How to Choose a Sleeping Bag (and the Pad That Keeps It Warm)

Your first overnight is booked, the forecast low is about 35°F (2°C), and you’ve heard about someone who shivered in a bag rated colder than that. Now the warmest bag you can find looks like the safe choice, though it costs more, weighs more and might leave you sweating.

That worry is fair, and it isn’t your fault. The labels aren’t written for a first-time buyer, and the big number on most bags means something narrower than it looks.

So the real question is how to choose a sleeping bag that’s warm enough for the coldest night you’ll actually meet, without paying for or hauling a winter bag you don’t need.

By the end, you’ll know which number on the label to read for the way you sleep, which pad to pair with it, and which fill and fit suit your trip, so one bag keeps you warm on your first night out.

How to Choose a Sleeping Bag, Starting From the Coldest Night You Expect

Start with the coldest night you realistically expect on your trip, rather than a season name. REI’s advice is to pick a bag rated a little lower than the typical lows you’ll see. Everything else on the label makes sense once you have that number.

Check the forecast low for the place you’ll actually sleep, on the dates you’ll be there. Then five more questions settle the rest, in the order you’ll meet them:

• Whether you sleep cold or warm, because it decides which line of the temperature rating you should read
• The pad under you, because a bag’s rating assumes you’re lying on an insulated one
• Down or synthetic fill, because your weather, your budget and your pack space pull in different directions
• The shape and size, because empty space inside the bag is air your body has to heat
• A few small habits and a liner, which add warmth for far less than a warmer bag costs

If you’re car camping, REI suggests choosing a bag rated 10 to 15 degrees (about 6 to 8°C) below the coldest temperature you’re likely to see. Real comfort rarely matches a lab rating, because your metabolism, your pad and the night itself all come into it.

If one bag has to cover both car camping and backpacking, REI’s advice is to choose a backpacking bag. That makes sense, because a bag light and compact enough to fit in one of the backpacking packs for a first overnight still does the job beside the car.

Then there’s the thought most first-timers have: “I’ll just buy the warmest bag to be safe.” A warmer bag than you need costs more, weighs more and takes up more of your pack.

It can also backfire. Therm-a-Rest points out that a bag that’s too warm makes you sweat, and that damp leaves you waking up cold.

I’d put that money into the pad first. Dry sleep clothes, a hat and a liner add margin too, and none of them commits you to carrying a winter bag all summer. All of them are covered further down.

If you already know your coldest night, you can skip ahead to the sleep system table in the pad section, which pairs that night with a pad and a bag rating.

Why a Bag Rated 30 Degrees Can Leave You Cold at 35

On most men’s and unisex bags, the big number is the limit rating: the lowest temperature at which a warm sleeper, curled up, still sleeps through in testing. The comfort rating is a warmer figure, for a cold sleeper lying relaxed. If you sleep cold, read the comfort line.

That’s where the 35-degree surprise comes from. Rab names its bags by the limit of comfort, and Therm-a-Rest explains that a bag’s published rating is its limit, so most people shouldn’t expect a 32°F (0°C) bag to keep them cozy at 32°F.

So if the forecast said 35°F, your bag said 30°F (-1°C) and you were still cold, the bag may have done exactly what its label described. It was rated for a warm sleeper curled up against the cold, not someone stretched out and comfortable.

A thin pad widens that gap, as the pad section explains.

Comfort, Limit and Extreme in Plain Words

Most bags tested to the standard show three temperatures, and each one describes a different kind of night:

• Comfort: the lowest temperature at which an average cold sleeper sleeps relaxed through a good night. It’s the warmer of the two numbers you’ll shop by.
• Limit: the lowest temperature at which a warm sleeper, curled up and fighting the cold, still sleeps through. This is the number most men’s and unisex bags are named by.
• Extreme: an emergency survival figure that you should never plan around. Rab describes it as up to six hours curled up, with frostbite possible, and Therm-a-Rest says the range carries a risk of hypothermia.

If a bag shows a single temperature with no “comfort” or “limit” next to it, REI says it’s likely the brand’s own estimate. Numbers from ISO 23537 and the older EN 13537 can be compared directly, because ISO is the newer version of the same standard.

Which Line to Read for How You Sleep

REI’s rule is simple: read the comfort rating if you sleep cold and the limit rating if you sleep warm. If you aren’t sure which you are, I’d read the comfort line. It’s easier to vent a bag that runs a little warm than to warm up one that doesn’t.

One labeling habit is worth knowing. Women’s bags generally list the comfort rating, while men’s and unisex bags generally list the limit, so two bags showing the same number may be rating different things. Compare comfort with comfort and limit with limit.

Both lines come from the same lab test. Therm-a-Rest’s explanation of the EN/ISO ratings describes the setup: a heated manikin wearing a base layer, lying in the bag on a pad, with the ratings worked out from measurements taken on it.

If you’d like to see how it works, REI’s video shows how its lab tests sleeping bags for their temperature ratings.

That doesn’t make the ratings a lie. They’re careful lab numbers taken under fixed conditions, which is why they’re a fair way to compare bags, even though REI is clear that a rating isn’t a guarantee of warmth.

Your own night adds things the lab doesn’t control. Therm-a-Rest lists what you ate and drank, what you’re wearing, how the bag fits and the pad under you, and the pad is the one that surprises people most.

Why Your Sleeping Pad Matters as Much as the Bag

A bag’s rating assumes you’re on an insulated pad, about R 5.5 by REI’s account, in long underwear and socks. Your weight flattens the insulation beneath you, so the pad does the work against the cold ground. On a thin pad, the bag may fall short of its rating.

Picture the night where the bag is warm enough on paper and you still feel the cold coming up through a thin foam pad. The top of you is fine, but your back and hips are cold, because the fill underneath is squashed flat and doing very little.

R-value is how well a pad resists heat loss to the ground, and a higher number means a warmer pad. The ASTM F3340 standard puts every pad on the same scale, so you can compare any two of them.

Major retailers now require ASTM ratings, as Therm-a-Rest explains in its page on the ASTM R-value standard. When you shop, look for “ASTM” next to the number.

REI’s lab paired pads and bags by the night’s low, and its guide to choosing backpacking sleeping bags sets the results out like this:

In Celsius, those four lows are about 10°C, 0°C, -7°C and -18°C. Find the row at or just below your coldest night, then read across to the pad and the bag.

For the bag column, REI says to read the comfort rating if you sleep cold and the limit rating if you sleep warm.

Pads run from about R 1 to 8, according to Therm-a-Rest. Three-season campers often choose R 2 to 4, and winter campers R 5 to 8 or two pads stacked. Stacked R-values add up, so a foam pad under an air pad warms the whole setup without replacing either one.

If your bag is nearly warm enough for the night you’re planning, I’d look at the pad before the bag. A better pad, or a foam pad slipped under yours, is often the cheaper fix, and it helps every bag you put on top of it.

Down or Synthetic: Matching the Fill to Your Weather, Budget and Pack Space

Down is lighter, packs smaller and lasts longer with care, but it costs more and loses insulating power when wet. Synthetic costs less, keeps insulating when damp and dries fast, but it’s heavier and bulkier. The right one depends on your weather, your budget and the room in your pack.

REI says down can last decades with care, and it costs more on average. Most down bags now use water-repellent treated down, but down still loses insulating efficiency when it gets wet.

Synthetic insulation keeps working when damp, dries fast and is non-allergenic, which is why REI points it toward damp climates. The tradeoff is weight and bulk, and REI notes it loses some warmth over years of compression.

If your trips are in a rainy region, or your budget for a first bag is tight, I’d lean synthetic. If you’ll carry the bag a long way, or your pack is on the small side, down earns its price in the weight and space it saves.

A synthetic bag also takes more room, and that room comes out of the bottom of your pack. That’s where the sleeping bag goes in your pack: under everything else, as the soft base of the load.

What Fill Power Tells You (and What It Doesn’t)

Fill power is a lab measure of how many cubic inches one ounce of down fills when it lofts, or puffs up to trap warm air. A higher number means more air trapped per ounce, so less down does the same job.

It doesn’t tell you how warm a bag is, though, because that depends on how much down is inside.

REI’s explanation of down fill power uses a clear example: 50 g of 900-fill down is about as warm as 100 g of 450-fill, and it packs about half as small. To compare how warm two bags are, use their ISO ratings.

Down isn’t the stiff, quilled feathers used in cheaper fills. Its warmth comes from air held in the plumules, the fine, fluffy clusters that loft up, which is what fill power is measuring.

Synthetic insulation has no fill power at all, so there’s nothing equivalent to compare. Synthetic bags are compared by their ISO ratings too.

One care habit protects either fill. Store the bag loose at home in a large mesh or cotton sack rather than squashed in its stuff sack, because long-term compression impairs the loft that keeps you warm.

Shape and Fit: Sizing a Bag So You Aren’t Heating Empty Space

Pick a bag several inches longer than you inside (REI says at least 6 ft 2 in for a 6 ft sleeper) and wide enough for your shoulders, hips or belly. Too much room leaves air your body has to heat. Too little squashes the fill and makes cold spots.

Mummy, Semi-Rectangular or Rectangular

Each of the three main shapes trades warmth for room in a different way:

• Mummy: tapered, with a hood that cinches around your head. It’s the warmest shape for its weight, and the snuggest.
• Semi-rectangular: a compromise between warmth and room, including shapes designed with side sleepers in mind.
• Rectangular: the roomiest and the least warm, most often used for warm-weather car camping.

Length and Girth

For length, REI’s rule is an interior several inches longer than you: at least 6 ft 2 in (188 cm) inside for someone 6 ft (183 cm) tall. For girth, the bag needs to clear your widest point, whether that’s your shoulders, your hips or your belly.

Getting it right matters in both directions. Therm-a-Rest explains that a bag that’s too big wastes your heat on empty space, while one that’s too small compresses the insulation and leaves cold spots.

If you sleep on your side or move around a lot, REI suggests a few more inches of room. If broad shoulders are your problem, check the girth instead. For side sleepers I’d look at width before length, since rolling over is what makes a bag feel tight.

Women’s bags are typically shorter, narrower at the shoulders and wider at the hips. That makes them a fit option for anyone whose body is shaped that way, rather than a rule about who should buy one.

REI also reports the industry is moving toward unisex bags in several lengths and widths. If you fall near the top of a size, REI suggests trying both, and the smaller one is usually warmer and lighter.

Zipper side is mostly a convenience choice, like your dominant hand or the side of the tent door. It matters most if you want to zip two bags together.

A hood cinched around your head adds warmth too, which is where the small habits come in.

Small Habits That Make the Same Bag Sleep Warmer

A few cheap habits stretch the bag you already have: dry clothes kept only for sleeping, a hat, a liner for a little extra warmth, and a closed bottle of warm water. Then try the whole setup in a backyard, a few steps from your own bed.

It’s tempting to slide into the bag in the shirt you sweated in all afternoon. Damp clothes keep pulling heat from you, so Therm-a-Rest’s advice is a dry set kept only for sleeping: long underwear, a top, a hat, and gloves and socks if you like.

The base layers you’d hike in work well here, and knowing what each hiking layer does helps you pick fabrics that stay dry rather than cotton.

If you’re still cold, add a midlayer or a down jacket, but leave yourself some room. Therm-a-Rest’s point is that stuffing yourself into the bag compresses its loft, and squashed loft holds less warmth.

The rest are small, and each one does a specific job:

• A hat, or the bag’s hood cinched around your head, because REI notes a hood adds warmth, and it costs nothing if your bag already has one
• A liner, which adds a little warmth, keeps the bag clean and cuts down on wear, REI says. Expect a small boost rather than a whole new temperature range. On hot nights, an uninsulated liner can be used on its own
• A closed bottle of warm water in the bag, one of Therm-a-Rest’s suggestions, with the lid checked before it goes in

These habits are the cheapest warmth you can add, and I’d use them before reaching for a warmer bag. They’re also the honest answer to the “warmest bag just in case” idea, because they’re margin you can adjust on the night itself.

Before you buy, write down the coldest low you expect on your trip. Then check the bag’s comfort or limit line, whichever fits how you sleep, and the pad’s ASTM R-value against the sleep system table above.