Sleeping Bag Temperature Ratings, Explained
An ISO sleeping bag rating gives a comfort temperature and a lower limit; cold sleepers go by comfort, warm sleepers by the limit, both with a margin.
By Adolph GreenContributing writer.
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The short answer
Most quality sleeping bags carry ratings from the ISO 23537 test. It gives a comfort temperature, a lower limit and an extreme figure. Pick a bag whose comfort rating (if you sleep cold) or lower limit (if you sleep warm) is at or below the coldest night you expect, with a margin to spare. The number in a bag's name may be only one of these figures, so read the full rating on the spec sheet.
How the ISO test works
A heated manikin wired with sensors and dressed in a base layer is zipped into the bag, laid on a foam pad and placed in a cold chamber. As the air cools, the lab measures how much energy keeps the manikin warm. A bag that needs a lot of energy input is not very warm. Therm-a-Rest notes that the European EN standard arrived in 2005, and the ISO version replaced it in 2017.
Comfort, limit and extreme
| Rating | What it describes |
|---|---|
| Comfort | Lowest temperature at which a standard woman in a relaxed posture does not feel cold |
| Lower limit | Lowest temperature at which a standard man, curled up against the cold, stays in balance without shivering |
| Extreme | Survival only: a strong risk of hypothermia; emergency use |
Therm-a-Rest explains that the published numbers sit at the bottom of three zones: comfort, transition and risk. Read them as ranges rather than one exact temperature.
Sea to Summit gives a real example: its Spark 15°F bag has a comfort rating of 29°F and a lower limit of 15°F. So a bag named "15 degrees" is comfortable for many people only down to about 29°F.
What temperature sleeping bag do I need?
- Find the lowest night temperature where and when you will camp.
- If you tend to sleep cold, match that to the comfort rating. NEMO and Sea to Summit both point cold sleepers, and most women, to this figure.
- If you sleep warm, the lower limit is a fair guide.
- Leave a safe margin between the expected low and the rating, as NEMO advises, because the test assumes more than many campers bring.
That last point matters. Sea to Summit says the test manikin wears heavyweight thermal layers and a cap and lies on a pad with an R-value of 5.38, more insulation than many pads give. With lighter clothes or a thinner pad, expect the bag to feel colder than its rating. NEMO adds that women's bags often hold extra insulation, making them roughly 10 to 15°F warmer than men's bags with a similar rating.
Mummy vs rectangular sleeping bag
| Mummy | Rectangular or relaxed | |
|---|---|---|
| Shape | Narrow, tapers to the feet | Roomy, wider at the hips and feet |
| Warmth for weight | Higher: less empty space to heat | Lower: more air to warm |
| Room to move | Tight; side sleepers may compress the fill | Easy to turn and bend knees |
Therm-a-Rest's advice cuts both ways: you need room to move, but a bag that is too roomy wastes heat on empty space, and one that is too small compresses the insulation and makes cold spots. Sea to Summit warns that side sleepers can find a classic mummy too tight and sells shapes from relaxed mummy to tapered rectangle for that reason.
Shopping for sleeping bags on Wiki Shopping
Sleeping bags sit in the sports and outdoors department. On the store page, look for the ISO comfort and limit figures, not only the number in the name.