Understanding sleeping bag temperature ratings is vital when it comes to choosing the perfect sleeping system for your adventures. With winter in full swing, many of us are eyeing up adventurous camping adventures in the cold, while some people might have wild camping as one of their 2022 new year's resolutions. Whether you are after the perfect, down-filled bag for Arctic expeditions or you’re looking for something for the warmer months, our guide to sleeping bag temperature ratings has got you covered.
Sleeping bags are important pieces of outdoor kit. The difference between a high quality sleeping bag and a cheap knock-off is the difference between a cozy night in your tent and the potential of a dangerously cold one. When it comes to choosing a sleeping bag, their labels often display a range of temperature ratings, which can be overwhelming at first.
This is because all bags, from the best sleeping bags to the most basic, go through internationally standarized testing. This is called the International Standard (ISO) and it allows campers to compare like for like when choosing their sleeping bag.
ISO 23537 establishes the range of nighttime air temperatures, measured outside the sleeping bag, at which the sleeping bag will work effectively. This range is typically illustrated in a color chart, running from red at the hottest, through amber and yellow to blue at the coldest. The maximum temperature, or upper limit, is the warmest at which an adult male can have a comfortable night’s sleep without excess sweating. Even with the best tent available, if you don't have a sleeping bag that's rated for the conditions, you may suffer.
Sleeping bag temperature ratings explained: comfort
The key sleeping bag temperature rating is called ‘Comfort’, which establishes the lowest temperature at which a camper, sleeping in a relaxed position, such as lying on their back, is ‘globally in thermal equilibrium’ – ie not feeling cold.
Sleeping bag temperature ratings explained: limit
The ISO also sets a ‘Limit’ sleeping bag temperature rating, at which a sleeping bag user in a curled-up position is ‘globally in thermal equilibrium’ and just not feeling cold.
Sleeping bag temperature ratings explained: extreme
The ‘Extreme’ sleeping bag temperature rating is only for survival without suffering frostbite, and shows the lowest temperature at which a sleeping bag may prove effective, even if this very low temperature still involves the risk of hypothermia.
Obviously, we strongly recommend choosing a bag that will keep you comfortable, as well as getting yourself one of the best sleeping pads to insulate you from the cold ground below.
It’s important to note that the ratings apply to adults only – ISO says it would be unethical to expose children and babies to such tests. The best children's sleeping bags soemtimes have their own rating, but often manufacturers don't display them. Nor do the ratings apply to sleeping bags designed for extreme climate zone conditions.
If you’re unsure of the lowest nighttime temperatures you are likely to encounter on your camping adventures, there is also a more generic labelling system, based on the seasons, just as with one-person tents. Depending on the sleeping bag manufacturer and retailer, there can be as many as six seasons in a four-season year.
One-season sleeping bags
One-season sleeping bags are for mid-summer camping at low-level altitudes, and for tropical travel. They are also good for sleepovers at home. Guides vary, but think temperatures of at least 5°C/41°F.
Two-season sleeping bags
In theory, the operating zone for two-season sleeping bags stretches from late spring to early autumn/fall in the UK, but this window narrows if you’re wild camping in the mountains, where temperatures can plummet overnight. You don't want to have forked out for a sleeping bag only to also need to wear your best down jacket to bed. A rough outside temperature guide would position the limit of two-season sleeping bags for nights of 0/32°F to 5°C/41°F.
Three-season sleeping bags
Designed to keep you warm from early spring to late autumn/fall, these sleeping bags are suited to committed campers happy to be under canvas on frost-free nights when the mercury slips as low as -5°C/23°F.
Four-season sleeping bags
These sleeping bags are insulated to keep you warm during winter camping trips at lower levels, typically to temperatures as low as -10°C/14°F. They will also cope with three seasons at higher mountain altitudes, which can become bitterly cold at night.
Expeditions to the world’s coldest climes, whether high-altitude mountaineering or even to the poles, demand the exceptional insulation of five-season (also known as four-plus season) sleeping bags to contend with temperatures as low as -40°C/-40°F. Five-season sleeping bags have the highest grade construction and insulation.
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After spending a decade as editor of Country Walking, the UK’s biggest-selling walking magazine, Jonathan moved to edit Outdoor Fitness magazine, adding adrenaline to his adventures and expeditions. He has hiked stages or completed all of the UK's national trails, but was once overtaken by three Smurfs, a cross-dressing Little Bo Peep, and a pair of Teletubbies on an ascent of Snowdon. (Turns out they were soldiers on a fundraising mission.)