A sleeping bag that is wrong for your trip is one of the fastest ways to ruin a night outdoors, either shivering through a bag rated too warm for the conditions or sweating and hauling extra bulk in one rated too cold. The good news is that choosing correctly comes down to a short list of factors rather than a huge amount of gear knowledge. Here is what actually matters, in the order it matters.
Start with a temperature rating, then go 10 degrees colder
Every sleeping bag lists a temperature rating, the lowest temperature at which an average sleeper should stay just barely comfortable. The practical rule experienced campers use: buy a bag rated about 10°F colder than the lowest temperature you expect to encounter. Ratings are tested under near-ideal conditions (a sleeping pad underneath, appropriate base layers, a tent), and most people run a little colder than the rating assumes, especially if you sleep cold naturally. A three-season bag rated around 20 to 35°F covers most spring-through-fall camping in temperate regions; a summer bag rated 35 to 50°F is lighter and packs smaller for warm-weather trips only; a winter bag rated below 20°F is purpose-built for cold-weather and high-altitude trips.
Understand EN/ISO ratings
Many bags now carry a standardized EN 13537 or ISO 23537 rating, which lists three numbers: a Comfort rating (temperature at which a cold sleeper, typically modeled as female, stays comfortable), a Lower Limit (temperature at which a warm sleeper, typically modeled as male, stays comfortable in a curled position), and an Extreme rating (survival only, not comfort, for a short duration). Shop by the Comfort or Lower Limit rating depending on which better matches how you sleep, and treat the Extreme rating as a safety margin, not a target.
Down vs. synthetic insulation
- Down offers the best warmth-to-weight and packs down smaller, making it the top choice for backpacking where every pound and every liter of pack space matters. Its major weakness is that it loses almost all insulating power when wet, though hydrophobic-treated down has narrowed this gap significantly. Down bags cost more and need more careful storage and cleaning.
- Synthetic insulation is cheaper, insulates reasonably well even when damp, dries faster, and is easier to care for, which makes it a better fit for car camping, humid climates, and anyone on a tighter budget. It is bulkier and heavier for an equivalent warmth rating, which matters more the farther you are carrying it.
If your trips are mostly drive-up car camping, synthetic is the simpler, more forgiving choice. If you are carrying the bag on your back for miles, and especially in dry climates, the weight and pack-size savings of down are usually worth the extra cost.
Mummy, semi-rectangular, or rectangular shape
Shape is a genuine warmth-versus-comfort tradeoff, not just a style choice. Mummy bags taper closely to the body, minimizing the air space your body has to heat, which makes them the warmest and lightest option per degree of rating; they are the standard for backpacking and winter use, though some people find the close fit restrictive. Rectangular bags are roomiest, sometimes unzip fully into a blanket, and are the most comfortable for people who move around a lot in their sleep, but the extra air space makes them meaningfully less warm for the same fill weight, and they are bulkier to pack. Semi-rectangular (sometimes called modified mummy) bags split the difference, tapering less aggressively than a true mummy while staying warmer and more packable than a rectangular bag, and are a solid default for most three-season car camping.
Fit matters more than people expect
A bag that is too roomy leaves cold air space your body has to work to heat, reducing effective warmth below the rating. A bag that is too tight compresses insulation and restricts the loft that actually traps warm air, which also reduces effective warmth. Check the bag’s listed dimensions against your height (most brands offer a regular and a long length) and shoulder girth if you are broad-shouldered or prefer to sleep with more room. If you can try one in-store, get inside it and check you can move your arms and turn without the fabric pulling taut.
Weight and pack size, if you are carrying it
For any trip where you are hiking in rather than driving to the campsite, weight and packed volume become real factors alongside temperature rating. A three-season backpacking bag typically packs to the size of a loaf of bread and weighs 2 to 3 pounds; car camping bags can be double that or more since weight is not a constraint. If you are pairing a new bag with a pack overhaul, our guide to hiking gear essentials covers how the rest of your kit fits together, and warm, dry feet matter just as much overnight as during the day; see our guide to choosing hiking socks for the layering logic that also applies to sleep socks on cold nights.
Extra features worth considering
- Draft collar and draft tube. An insulated collar around the neck and a tube of fabric behind the zipper both stop warm air from escaping through gaps; look for these on any bag rated below freezing.
- Hood with a cinch cord. Significant heat is lost through the head; a well-fitted, adjustable hood matters more on colder-rated bags.
- Two-way zippers or zip-together compatibility. Useful for venting on warm nights or combining two bags into a double for couples camping together.
- Stuff sack and compression sack. Most bags include a basic stuff sack; a separate compression sack is worth buying if pack space is tight.
Caring for your sleeping bag
Air out and fully dry a bag after every trip before storing it, since trapped moisture breaks down insulation loft over time, especially in down. Store it loose in a large cotton or mesh storage sack rather than tightly compressed for long-term storage; permanent compression flattens the loft that creates warmth. Wash according to the manufacturer’s instructions, using a down-specific detergent for down bags, and tumble dry on low with clean tennis balls to help redistribute clumped down evenly. A well-cared-for bag, down or synthetic, can easily last a decade or more of regular use.
Matching a bag to your actual trips
If you mostly car camp in mild weather from late spring through early fall, a synthetic, semi-rectangular bag rated 30 to 40°F covers the vast majority of nights comfortably and affordably. If you are backpacking regularly and want one do-it-all bag, a down mummy rated around 20°F is the most common recommendation, since it is warm enough to stretch into shoulder-season trips while staying light and packable. If you camp year-round across a wide temperature range, most experienced campers end up owning two bags rather than one: a lighter summer bag and a warmer three-season or winter bag, since no single bag comfortably covers both a 75°F night and a 15°F one without either overheating or freezing you.
Frequently Asked Questions
What temperature rating sleeping bag do I need?
Pick a bag rated about 10°F colder than the lowest overnight temperature you expect on your trips, since ratings assume near-ideal conditions like a sleeping pad and appropriate base layers underneath. Most three-season camping in temperate climates is well covered by a bag rated 20 to 35°F.
Is down or synthetic insulation better for a sleeping bag?
Down is warmer for its weight and packs smaller, making it better for backpacking, but it loses insulating power when wet and costs more. Synthetic insulation is cheaper, still insulates when damp, and dries faster, making it a more forgiving choice for car camping and humid climates.
Should I buy a mummy or rectangular sleeping bag?
Mummy bags are warmer and lighter because they minimize the air space your body has to heat, making them the standard choice for backpacking and cold-weather trips. Rectangular bags are roomier and more comfortable for restless sleepers but are less warm per pound and bulkier to pack, so they suit mild-weather car camping better.
How do I know if a sleeping bag fits properly?
Check the bag’s listed length against your height and choose a long size if you are near the top of a regular size’s range. Inside the bag, you should be able to move your arms and shift position without the fabric pulling taut; too roomy wastes warmth on unheated air space, and too tight compresses the insulation and reduces its loft.