimate. Many are honest and consistent within a brand, but they can't be compared directly with
ISO figures, and they vary from maker to maker. For cross-brand comparisons, fill power and fill
weight tell you more (see down fill power explained),
and so do other buyers' reviews.
How to choose a rating
Find the coldest overnight low you expect: check historical lows for your route and season,
and remember that temperatures fall about 3 to 5°F for every 1,000 ft you climb.
Choose a bag whose comfort rating is at or below that temperature, or whose
limit rating is about 10°F below it.
If you sleep cold, go another 5 to 10°F warmer. If you sleep hot, you can shave a little.
Your sleeping pad: the ISO test assumes an insulated pad. On a thin pad, no bag
performs to its rating. See R-values explained.
Fit: too roomy means more air to heat; too snug compresses the insulation. Check
the shoulder and hip girth and the length on each product page.
What you wear: a warm hat, dry socks and your insulated jacket can add several
degrees. Sleeping in damp hiking clothes does the opposite.
Food and water: a fatty snack before bed fuels your body's heat; dehydration
makes you feel colder.
Site and shelter: cold air drains downhill and pools in valleys, and wind strips
heat. A sheltered spot a little above the valley floor can be noticeably warmer than the forecast
low.
Moisture: over several nights, down bags gain moisture and lose loft. Air your
bag out when the sun is out.
The quick version
Use the comfort rating if a bag has one, especially if you sleep cold.
Choose 10°F below the coldest night you expect if you only have the headline number.
Match the bag with a pad of at least R 3 to 4 for three-season use.
