How to choose a sleeping bag: find the coldest night you expect, then buy an ISO- or EN-tested bag rated below it. Read the lower limit if you sleep warm and the comfort rating if you sleep cold. Pair it with a pad: R-value under 2 for 50°F nights, 4 or more at 20°F. Then pick shape, fill and length.
A sleeping bag’s temperature rating is a lab result, and the lab tests every bag on an insulated pad. On a thinner pad, the same bag sleeps colder than its number. So choose the bag and the pad together, starting from the night you’ll actually face.
What temperature rating and pad R-value do you need?
Look up the usual overnight low for your campsite and month, then read across. Use the lower-limit column if you sleep warm and the comfort column if you sleep cold; Therm-a-Rest’s rule of thumb is limit for most men, comfort for most women. When in doubt, REI advises the warmer bag, since you can always unzip it.
| Expected overnight low | Warm sleeper: bag’s lower limit | Cold sleeper: bag’s comfort rating | Pad R-value | Cold sleeper’s pad |
|---|---|---|---|---|
| 50°F (10°C): warm | 30°F or lower | 30°F or lower (roughly a unisex bag named 20°F) | Under 2 | 2 to 3.9 |
| 32°F (0°C): cool | 20°F or lower | 20°F or lower (roughly a bag named 10°F) | 2 to 3.9 | 4 to 5.4 |
| 20°F (−7°C): cold | 15°F or lower | 15°F or lower (roughly a bag named 5°F) | 4 to 5.4 | 5.5 or more |
| 0°F (−18°C): very cold | 0°F or lower | 0°F or lower (usually the maker’s own rating; see below) | 5.5 or more | 5.5 or more, plus a foam pad |
Lows, bag ratings, pad bands and which rating to read: REI Co-op’s Magnusson Lab table, in REI’s sleeping pad (March 2026) and backpacking bag guides. Cold sleeper’s pad: our rule of thumb (one band up), following REI’s and Therm-a-Rest’s advice to choose a warmer pad if you sleep cold. “Roughly a bag named”: the 11°F median gap in the fact box. R-values per ASTM F3340; °C rounded.
R-values add up, so you can stack pads. Both Therm-a-Rest and REI say to add the R-values of stacked pads. Therm-a-Rest’s Z Lite SOL foam pad (R 2, 14 oz) under its NeoAir XLite NXT (R 4.5, 13 oz) makes R 6.5: inside the 5.5-plus band for 0°F nights, for 1 lb 11 oz in all.
The table builds in its own margin: on a 50°F night with a thin pad, it asks for a 30°F bag. REI’s car-camping advice is similar. Add a 10 to 15°F buffer to the coldest temperature you expect, so a Rocky Mountain summer night near 30°F calls for a 15°F or 20°F bag.
Ground temperature counts as well. REI notes that spring trips, before the earth has warmed, call for a warmer pad than fall trips at the same air temperature.
What do comfort, limit and extreme ratings mean?
EN 13537 was the first shared lab standard for sleeping bags, and ISO 23537 replaced it. REI says the two are close enough to compare an old EN-rated bag with a new ISO one. The current edition, ISO 23537-1:2022, covers adult bags with a limit of −4°F (−20°C) or warmer.
How the ISO test works: a heated manikin in base layers and socks lies in the bag, on an insulated pad, in a cold chamber. The power it takes to keep the manikin warm gives three temperatures:
- Comfort: a “standard woman” (25 years old, 132 lb, 5 ft 3 in) sleeps relaxed without feeling cold.
- Lower limit: a “standard man” (25, 154 lb, 5 ft 8 in) is curled up and fighting the cold, but not shivering.
- Extreme: the standard woman shivers and risks hypothermia. It’s a survival figure, and Sea to Summit doesn’t publish it.
Definitions and body figures: Therm-a-Rest, quoting ISO 23537; Sea to Summit.
Why are most bags named by their limit rating?
Because unisex and men’s bags are labeled with it. Therm-a-Rest says the number in its bag names is the limit. Sea to Summit lists unisex bags by lower limit and women’s bags by comfort, and REI describes the same split. NEMO’s men’s Disco 15, for example, has a 16°F limit and a 27°F comfort rating.
Names are rounded, too: REI notes that brands round to a number ending in 0 or 5, with no rule on which way. Not every brand follows the pattern. NEMO’s genderless Coda carries both numbers in its name; the Coda 10/20 has an 11°F limit and a 23°F comfort rating.
Did you know? On 26 ISO- or EN-rated bags from NEMO, Sea to Summit and Therm-a-Rest, the comfort rating ran 6 to 14°F warmer than the lower limit, with a median of 11°F. So a unisex bag named 20°F keeps a cold sleeper comfortable only to about 30°F. (Our count from the makers’ spec pages, October 2026.)
Why does your pad change the bag’s rating?
The lab rates every bag on an insulated pad, and the pad’s R-value is part of the result. Accounts of that pad differ: R 4 in Therm-a-Rest’s description of the test and REI’s pad guide, above R 5 per Sea to Summit, and about R 5.5 per REI’s backpacking guide.
Either way, the number on the bag assumes a well-insulated pad. Sea to Summit puts it plainly: on a pad with a lower R-value, the bag “may not attain its ISO rating.” That’s why the pad column climbs as the nights get colder.
Can you trust a bag without an ISO rating?
Treat it as the maker’s estimate. REI says a rating that doesn’t say “comfort” or “limit” is probably not an ISO or EN result. The standard also leaves some bags out: ISO 23537-1 excludes children’s bags and expedition bags, and stops at a −4°F limit. Bags built for colder nights carry the maker’s own number.
What is a sleeping pad R-value, and do R-values add up?
What R-value means: R-value measures how well a pad resists heat flowing from your body into the ground. Higher is warmer. Since pad makers adopted the ASTM F3340 test, which Therm-a-Rest helped develop, an R 4 from one brand means the same as an R 4 from another.
Two rules make R-value useful. A pad of R 2 insulates twice as well as one of R 1, and R-values add when you stack pads (REI). A closed-cell foam pad is around R 2, so sliding one under any pad rated below R 5.5 moves you at least one band up the table.
Some reference points from Therm-a-Rest’s own pads:
- Z Lite SOL closed-cell foam: R 2, 14 oz (Regular).
- NeoAir XLite NXT air pad: R 4.5, 13 oz (Regular).
- NeoAir XTherm NXT air pad: R 7.3, 15 oz.
Therm-a-Rest’s season chart puts summer at R 1, 3-season at R 2, all-season at R 4 and extreme cold at R 6. For winter, REI suggests foam under an inflatable: it adds warmth, guards against punctures and keeps working if the air pad fails.
What temperature rating do you need for summer, 3-season and winter?
Season labels are a maker’s shorthand; the overnight low is what counts. As starting points, from Therm-a-Rest’s and REI’s guidance:
- Summer: Therm-a-Rest suggests a 45°F quilt for summer backpacking at lower elevations. REI’s table is more cautious for a 50°F night on a thin pad, asking for a 30°F bag. A pad of R 1 to 2 is enough.
- 3-season (spring to fall): 20°F is a very popular 3-season rating, and Therm-a-Rest calls 32°F ideal for most people in temperate, sub-alpine places. Pair it with R 2 or more.
- Winter: 0°F is Therm-a-Rest’s benchmark winter rating, and REI pairs 0°F nights with a pad of R 5.5 or more. Therm-a-Rest rates all its quilts as 3-season only, because the open bottom can’t hold enough heat. Our winter camping guide covers the rest of a cold-weather setup.
Your body shifts the seasons. Therm-a-Rest says a warm sleeper may stretch a 3-season bag into late fall, while a cold sleeper may need a winter bag for the same trip. Layering is the alternative: Therm-a-Rest puts its Parsec 20°F bag with a Corus 32°F quilt on top at about 5°F.
Mummy, rectangular or quilt: which sleeping bag shape should you choose?
Your body has to heat every bit of air inside the bag, so less empty space means more warmth. REI’s rule of thumb: look for the smallest bag you can fit in and still feel truly comfortable.
| Shape | Fit | Warmth for its weight | Best for |
|---|---|---|---|
| Mummy | Snug, tapered, with a cinchable hood | Highest | Backpacking, cold nights, back sleepers |
| Semi-rectangular (“modified mummy”, barrel, spoon) | More room at the knees and elbows | Middle | Side sleepers and restless sleepers |
| Rectangular | Roomiest and least confining | Lowest | Warm-weather car camping |
| Quilt | No insulation underneath; straps to the pad | High, but relies on the pad beneath | 3-season backpacking on a well-insulated pad |
Shapes and uses from REI’s camping and backpacking bag guides and Therm-a-Rest’s sleeping bag and quilt guide, October 2026.
The same rating doesn’t mean the same warmth. The lab manikin never moves, so a semi-rectangular bag sleeps colder than a mummy rated the same: each time you turn, the roomier bag pumps in cold air (Therm-a-Rest). Sea to Summit adds the reverse: ultra-narrow mummies test well but can sleep colder outdoors, where knees and hips crush the insulation.
Quilts drop the insulation under your back, which your weight flattens anyway (Therm-a-Rest). That makes them lighter, and it makes the pad do all the work underneath. In a hammock your weight flattens the bag’s underside too; our guide to sleeping pads for hammocks covers what goes beneath you.
Couples can zip two compatible bags together, one with a left-hand zip and one with a right-hand zip, or share a double bag (REI). Our best double sleeping bags compares the two-person options. A small inflatable pillow tucks into a mummy hood; our camping pillow guide covers sizes.
Down or synthetic: which sleeping bag insulation is better?
Down for weight and packed size, synthetic for damp trips and tight budgets. Down is lighter, packs smaller and, with care, keeps its loft for decades (REI). Its weakness is water, so most down bags now use water-repellent treated down.
Synthetic fill costs less, keeps insulating when damp, dries faster and is non-allergenic. It is heavier and bulkier for the same warmth.
What does fill power mean?
Fill power is how many cubic inches one ounce of down lofts to in a lab cylinder: an ounce of 800-fill down fills 800 cubic inches. Higher numbers mean more warmth per ounce, and REI calls 900 “about as good as down gets.”
Fill power isn’t warmth, though. Warmth depends on how much down is in the bag, so compare bags by their ISO rating, and use fill power to explain why one bag weighs less than another with the same rating. Synthetic fills have no equivalent test.
When does synthetic win?
- Wet and humid trips. REI calls synthetic “the bag insulation of choice for damp climates” because it keeps insulating when wet. Treated down narrows the gap, but Therm-a-Rest still rates synthetic better if the bag gets wet.
- Drying. In a dryer, REI expects a synthetic bag to take at least an hour and a down bag several hours.
- Car camping on a budget. When packed size doesn’t matter, an REI sales specialist calls synthetic “a great option.”
- Allergies. Synthetic fill is non-allergenic.
The trade-off is lifespan. After years of being stuffed into a sack, synthetic fibers break down and lose some warmth, faster than down does (REI).
How long should a sleeping bag be?
Long enough that your feet don’t press against the end, and no longer. REI’s guideline is a bag several inches longer inside than you are: at 6 ft tall, at least 6 ft 2 in. Makers publish a “fits up to” height for each size, so match yours to it.
| Your height | Size | NEMO | Therm-a-Rest | Sea to Summit | Kelty |
|---|---|---|---|---|---|
| Up to 5 ft 6 in (168 cm) | Short, or women’s Regular | Coda Short; Disco women’s Regular | Boost 650 Short | Women’s Spark and Ascent Regular (to 5 ft 7 in) | Cosmic Down Short; women’s Regular (to 5 ft 8 in) |
| Up to 6 ft (183 cm) | Regular, or women’s Long | Regular (Disco, Forte, Coda); Disco women’s Long | Regular (Parsec, Boost 650) | Trek Regular; Women’s Ascent Long; Spark and Hamelin Regular and Women’s Spark Long (to 6 ft 1 in) | Cosmic Down Regular |
| Up to 6 ft 6 in (198 cm) | Long | Long | Long | Long | Long |
| Over 6 ft 6 in | Check the maker’s chart | — | — | — | — |
“Fits up to” heights from the makers’ US product pages, 22 October 2026: NEMO Disco, Forte, Coda; Therm-a-Rest Parsec, Boost 650; Sea to Summit Spark, Women’s Spark, Trek, Hamelin, Women’s Ascent; Kelty Cosmic Down 20, Women’s Cosmic Down 20. A height in a cell replaces the row’s. Other models may differ.
The bag itself runs longer than the height it fits. Therm-a-Rest’s Parsec Regular is 80 in long for sleepers up to 72 in, and Kelty’s Cosmic Down Regular is 78 in for 6 ft. If you’re at the top of a size, REI suggests trying both; the smaller one is usually warmer and lighter.
Check girth too. It has to clear your widest point, whether that’s shoulders, hips or belly (REI). NEMO’s men’s Disco Regular measures 62 in around the shoulders and its Regular Wide 70 in, and Therm-a-Rest cuts its coldest bags roomier for extra layers.
Are women’s sleeping bags different?
Yes, mainly in the rating they’re named by. The ISO test rates every bag the same way. Its comfort rating is calculated for a “standard woman”, and brands name women’s bags by that number while unisex bags carry the limit (Sea to Summit, REI). So a women’s 15°F bag is usually warmer than a unisex 15°F bag.
NEMO’s spec sheets show the difference. The women’s Disco 15 is comfortable to 17°F, the men’s Disco 15 to 27°F. The women’s Regular carries 910 g of down to the men’s 642 g, though it fits sleepers only up to 5 ft 6 in, not 6 ft. That’s why women’s bags usually weigh more for their rating (REI).
The cut changes too: shorter, narrower at the shoulders and wider at the hips (REI), with extra insulation “where women need most warmth” (Sea to Summit). REI says the industry is moving toward genderless bags in more lengths and widths. Anyone who sleeps cold can shop by the comfort rating; Sea to Summit says cold-sensitive men should.
How do you wash and store a sleeping bag?
- Store it loose. Take it out of the stuff sack and keep it in a large cotton or mesh sack (REI suggests about 90 liters) in a cool, dry closet. Avoid garages, attics and car trunks. Months of compression cost loft, and synthetics suffer more.
- Dry it after every trip. Unzip it and hang it for 6 to 8 hours, out of direct sun, before you put it away.
- Wash it when it’s grimy or losing loft, in a front-loading machine, since a top-loader’s agitator can snag and damage it. Use a down-specific or technical wash on a gentle cycle, and rinse twice.
- Dry it on low heat, with two or three tennis balls for down. Skip dry cleaning: the solvents strip the natural oils that help down loft.
- Use a liner. It keeps body oils out of the bag and adds a little warmth.
Care steps from REI’s washing and storage guides. Check your bag’s label first.
Then add the bag, pad and liner to our camping checklist so none of them stays home.
