Sleeping Bag Temperature Ratings Decoded: How to Pick the Right Bag for Australian Autumn and Winter Camping

You've stuffed the new sleeping bag into the boot. The label says “rated to 0°C”. You wake up at 3 AM in May at Thredbo Diggings shivering and wondering whether you've been dudded. You haven't been dudded. You've been reading the rating wrong.

Sleeping bag temperature ratings are one of the most misunderstood specs in camping gear. They're standardised, they're tested in a lab, and they mean exactly what they say — but only if you know which of the three numbers on the tag actually matters for you.

TL;DR — the punchline first

• Australian and European bags use the ISO 23537 standard (which replaced EN 13537 in 2017). Most bags list three numbers: Comfort, Limit, and Extreme.
• Comfort is the temperature an average cold-sleeper (modelled as an adult woman) can sleep relaxed and unbothered all night. This is the number to buy by.
• Limit is the temperature an average warm-sleeper (modelled as an adult man) can survive in a curled-up position. Not “comfortable” — survive.
• Extreme is the survival floor before frostbite and hypothermia start. Pretend it doesn't exist when shopping.
• For most Australian autumn/winter camping below 1,000 m elevation, buy a bag with a Comfort rating of -5°C to +5°C. For high country and alpine, drop that to -10°C Comfort or lower.

What the three numbers actually mean

The ISO 23537 test puts a heated thermal manikin (essentially a metal mannequin with skin sensors and internal heaters) inside the sleeping bag in a climate-controlled chamber. The chamber temperature drops, and the lab measures how much heat the bag retains around different body zones. Three temperatures fall out of the test:

Comfort (T-comf)

The lowest air temperature at which a “standard cold sleeper” — modelled as a 25-year-old woman, 60 kg, 160 cm — can sleep in a relaxed posture (lying on her back, arms beside her body) through an eight-hour night without feeling cold. If you sleep cold, are female, are slim, are tired, or have just done a hard day's hiking and your blood sugar is low — buy by this number.

Limit (T-lim)

The lowest air temperature at which a “standard warm sleeper” — modelled as a 25-year-old man, 70 kg, 173 cm — can sleep in a curled-up position (knees drawn up, arms tucked) for eight hours without feeling cold. This is the marketing-headline number that most cheap bags slap on the front of the stuff sack. It's optimistic for almost everyone in real-world conditions.

Extreme (T-ext)

The lowest air temperature at which the same standard woman can survive six hours in the bag without dying of hypothermia. Strong shivering, risk of frostbite, definitely not sleeping. This number exists for safety calculation only — never plan to camp anywhere near it.

What temperature should you actually buy?

The honest rule of thumb: take the coldest overnight temperature you realistically expect to camp in, then add 5°C to it. That's the Comfort rating you want.

So if you're heading to camp at Thredbo Diggings in May where the Bureau of Meteorology is forecasting overnight lows of 0°C, you want a bag with a Comfort rating of -5°C. If your trip is to Bermagui on the south coast in July (lows around 6°C), a Comfort rating of around 0°C is plenty.

Quick-reference Australian seasonal guide

Down vs synthetic — the trade-off that actually matters

For the same warmth rating, a quality down bag is significantly lighter, packs smaller, and lasts longer than a synthetic. A synthetic bag is cheaper, dries faster, and keeps insulating when wet. The shorthand:

Buy down if

• You hike in and weight matters
• You camp mostly inland in dry conditions
• Budget allows ($300–$700+ for a quality 0°C bag)
• You'll look after it (store uncompressed, keep dry)

Buy synthetic if

• You drive in and weight is no issue
• You camp coastal, in humid Top End conditions, or somewhere damp
• Budget is tight ($100–$250 for a comparable rating)
• The bag will be used by kids, the dog will get on it, or it'll cop abuse

For most touring 4WD and caravan owners who don't carry their bag on their back, a quality synthetic bag is the smart pick. The weight penalty is irrelevant when it lives in the canopy or the front boot of the van.

The cheap upgrade everyone forgets: a liner

A silk or thermolite sleeping bag liner adds roughly 5–8°C of comfort to whatever bag you already own, weighs 100–200 g, and costs $30–$80. It also keeps the inside of the bag clean (you wash the liner, not the whole bag) and doubles as a sheet on warm nights when the bag itself is too much. If you're on the borderline of needing a colder bag, buy the liner first and see if it solves the problem.

Five real-world tips from autumn campers

• Eat before bed. Your body burns calories overnight to stay warm. A cold camper at 2 AM is often a hungry camper. A small handful of nuts or a snack bar before zipping up makes a real difference.
• Pre-warm the bag with a Nalgene of hot water. Boil the kettle 30 minutes before bed, fill a hard-sided bottle (Nalgene, Klean Kanteen) with hot water, drop it into the foot of the bag. Toasty toes for the first three hours, then drink the warm water in the morning.
• Get off the ground. Half your warmth loss is conducted into the cold ground through compressed insulation. A self-inflating mat with an R-value of 4 or more (or two thinner mats stacked) makes a winter bag actually work.
• Wear dry clothes, not the day's gear. Sweaty thermals will make you cold. Stash a dedicated dry pair of merino base layers in the bag's stuff sack just for sleeping in.
• Beanie or hood — not bare head. Up to 10% of body heat goes through your head. A mummy bag with a drawcord hood does the job. With a rectangular bag, wear a beanie.

Outcamp gear that pairs well

A solid sleeping bag is half the system — the other half is the mat under it and the lighting around it for those long autumn evenings. The camping and hiking gear collection has the head torches, swag accessories and small touring essentials that round out a winter sleep setup. If you're pairing the bag with an off-grid power source for an electric blanket or a 12V heated mat in the caravan, the 12V accessories collection covers the wiring side.

The bottom line

Don't buy a sleeping bag by the number on the front of the stuff sack. Find the Comfort rating, add 5°C to your worst-case overnight low, and you'll sleep through the night instead of staring at the tent ceiling at 3 AM doing maths.

Got a sleeping setup that's worked for you through an Aussie winter? Drop your bag, mat, and worst-night experience in the comments below — those real-world stories help the next person more than any rating chart ever will.