Reading a Sleeping Bag Rating: Comfort, Limit, Extreme
Three numbers, three meanings — and two of them are not the one you should buy against. Here is what the standard ratings actually measure.
A sleeping bag rating has three numbers. The comfort rating is the lowest temperature a cold-sleeping adult woman can sleep through; the limit rating is the lowest temperature a standard adult man can sleep at without waking; the extreme rating is survival only, not sleeping. Buy against the comfort rating, then subtract 9 °F (5 °C) of margin for the temperature you actually expect.
Two bags rated "20 °F" can be thirty degrees apart in real-world warmth. The rating is only comparable when it comes from the same test standard, and most shoppers never check whether it does.
The three numbers
The current standard is ISO 23537 (it replaced EN 13537 in 2016). It produces three figures, tested on a heated mannequin:
| Rating | What it means |
|---|---|
| Comfort | The lowest temperature at which the standard adult woman can sleep through the night in a relaxed posture |
| Limit | The lowest temperature at which the standard adult man can sleep for eight hours without waking |
| Extreme | Survival only, for a short period, at serious risk of injury. Never a sleeping target |
The test subject is a 25-year-old woman of 5 ft 3 in / 132 lb for the comfort figure, and a 25-year-old man of 5 ft 8 in / 154 lb for the limit figure. If you are not either of those people, the ratings are a reference point, not a measurement of you.
Buy against the comfort rating. The limit rating is the number manufacturers love to quote, and for a cold sleeper it is optimistic by a full category.
The assumption almost nobody reads
The standard assumes you are:
- wearing a base layer and socks,
- lying on a pad with an R-value of 5,
- well fed, hydrated and not exhausted.
Break any of those and the rating no longer describes your night. This is why two people in identical bags, on different pads, have completely different experiences. Your pad is half your sleep system. An R-5 pad under a 20 °F bag is warmer than an R-2 pad under a 0 °F bag.
Fill: down versus synthetic
- Down (700–900+ fill power): the best warmth-to-weight and the best compressibility. It loses most of its loft when wet, which is why down bags live in a waterproof stuff sack or a pack liner.
- Hydrophobic down: treated to resist moisture. It keeps more loft in humid, damp conditions and dries faster, at some cost in durability.
- Synthetic: keeps insulating when wet and is cheaper and easier to wash. Heavier and bulkier for the same warmth, and it packs down over years of use.
How to pick a number
- Find the expected low, not the average low. Look at the actual forecast for your dates and elevation, then plan for the coldest night plus a safety margin.
- Subtract 9 °F (5 °C) from that low and buy the comfort rating that covers it. Cold sleepers, people who run thin, and anyone sleeping above treeline should subtract more.
- Choose a women's-specific bag if the published comfort rating is a concern. Women's bags typically use a higher comfort figure, more insulation in the footbox, and a shorter length — they are not simply "men's bags in smaller sizes."
- Budget for the pad. A warm bag on a cold pad is the most common way to be cold at 30 °F in a 20 °F bag.
A rating tells you what the bag can do in a laboratory. Your pad, your dinner, your hydration and your base layer decide what happens on the mountain.