Six shapes, from the tapered mummy to the half bag that stops at your waist. One buys you warmth for its weight, the next buys you room to roll over, and choosing between them decides more about the night than the number on the label.
Explainers·Published September 4, 2026·12 min read
Lie down in a mummy bag and the fabric is against you nearly everywhere. Pull the hood cord and all that's left is a small warm opening for your face. Lie down in a rectangular bag and you can cross your ankles, roll onto your stomach, throw an arm out sideways and never touch a seam.
That gap is the shape, and it decides more about how a night feels than the number stitched on the label does. There are six worth knowing, and they line up neatly from tightest to loosest. Every one hands you something you want and charges for it somewhere else.
What are the main types of sleeping bags?
Six families cover almost everything sold: mummy, semi-rectangular, rectangular, double, quilt and half bag. They run from the tightest cut to the loosest, and each one trades warmth for its weight against room to move. The table sets out what you get and what it costs.
Shape
What it buys you
What it costs
Where it belongs
Mummy
The most warmth for its weight, and a hood that seals around your face
The least room to move of anything here
Backpacking, cold nights, anything you carry far
Semi-rectangular
Room at the shoulders, elbows and knees without a full rectangle's dead air
A few ounces, and a little warmth against a mummy of the same fill
Three-season trips for side sleepers and people who turn over
Rectangular
The most room, and a zip that opens the whole thing flat into a blanket
The heaviest and bulkiest way to reach a given temperature
Car camping, cabins, warm summer nights
Double
Two people under one piece of insulation, with no cold seam between them
Bulk, and one thermostat shared by two bodies
Couples car camping, and shared trips where one person carries it
Quilt
The lightest way to cover the top of you, with no back panel, no hood and no zipper
Your sleeping pad becomes the only thing underneath you
Ultralight backpacking, paired with a pad rated for the night
Half bag
Legs and hips only, for less weight and bulk than anything else here
You have to be carrying an insulated jacket, and wearing it
Alpine bivouacs and fast, light nights out
These are families rather than sealed categories. Plenty of bags sit between two rows, and the makers name them whatever they like.
How does each sleeping bag shape work?
A mummy tapers and adds a hood. A semi-rectangular relaxes that cut at the shoulders and knees. A rectangle holds one width throughout. A double covers two people, a quilt drops the back panel, and a half bag stops at the waist. Here is what each does to a night.
The head end is where a mummy earns its name: a hood that cinches down, and a cut that tapers away from the shoulders.·Photo: Smack — Public domain, cropped
01
Mummy
The default for anything you have to carry
Cut
Tapers from the shoulders down to a narrow footbox
Hood
Built in, and cinches around the face
Trades away
Room to move
A mummy is cut narrow on purpose. Your body has to heat the air inside the bag before that air can insulate you, so a smaller volume warms faster and stays warm with less down in the walls. Therm-a-Rest describes the tapered fit plainly as the warmest of the shapes it makes, and nearly every ultralight bag is some version of it.
The hood does more work than it looks like it's doing. Once the bag is closed, your head and neck are the one opening left, which is why a mummy has a cinchable hood, and why the hoodless shapes further down need you to supply a hat or a jacket hood instead.
Who it suits: Back sleepers, cold sleepers, and anyone counting ounces. If you turn over all night you'll feel the walls, and the next two shapes exist because of that.
02
Semi-rectangular
Barrel cuts, modified mummies and the spoon
Cut
Tapered, but wider through the shoulders and hips
Hood
Usually, though often less closely fitted
Trades away
A little warmth per ounce
This is the middle of the range, and the widest family in it. A barrel or modified mummy keeps the tapered footbox and relaxes everything above it. Therm-a-Rest calls the result a go-between for fair-weather campers who want warmth and extra room. REI's line for anyone who tosses and turns is that a mummy will feel restrictive, and this is the shape that gives the room back.
NEMO's spoon shape is the interesting version. Instead of adding width evenly, it flares only at the elbows, knees and hips, so a side sleeper gets room exactly where a mummy pinches. That's worth more than it sounds: an accelerometer study of 664 Danish adults found they spent 54.1 percent of their time in bed on their side, against 37.5 percent on their back.
Who it suits: If you've ever spent a night fighting a mummy bag instead of sleeping in one, this is the shape to look at.
03
Rectangular
The one that unzips into a blanket
Cut
The same width from head to foot
Hood
Rarely, and often just a fold of fabric
Trades away
Warmth for its weight, and packed size
A rectangle has no taper to save it. You heat all that extra volume yourself, and REI's summary is that these bags feel the least confining and typically sacrifice the most warmth. The zip usually runs the full length and around the bottom, so the bag opens out flat and becomes a duvet on a hot night.
None of that is a flaw when the bag is going in a car. Packed size stops mattering, a heavier shell lasts longer against tent floors and picnic tables, and the room is what you came for. It's a bad backpacking bag and a very good camping one.
Who it suits: Car campers, cabins and kids. Also anybody who finds a tapered bag claustrophobic enough to lie awake in one.
04
Double
One piece of insulation for two people
Cut
Wide enough for two, usually rectangular
Hood
Two, one, or none depending on the model
Trades away
Bulk, and separate temperature control
A purpose-built double removes the cold seam you get from two singles side by side, and two bodies under one set of baffles genuinely warm the space faster than either would alone. Some doubles are built so the top layer detaches and works as a blanket, which is what makes them tolerable in summer.
The other route is to zip two bags together, and it comes with a condition: you need one bag with a right-hand zip and one with a left, from models the maker says are compatible. Worth checking at home rather than at the campsite with two bags that won't join.
Who it suits: Couples who sleep at roughly the same temperature. If one of you runs cold and the other kicks the covers off, two singles and two thermostats will end better.
A quilt from below. There's no closed back: the pad sits inside it, held by elastic straps.·Photo: Schoschi — CC BY 4.0, cropped
05
Quilt
A bag with the back, the hood and the zipper removed
Cut
Open along the back, closed into a footbox
Hood
None. You wear a hat or a hooded jacket
Trades away
Your margin if the pad underperforms
The reasoning is that insulation under your body spends the night crushed flat, and crushed insulation does very little, so a quilt drops it and hands that job to the sleeping pad. Straps run under the pad to keep the quilt from sliding off when you turn.
You save real weight and you take on real risk, because nothing sits between you and the ground now except the pad you picked. We've got a separate piece on this, What is a backpacking quilt, which goes into pad ratings, draft control and who should stay in a bag instead.
Who it suits: Ultralight backpackers who already own a warm pad. Restless sleepers get on well with quilts too, since you rotate under one instead of inside a bag.
A winter bivouac with the jacket hood up. A half bag is what you get once you decide the jacket can cover your top half on its own.·Photo: Rolf Steinmann — Public domain
06
Half bag
Stops at the waist, on purpose
Cut
Covers the feet, legs and hips, and ends
Hood
None. Your jacket's hood is the hood
Trades away
Everything above the waist
Also called an elephant's foot, or a pied d'elephant. It's an old alpine idea that reached Britain from the continent: climbers already carrying a warm down jacket figured out that the top half of a sleeping bag was insulation they were paying to carry twice.
PHD, who still make them in Britain, describe the idea as using the jacket for the top half and the bag for the legs. That makes the jacket part of the sleep system rather than an extra, and it's why a half bag has no rating that means anything on its own.
Who it suits: Alpine starts and planned bivouacs, where the trip already demands a serious down jacket. Nobody else, really.
Which sleeping bag shape should you choose?
Carrying it far and sleeping on your back: mummy. It's the most warmth you can get for the weight.
Carrying it far and sleeping on your side: semi-rectangular, and look specifically for room at the elbows and knees.
Driving to the campsite: rectangular. The room is free once packed size stops mattering.
Sharing with one other person who sleeps at roughly your temperature: a double, or two compatible singles zipped together.
Counting every ounce, and you already own a warm pad: a quilt.
Already carrying a down jacket to a cold bivouac: a half bag, and wear the jacket.
Where does a sleeping bag's insulation sit?
A sleeping bag's insulation sits wherever its shape decided it was worth putting. Every one of the six makes a call about which parts of you get wrapped and which parts something else is already covering, and that call is most of the difference between them.
A mummy insulates all the way around and accepts that the underside gets flattened. A quilt looks at that same flattened underside and deletes it, so the pad takes over. A top bag splits the difference: it builds a sleeve, and the pad slots into the bag itself. A half bag deletes the top instead, because your jacket is already there.
That's why comparing two bags on temperature ratings alone can mislead you. The rating describes the bag. What keeps you warm is the bag plus the pad plus what you're wearing, and different shapes move the work around between those three.
A top bag from underneath: the pad slides into a sleeve where the bottom insulation would otherwise be.·Photo: Schoschi — CC BY 4.0
What do comfort, limit and extreme mean on a sleeping bag?
All three come from one lab test, and they describe three different nights. The current standard is ISO 23537-1:2022. It replaced the 2016 edition, which had replaced the old European norm EN 13537.
The figure
Who it describes
What it means
What to do with it
Comfort
A standard woman: 25 years old, 1.60 m, 60 kg
The lower end of the comfort range, where a sleeper relaxed on their back is at the threshold of feeling cold
Buy on this one if you sleep cold
Limit
A standard man: 25 years old, 1.73 m, 73 kg
The point where a sleeper curled up is at the threshold of feeling cold
Recognize it: most bags are named after this number
Extreme
The same standard woman
A very low temperature at which, the standard says, harm from hypothermia is possible
Ignore it when shopping. It is a survival figure, not a use figure
The test uses a heated manikin in a climate-controlled room, dressed in a long-sleeved top, long johns and knee-length socks, and the standard covers bags down to a limit temperature of minus 20 Celsius. The figures are published in Celsius even where a bag is sold with a Fahrenheit name.
One line in the standard matters more here than anywhere else. It says the manikin test suits mummy-shaped bags that fit the manikin without being tight, so a number printed on a rectangular or a double bag rests on a method built around a mummy.
How are fill and shell different from shape?
Shape, fill and shell are three separate choices, and mixing them up is how people end up comparing a light bag to a warm one and concluding something about mummies. Any of these shapes can be built with either fill and almost any shell.
Down gives more warmth per ounce, packs smaller, and lasts many years if you take care of it. Synthetic costs less and keeps more of its warmth when damp, so it minds a wet tent far less. Andrew Skurka's read on treated down is worth keeping: it isn't warm when wet, but it holds up better in humid air and dries faster than untreated down.
Two numbers describe the down itself, and people mix them up constantly. Fill power is how high an ounce of it lofts, so it measures quality. Fill weight is how many ounces are in there. Buy a high fill power with very little of it and you've bought a light bag rather than a warm one.
The shell gets talked about least. A thin, low-denier fabric saves real weight and tears more easily; a heavier one survives being dragged in and out of a car all summer. Neither has anything to do with the cut of the bag it's wrapped around.
Should you choose a roomier shape or a tighter one?
This comes down to how you sleep, not how experienced you are. Read both lists and take the one that describes more of your nights.
Go roomier
+You sleep on your side, or you change position several times a night.
+You've woken up feeling pinned rather than cold.
+You sleep with your knees bent or your arms above your head.
+The bag is going in a car, so its packed size costs you nothing.
+You're broad through the shoulders, and a standard mummy is tight before you even zip it.
+You'd rather add a warm layer than give up the room.
Go tighter
·You sleep on your back and stay there.
·You sleep cold and want every degree the fill can give you.
·You're carrying it for days, where a few ounces and a smaller stuff sack add up.
·Your nights are genuinely cold rather than occasionally chilly.
·You want the hood, and you want it to seal.
·You're pairing it with a pad you haven't checked, and you want the margin a full bag gives you.
No shape here is better than the others. Each one has picked which parts of you it insulates, and left the rest to your pad, your jacket or your hat.
The tapered, hooded shape. It is also the shape the rating test is built around, which is worth knowing before you compare its number to a rectangle's.
A stuffed tube across the neck that seals the gap at your shoulders. Below freezing it's the difference between a bag that works and one you spend the night cinching tighter.
Your sleeping pad's resistance to heat passing through it, tested to a common standard so brands can be compared. It matters more the less bag there is underneath you.
What should you check before buying a sleeping bag?
Once you know the shape, these are the questions that separate two bags that look identical on a product page.
The shape and fit
Shoulder and hip girth, checked against your own buildA bag that's tight before you move is a bag you'll fight all night.
Length, and whether the maker offers a short or long
Where the extra room is, if the bag claims any
Which side the zip is on, if you may ever pair it with another bag
The numbers
Whether the headline figure is comfort or limitIf the maker doesn't say, assume the more optimistic one.
Fill weight in ounces, not just fill power
Whether there is a draft collar, and a draft tube behind the zip
Down or synthetic, judged on how wet your trips really get
The rest of the system
Your pad's R-value, especially with a quilt or a top bagThe less insulation under you, the more the pad decides the night.
A hat, or a hooded jacket, for any hoodless shape
Dry base layers saved for sleeping and nothing else
If you have picked a shape
Our own ranked guides, for once you've moved from which shape to which bag.
Six shapes cover almost everything sold: mummy, semi-rectangular, rectangular, double, quilt and half bag. They run from tightest to loosest, and the cut is a trade between warmth for its weight and room to move. Fill type and shell fabric are separate decisions that any of the six can be built with.
Which sleeping bag shape is warmest?
A mummy, for a given amount of insulation. The taper leaves less air for your body to heat and less surface area to lose heat through, and the hood closes the biggest remaining gap. A rectangular bag with the same fill will feel colder, because you're heating considerably more space inside it.
Is a mummy bag worth it if I sleep on my side?
Often not, and there's a shape built for the problem. Semi-rectangular and spoon cuts add room at the elbows, knees and hips without a rectangle's dead air. Accelerometer studies put adults on their side for more of the night than in any other position, so a bag you can turn over in is worth a few ounces.
Can you zip two sleeping bags together?
Yes, if they're made to. You need one bag with a right-hand zip and one with a left, and the maker generally has to list them as compatible, since zipper length and tooth size have to match. A purpose-built double avoids the cold seam you get down the middle of two joined singles.
What does the extreme rating on a sleeping bag mean?
Treat it as a survival figure rather than a use figure. Between the limit and the extreme temperature the standard expects a strong sensation of cold and a real risk of hypothermia, which is why that band is labeled a risk range. Shop on the comfort figure if you sleep cold, and on the limit figure if you sleep warm.
Is a quilt a type of sleeping bag?
Usually, yes, though it's really a bag with the back, hood and zipper taken off. The insulation you lost was being crushed flat under you anyway, so the pad takes over that job entirely. We wrote a separate article on it, What is a backpacking quilt, covering how pad ratings and draft control work once you make that swap.
Do I need a women's specific sleeping bag?
Sometimes, and the labels are changing. Women's models have typically been cut narrower at the shoulders, wider at the hips and given extra insulation at the feet. REI notes the industry is moving away from men's and women's models toward sizing anyone can pick from, so read the girth numbers rather than the label on the tag.
Sources
Facts on this page were checked against these. Conditions, fees and rules change — check the official page before you go.
