How to Choose a Sleeping Bag (Temperature Rating Guide)
You’re standing in a gear shop staring at two sleeping bags. One says “20°F comfort rated.” The other says “20°F lower limit.” They’re the same temperature, right? Not even close. One will keep you comfortable at 20°F. The other might leave you shivering, questioning every life choice that led you to this frozen campsite.
Sleeping bag temperature ratings are one of the most misunderstood specs in outdoor gear. Manufacturers have historically played fast and loose with their numbers, and even with standardized testing, the ratings require interpretation. This guide breaks down exactly what those numbers mean, how to match a bag to your trips, and how to avoid the expensive mistake of buying the wrong one.
Understanding Temperature Ratings: What the Numbers Actually Mean
Before 2005, sleeping bag temperature ratings were essentially marketing claims. A company could slap “rated to 0°F” on a bag with no standardized testing behind it. The European Norm (EN) 13537 standard changed that, and its successor, ISO 23537, refined it further. Today, most reputable manufacturers test their bags using one of these standards.
The Three Ratings You Need to Know
When a bag is tested to EN/ISO 23537, it receives three temperature ratings measured on a heated mannequin in controlled conditions:
- Comfort Rating: The temperature at which a “standard” cold sleeper (modeled on an average woman) can sleep comfortably in a relaxed position. This is the most conservative and most reliable number.
- Lower Limit Rating: The temperature at which a “standard” warm sleeper (modeled on an average man) can sleep for eight hours in a curled position without waking. This is the number most manufacturers put on the label.
- Extreme Rating: The temperature at which the bag prevents hypothermia for six hours but does not guarantee comfort. You will be cold. You may be dangerously cold. Ignore this number for planning purposes.
Here’s the critical takeaway: when a bag says “rated to 20°F,” that’s almost always the lower limit rating. The comfort rating might be 30°F or higher. If you tend to sleep cold, or if you’re a woman (whose metabolic heat production trends lower during sleep), shop by the comfort rating, not the headline number.
What About Bags Without EN/ISO Testing?
Some manufacturers, particularly budget brands, skip standardized testing and assign their own ratings. These self-assigned ratings are less reliable. If a bag doesn’t specify EN 13537 or ISO 23537 testing, add 10–15°F to the stated rating as a safety buffer. A bag “rated to 30°F” without standardized testing might only be comfortable at 40–45°F.
Down vs. Synthetic Fill: Which Insulation Is Right for You?
The fill inside your sleeping bag determines its warmth, weight, packability, and durability. Both down and synthetic have legitimate strengths, and the best choice depends on where and how you camp.
Down Fill Power Explained
Fill power measures how many cubic inches one ounce of down occupies when allowed to fully loft. Higher fill power means more warmth per ounce and better compressibility. Here’s what the numbers mean in practice:
- 550–600 fill power: Budget down. Heavier and bulkier, but still warmer than most synthetics at the same weight. Common in car-camping bags from brands like Kelty and Teton Sports.
- 650–700 fill power: Mid-range. A solid choice for three-season backpacking without breaking the bank. Brands like REI Co-op and Marmot use this range extensively.
- 800–850 fill power: Premium. Excellent warmth-to-weight ratio. This is the sweet spot for serious backpackers. Think Feathered Friends Swallow or Western Mountaineering Apache.
- 900+ fill power: Ultra-premium. Marginal gains over 850 at significantly higher cost. Worth it for ultralight thru-hikers or alpine climbers where every gram counts. The Western Mountaineering HighLite (850+) and Montbell Down Hugger 900 #3 sit in this territory.
Down Pros and Cons
- Pros: Best warmth-to-weight ratio, compresses smaller, lasts 10–20 years with care, regains loft after compression
- Cons: Loses insulation when wet, slower to dry, more expensive, requires DWR treatment or dry bag for moisture protection
Synthetic Pros and Cons
- Pros: Retains warmth when damp, dries faster, less expensive, hypoallergenic, easier to care for
- Cons: Heavier and bulkier at the same warmth level, breaks down faster (3–5 years of heavy use), doesn’t compress as well
The bottom line: Choose down if you prioritize weight and packability and can keep it dry. Choose synthetic if you camp in consistently wet conditions, need a budget-friendly option, or want a low-maintenance bag for occasional use.
Shape and Style: Mummy, Semi-Rectangular, and Quilts
Sleeping bag shape directly affects thermal efficiency, comfort, and packed weight. There’s no universally best shape — just the right one for your priorities.
Mummy Bags
Mummy bags taper from shoulders to feet, minimizing dead air space that your body has to heat. They’re the most thermally efficient shape and the standard for backpacking. The tradeoff is restricted movement — if you’re a side sleeper or feel claustrophobic in tight spaces, a mummy bag can be uncomfortable. Most mummy bags weigh between 1.5 and 3.5 pounds for three-season models.
Semi-Rectangular Bags
Semi-rectangular bags offer more room in the legs and torso while still tapering slightly at the feet. They’re warmer than full rectangular bags but not as efficient as mummies. Ideal for car camping or for backpackers who prioritize sleep comfort over minimal weight. Expect them to weigh 10–20% more than a mummy at the same temperature rating.
Quilts
Backpacking quilts eliminate the insulation underneath you (which gets compressed by your body weight and provides minimal warmth anyway). They typically have no hood and use straps or elastic to attach to your sleeping pad. The result is a lighter, more versatile system — a quilt rated to 20°F might weigh just 20–24 ounces compared to 30–40 ounces for a comparable mummy bag. Brands like Enlightened Equipment, Nunatak, and Katabatic Gear have built devoted followings among thru-hikers. The caveat: quilts require a good sleeping pad and some technique to avoid drafts.
Choosing Your Temperature Rating: A Practical Guide
The right temperature rating depends on when and where you camp. Use this table as a starting point, then adjust based on whether you sleep warm or cold.
| Season / Use | Temperature Rating | Best For | Example Bags |
|---|---|---|---|
| Summer / Warm Weather | 35–50°F (1–10°C) | Low-elevation summer camping, desert trips, tropical environments | Sea to Summit Spark SP II, Therm-a-Rest Corus 35 |
| Spring / Fall (3-Season) | 15–30°F (-9 to -1°C) | Most backpacking trips, shoulder seasons, moderate elevations | REI Magma 15, Feathered Friends Swallow 20, Kelty Cosmic 20 |
| Winter / Alpine | 0°F and below (-18°C+) | Winter camping, high-altitude mountaineering, snow trips | Western Mountaineering Puma, Marmot Col, Nemo Sonic 0 |
The versatility sweet spot: If you can only own one backpacking sleeping bag, a 20°F bag is the most versatile choice. You can vent it in summer and it’ll handle most three-season conditions. Pair it with a liner for shoulder-season warmth or layer clothing inside for occasional cold snaps.
If you tend to sleep cold, subtract 10°F from your expected low temperature and buy that rating. If you sleep warm, the listed lower limit rating should work fine.
Features That Actually Matter
Beyond temperature rating and fill, a handful of design features separate a good sleeping bag from a great one.
Draft Collar and Zipper Baffles
A draft collar is an insulated tube around the neck/shoulder area that prevents warm air from escaping out the top of your bag. Zipper baffles — insulated strips behind the zipper — prevent cold air from seeping through the zipper teeth. Both are essential in bags rated below 30°F. If a bag at that rating lacks these features, look elsewhere.
Hood Design
You lose significant heat through your head. A well-designed hood cinches snugly around your face with a single drawcord you can adjust without leaving the bag. Some hoods have differential cut (more insulation on top) to maintain loft. In summer bags, a hoodless design saves weight.
Pad Attachment
Pad loops, sleeves, or straps keep you on your sleeping pad throughout the night. This prevents you from rolling off the pad onto the cold ground — a common cause of middle-of-the-night cold. Quilts almost always include pad attachment; it’s a valuable feature in mummy bags too.
Storage and Compression
Store your sleeping bag in a large, breathable cotton or mesh sack at home — never compressed. For the trail, a compression sack or stuff sack reduces packed volume. Down bags compress significantly more than synthetic. Some ultralight bags ship with a minimal stuff sack; you may want to buy a separate compression sack for tight pack fits.
Budget Guide: Good Sleeping Bags at Every Price Point
You don’t need to spend $500 to get a good night’s sleep outdoors. Here’s what your money gets you at each tier.
Under $100: Getting Started
At this price, you’ll find synthetic bags with self-assigned temperature ratings. They work fine for car camping and casual summer backpacking. The Kelty Cosmic 20 (synthetic version, around $80–90) and Teton Sports Tracker +5°F are reliable options. Expect weights around 3–4 pounds and bulky packed sizes. These are not thru-hiking bags, but they’ll serve beginner backpackers well for weekend trips.
$150–$250: The Sweet Spot
This range opens up entry-level down bags and high-quality synthetics. The REI Co-op Magma 30 (down, ~$200) and Marmot Trestles Elite Eco 20 (synthetic, ~$160) deliver performance that punches well above their price. You’ll get EN/ISO-tested ratings, better draft collars, and weights under 2.5 pounds. For most backpackers doing a few trips per year, this tier is all you need.
$300+: Serious Performance
Premium bags from Feathered Friends, Western Mountaineering, Enlightened Equipment, and Katabatic Gear use 800–950 fill power down, ultralight fabrics, and meticulous construction. A Feathered Friends Swallow 20 weighs about 28 ounces and packs to the size of a football. Western Mountaineering’s UltraLite pushes warmth-to-weight ratios to their limit. These bags last decades with proper care, making their per-night cost surprisingly reasonable for frequent users.
Pro Tips for Sleeping Warmer
Your sleeping bag doesn’t generate heat — it traps the heat your body produces. These strategies maximize your warmth regardless of what bag you own.
Use the Right Sleeping Pad
Your sleeping pad’s R-value matters as much as your bag’s temperature rating. An R-value of 2–3 works for summer. Three-season trips need R-4 or higher. Winter camping demands R-5 to R-7. A Therm-a-Rest NeoAir XTherm (R-6.9) under a 20°F bag keeps you warmer than a 0°F bag on a foam pad with an R-value of 2. The ground steals heat far faster than the air.
Wear Camp Clothing
A clean, dry base layer inside your bag adds measurable warmth. Merino wool or synthetic long underwear, warm socks, and a beanie can effectively lower your bag’s comfort rating by 5–10°F. Avoid wearing your hiking clothes to bed — sweat-damp fabric chills you. Change into dedicated sleep layers.
The Hot Water Bottle Trick
Fill a Nalgene bottle with hot water, seal it securely, and tuck it between your thighs or at your feet before getting in the bag. This pre-heats your bag and provides a heat source for the first few hours of the night — exactly when temperatures drop fastest. Make sure the lid is tight. A leak inside a down bag is a disaster.
Eat Before Bed
Your body generates heat through digestion. A calorie-dense snack — peanut butter, cheese, chocolate, or a handful of nuts — before sleep gives your metabolism fuel to burn through the night. This makes a noticeable difference in cold conditions. Some backpackers swear by a spoonful of coconut oil for sustained overnight warmth.
Manage Moisture
Exhale outside the bag, not into it. Breathing into your sleeping bag introduces moisture that degrades insulation over time, especially in down bags. On multi-day winter trips, air your bag out during the day when possible. A vapor barrier liner is an option for extreme cold but comes with comfort tradeoffs.
Frequently Asked Questions
What does “EN comfort rating” mean on a sleeping bag?
The EN comfort rating is the temperature at which a cold sleeper (based on a 25th-percentile female metabolism) can sleep comfortably for eight hours. It’s the most conservative rating from EN 13537 or ISO 23537 testing and the safest number to use when choosing a bag. If you see only one temperature on a label, it’s usually the lower limit rating, which is 10–15°F colder than the comfort rating. Always check the full rating breakdown in the product specs.
Can I use a sleeping bag rated warmer than the conditions?
Yes, and it’s generally a safer bet than going too cold. Most bags can be vented by unzipping the foot box or leaving the zipper partially open. You can also sleep with arms outside the bag or use it as a blanket on warm nights. It’s far easier to cool off in an over-rated bag than to warm up in an under-rated one. That said, carrying a 0°F bag on a July trip adds unnecessary weight.
How long does a sleeping bag last?
A quality down bag, properly stored (loose, not compressed) and kept clean, can last 15–20 years or more. Synthetic bags degrade faster — expect 5–8 years of regular use before the insulation loses significant loft. Signs your bag is wearing out include visible cold spots, permanently compressed baffles, and needing more clothing to stay warm at temperatures that used to be comfortable.
Should I get a women’s-specific sleeping bag?
Women’s sleeping bags are shorter (typically fitting up to 5’6″), narrower in the shoulders, wider in the hips, and most importantly, they add extra insulation — especially in the foot box and core area. If you’re a cold sleeper regardless of gender, a women’s bag is worth trying. The extra insulation typically adds only 2–4 ounces but can make a 20°F bag feel like a 15°F bag.
What’s the difference between fill weight and fill power?
Fill power (measured in cubic inches per ounce) tells you the quality of the down — how much it lofts. Fill weight (measured in ounces) tells you how much down is actually in the bag. A bag with 850-fill-power down and 16 ounces of fill weight will be warmer than one with 850-fill-power down and 12 ounces of fill weight. You need both numbers to compare bags accurately. Two bags with identical temperature ratings may use different combinations of fill power and fill weight to achieve the same warmth.
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