How to choose a sleeping bag, in the order the decisions matter

Buy on the comfort rating, then work down a fixed list: temperature target, fill type, fill weight and fill power, shape and fit, draught features, shell fabric, size, and last of all the features you are being sold. Most bad purchases come from running that list backwards, starting with price, weight or the number printed largest on the hangtag.

Step 1. What temperature are you actually buying for?#

Under ISO 23537 one manikin test produces three numbers: comfort models a standard adult woman sleeping relaxed, limit models a standard adult man curled up and just holding on, and extreme is a survival figure. Comfort is the buying number, and the breakdown is in sleeping bag temperature ratings, decoded.

Set the target like this. Take the coldest overnight low you expect to camp in, subtract 3 degrees C of margin, and that is the comfort rating you are shopping for. The margin is not padding. Forecast lows are issued for weather stations, and sheltered valley campsites on clear, still nights routinely sit 3 to 5 degrees colder as cold air pools.

Then normalise whatever the label says:

• ISO or EN comfort quoted: use it.
• Only a limit rating quoted: add 6 degrees C.
• No standard quoted at all: add 8 degrees C and treat the result as an estimate.

Two other inputs belong in the same decision. The ISO test runs on a specified mattress, so a cold pad invalidates the rating: see the sleeping pad R-value guide. And if you sleep cold, add another 3 degrees before shopping, for the reasons in why you sleep cold. The sleep system calculator adds these up.

Step 2. Down or synthetic?#

Down gives more warmth per gram, packs far smaller, lasts decades if kept clean and dry, and costs more. It loses loft as it takes up moisture, so a damp bag is a colder bag. Synthetic costs less, holds most of its loft when damp, dries faster, and weighs and bulks noticeably more for the same rating. It also loses loft permanently with compression cycles, so it wears out sooner.

The decision rule is about drying, not rain. Ask how many consecutive nights the bag goes without a chance to air in sun or wind. Under about three nights in a dry climate, down is the better buy. Humid, coastal or canoe trips, or any trip where the bag lives in a damp shelter, favour synthetic. Down or synthetic insulation works through the comparison.

If you buy down, the maintenance is not optional: body oils and dirt cost loft over years, and periodic washing restores most of it, as covered in washing down gear.

Step 3. How much fill, and at what fill power?#

Fill power is a loft measurement, defined under the IDFB steam-conditioned method as the volume in cubic inches that one ounce of down occupies. So 850 fill power lofts 850 divided by 650, about 31 percent higher per gram than 650. Turned around, the same loft needs about 1 divided by 1.31, or roughly 76 percent of the mass. That is the whole benefit: fill power buys lower weight and smaller packed size, not extra warmth.

The 0 and minus 10 rows are the ranges commonly published. The others are extrapolated using the 76 percent mass ratio for 850 fill power and a factor of 1.5 to 2 for synthetic, so treat them as a sanity check rather than a specification.

The table also gives a useful derived number. Midpoints of 600 g at 0 degrees C and 950 g at minus 10 degrees C mean 350 g buys 10 degrees C, or about 35 g of 650 fill power down per degree C. Use it to price any specification difference: a bag with 100 g less fill than a competitor at the same claimed rating is either better built or about 3 degrees C optimistic.

Two warnings. Fill power without a fill weight tells you nothing, since a bag can hold a tiny amount of superb down. And above 900 fill power the gain over 850 is under 6 percent of loft per gram, for a large price step.

Steps 4 and 7. Shape, length and width#

A mummy cut is warmer per gram because it encloses less air and presents less surface area to the cold. A rectangular bag with identical fill is both heavier and colder. Between them sit semi-rectangular cuts, which trade a few degrees for room to move.

Length is the cheap mistake. A bag that is too long leaves air below your feet for your body to heat all night, worth roughly 2 degrees C. A bag that is too short compresses the fill at the footbox and the shoulders, which is worse. Published lengths usually fit to about 168 cm (short), 183 cm (regular) and 198 cm (long). Buy the shortest one in which you can lie with the hood closed above your head and your toes not pressing the end.

Width is quoted as shoulder girth, typically 145 to 160 cm on a regular mummy and 165 to 175 cm on a roomy cut. If you turn a lot, size up in girth rather than length: girth costs far less warmth than extra footbox air. If you rotate inside the bag rather than with it, read quilts against sleeping bags first.

Women's specific bags are a real specification difference, not a colourway. They typically add fill at the feet and hips, run shorter and wider at the hip, narrower at the shoulder, and are rated to comfort.

Steps 5 and 6. Draught features, baffles and shell fabric#

Baffle construction decides how much of the fill works. Sewn-through construction stitches the shell straight to the lining, so insulation is pinched to zero along every seam line. Box baffles use fabric walls between the layers, holding an even depth of fill everywhere.

The penalty is bigger than it looks. A lofted bag panel has a conductance near 1 watt per square metre per kelvin. A bare seam is limited only by the air films either side, near 8 watts per square metre per kelvin, so it leaks around eight times faster per unit area. If seams make up 3 percent of the top surface, that adds roughly 24 percent to the heat loss through it. Sewn-through is a warm weather construction and nothing else.

After construction come the draught features, worth several degrees on a cold bag and nothing on a summer one: a draft collar sealing around the shoulders, an insulated baffle along the zip, and a hood with drawcords closing to a face opening.

Shell fabrics are typically 10D to 30D nylon or polyester, often with a water resistant finish. Denier is the mass in grams of 9,000 metres of yarn, so 10D runs about 25 to 30 g per square metre and 30D about 45 to 55. A bag carries roughly 5 square metres across shell and lining, so the choice is worth 100 to 130 g, at the cost of a shell that snags more easily. Waterproof breathable shells add weight, cut breathability and trap moisture in the fill.

Step 8. What not to pay for#

• The extreme rating. It describes shivering survival, not sleep, and has no place in a purchase decision.
• Fill power above 900. Under 6 percent more loft per gram than 850, for a disproportionate price.
• A waterproof breathable shell on a bag that will live inside a tent.
• A heavy compression sack. The large breathable storage sack matters more, since months of compression destroy loft.
• Stash pockets, pillow sleeves and zip garages. Typically 20 to 50 g of features you will not use.
• Antimicrobial and odour treatments, which are not thermal specifications.
• A draft collar on a summer bag. Weight you will push aside all night.
• Any temperature number with no standard named. Add 8 degrees C and re-price it.

Before you pay: the inspection you can run on a spec sheet#

On the specification, in this order:

1. Is an ISO 23537 or EN 13537 comfort figure published? If only limit, add 6 degrees C.
2. Is fill weight in grams published? Without it, no warmth claim can be checked.
3. Is fill power published with a method named, such as the IDFB regulation? A bare number is a claim.
4. Compute the fill fraction: fill weight divided by total weight. A three-season down bag typically lands at 45 to 60 percent, so 600 g of fill in a 1,150 g bag is 52 percent. Under 40 percent means you are carrying shell, zip and hardware.
5. Is baffle construction stated? If the page does not say box baffle, assume sewn through.
6. Are draft collar, zip baffle and hood drawcords named, rather than implied by a photograph?
7. Are length and shoulder girth in centimetres published, and does your height sit inside the stated fit?

In hand, if you can get to one:

• Get in it on the floor, on your side, knees drawn up. Can you turn without dragging the bag round? Can you close the hood one-handed in the dark?
• Press the footbox around your feet. If your toes flatten the fill, the bag is too short.
• Pinch a top seam. If the two fabric layers meet with no fill between them, it is sewn through.
• Hold the bag to a window. Even light means even fill, and bright patches mean migration.
• Cinch the draft collar and check for a gap at the collarbone.

Common mistakes when buying a sleeping bag#

Buying to the forecast rather than 3 degrees below it. This is the single most common cause of a cold night in an adequate bag.

Matching a good bag to a thin pad. The rating assumes a mattress, so fixing a downward heat loss by buying a warmer bag replaces the wrong component.

Buying long to leave room. Extra length is extra air to heat, roughly 2 degrees C of it, and it never becomes useful.

Reading fill power as a warmth rating. It is a loft measurement per gram. Warmth comes from fill weight multiplied by loft, so both numbers are needed.

Adding a liner instead of buying the right bag. A silk liner adds about 1 degree C and a thermal one about 3, which does not close a 6 degree gap.

Storing it compressed. Months in a stuff sack cost loft permanently, in down and synthetic alike. That, plus sleep clothing from the layering system, decides how the bag performs in year five.

Frequently asked questions#

What temperature sleeping bag do I need for three season camping?#

For spring to autumn lowland camping with lows around 0 to 5 degrees C, buy a comfort rating near minus 3 degrees C, which gives about 3 degrees of margin, and pair it with a pad of R-3 or better. If your trips run colder, or you sleep cold, take the comfort rating down a further 3 degrees.

How much down should a 0 degree sleeping bag have?#

Typically 500 to 700 g of 650 fill power down, or 350 to 450 g at 850 fill power, for a comfort rating near 0 degrees C. Synthetic equivalents usually need 900 to 1,200 g. If a bag claims that rating with far less fill, the difference has to come from construction and draught features, and it is worth asking which.

Is 850 fill power worth paying for over 650?#

It buys weight and packed size, not warmth. The same loft needs about 76 percent of the mass at 850, so a 0 degrees C bag drops roughly 150 to 250 g of fill. Carried on your back that is a real saving, visible in your base weight. For car camping it buys nothing you will notice.

Should I buy a longer sleeping bag for extra room?#

No. Air below your feet has to be heated all night, and a bag one size too long costs roughly 2 degrees C. If you want room to move, buy extra shoulder girth instead, typically 165 to 175 cm on a roomy cut against 145 to 160 cm on a standard mummy. Girth costs far less warmth than length does.

Do I need a women's specific sleeping bag?#

If the fit matches you, it is usually the better buy. These bags typically carry extra fill at the feet and hips, run shorter with a wider hip and narrower shoulder, and are rated to comfort. The specification difference is real. The test is whether the length and girth suit you, not what the label says.

How can I tell if a bag is box baffled or sewn through?#

Look for stitch lines running straight through both faces: on a sewn-through bag you can pinch a seam and feel shell and lining touch with no fill between. Box baffled bags hold the layers apart with fabric walls, so loft stays even across the seam. Below about 5 degrees C, box baffle is the minimum.