Most sleeping bags carry two or three temperatures on the label, and they don't mean the same thing. Choose a bag on the wrong one and you can be awake at 2 am, shivering in a bag that was, technically, rated for the night.
Here's what each number means, where it comes from, and which one to plan on.
The three numbers on the label
Bags rated to the European standard EN 13537, or to ISO 23537-1, the international standard that replaced it, show a comfort, a limit and an extreme temperature. Some also show a maximum temperature: the warmest night before you start to sweat.
| Rating | What the standard describes | How to use it |
|---|---|---|
| Comfort | The lowest temperature at which a sleeper lying relaxed, on their back, is at the threshold of feeling cold. EN 13537 modelled it on a "standard woman". | Plan on this one, especially if you're a woman or you sleep cold. |
| Limit | The lowest temperature at which a sleeper curled up tight is at the threshold of feeling cold. EN 13537 modelled it on a "standard man". | A fair guide for warm sleepers. For everyone else, it's your safety margin. |
| Extreme | A very low temperature at which hypothermia and harm to health are possible. The standard calls it a point of danger that can lead to death. | Never plan on it. It's a survival figure. |
ISO labels also mark the gap between comfort and limit as the "transition range", and the gap between limit and extreme as the "risk range", with a warning to expect a strong sensation of cold there.
Where the numbers come from: EN 13537 and ISO 23537
Nobody sleeps out in the cold to rate a bag. A lab puts a heated thermal manikin inside it, in a climate-controlled room, and measures how much heat escapes. A physiological model then turns that heat loss into the comfort, limit and extreme temperatures.
EN 13537 came into use in Europe in 2005. ISO 23537-1 was first published in 2016 and replaced it, and the current edition is ISO 23537-1:2022, which tightened the test conditions so results are more consistent between labs.
A few details of the test matter when you read a label:
- EN 13537's "standard woman" is 25 years old, 1.60 m tall and 60 kg. Its "standard man" is 25, 1.73 m and 73 kg. If you're older or smaller, or tired and underfed after a long day, you'll probably feel colder than they would.
- In the current ISO edition, the manikin wears a thin two-piece base layer and knee-length socks. Nothing warmer.
- It lies on a foam mat with a thermal resistance of 0.85 m²·K/W. That's roughly R 4.8 in the units mat makers quote. It's measured by a different method, so treat the conversion as a ballpark, but it's a well-insulated mat: warmer than most summer mats.
- ISO 23537-1:2022 covers bags with a limit temperature of −20°C or warmer, and its manikin test is written for mummy-shaped bags. Ratings for quilts, envelope bags and expedition bags colder than that sit outside what the test was designed for. We cover what that means for quilts in quilt vs sleeping bag.
Why women and cold sleepers should use the comfort figure
The comfort figure describes someone lying relaxed and just not cold. The limit describes someone curled into a ball to hold in heat, and also just not cold. Neither describes a warm, deep sleep.
Because comfort was modelled on a woman and limit on a man, many people read the limit as "the men's rating". It's a risky shortcut. Plenty of men sleep cold, and on a long walk most people sleep colder on night four than on night one.
Our advice is the same for everyone: plan on the comfort figure, and if you know you feel the cold, add a 5°C margin. The limit then tells you how much buffer you have on the night the forecast gets it wrong.
Reading a real listing: two of our bags
The maker rates our Cozy 300 at comfort 0°C and limit −4°C, for 400 g of 80/20 duck down. Read that as a bag for nights down to about 0°C if you sleep warm, with −4°C as a buffer you shouldn't plan on. So we plan on about +3°C.
For our Goose Down Mummy Sleeping Bag, we give our own planning guide for each fill weight: about +8°C with 400 g of down, −1°C with 800 g and −8°C with 1,200 g. Three times the down, roughly sixteen degrees warmer.
None of the bags we sell has an EN or ISO lab report, which is why we also publish the fill weight of every bag. Grams of down is a physical number you can compare between brands, and some brands don't rate to either standard at all, using their own scales instead.
What changes the number on the night
Your mat
The rating assumes a mat of roughly R 4.8 under you. The down beneath your body is crushed flat and does very little, so on a thin summer mat of about R 2 you'll lose heat into the ground and feel colder than the label says. On frosty ground, use a mat of R 4 or more, and closer to R 5 if you want the bag to live up to its label.
Our guide to mat R-values covers what to use when, and our sleeping mats list each maker's R-value.
What you wear
The test manikin wears thin thermals and socks. A dry beanie, dry socks and a light down jacket add real warmth, though no one can give you a reliable figure in degrees. Don't wear so much that you press the down flat from inside, and never sleep in clothes that are damp from the day.
A liner keeps the down clean and adds a little warmth. Our ultralight liner is mostly about keeping the bag clean; the fleece liner adds real warmth.
Where you pitch
Wind through the tent, a damp night, or a camp in a valley where cold air settles on clear, still nights will all make a bag feel colder. Forecasts are for towns, and camps in hills and valleys can be several degrees colder.
You
Eat before bed, drink enough water, and get into the bag warm. A bag holds heat in; it can't make it.
What rating to pick for Australian conditions
These are the comfort ratings we'd plan on, based on long-term Bureau of Meteorology station records. Our region-by-region guide has the numbers behind them.
- Summer on the coast from about Sydney north: comfort around 10°C.
- Summer on the Victorian and Tasmanian coasts: comfort around 5°C.
- Winter in the Blue Mountains or the Grampians, and summer in the high country: comfort around 0°C.
- Autumn and spring in the Alps, Tasmania's alpine walks, and the Red Centre in winter: comfort −5°C to −10°C.
- Winter in the Alps and snow camping: comfort −10°C or colder, on a mat of R 5 or more.
Snow camping takes experience and gear well beyond a warm bag: a four-season tent, navigation skills and a plan for bad weather. Before any alpine or winter trip, check the park's alerts and the forecast for where you'll camp.
The short version
- Plan on the comfort rating, and add 5°C if you sleep cold.
- Treat the limit as your margin. Ignore the extreme.
- Match your mat to the season: the rating assumes a warm one.
- Compare fill weight as well as the label.
- If you're torn between two bags, take the warmer one.
Our sleeping bag guide walks you through the choice, and every one of our down sleeping bags lists the maker's comfort and limit figures with its fill weight.
Sources
- ISO 23537-1:2022, Requirements for sleeping bags, Part 1: definitions, scope, test garments and test mat.
- EN 13537: the standard man and woman, and the extreme rating.