Sleeping bags are unusual in outdoor gear: there is an actual international standard for temperature rating. ISO 23537 (which replaced EN 13537) puts a bag on a heated mannequin and produces two numbers — a comfort rating, at which a standard cold sleeper is comfortable, and a limit rating, at which a standard warm sleeper is curled up and just about not shivering.
The number printed large on the stuff sack is usually the limit. That gap is why so many people are cold in a bag rated ten degrees below the night's low. As a working rule, buy a bag rated 10 to 15°F below the coldest temperature you actually expect.
And note what happens when a bag quotes no rating at all. In the one category where a comparable standard exists, declining to state a number is the most informative thing a listing can do.
Choosing a bag
Down versus synthetic
Down has a far better warmth-to-weight ratio and compresses dramatically smaller, which is why backpackers buy it. It also loses most of its loft when wet and dries slowly. Synthetic is heavier and bulkier and keeps insulating when damp, which is why it is the right choice for canoe trips, humid climates and anyone hard on gear.
Fill power measures down's loft per ounce: 550 is entry-level, 800+ is premium. Higher fill power means the same warmth in a smaller, lighter package — it does not mean warmer by itself.
Mummy versus rectangular
A mummy cut with a hood and a draft collar is substantially warmer for the same fill, because there is less air to heat and less exposed head. A rectangular bag is more comfortable to move in and considerably colder. If you dislike mummy bags, buy a warmer rating rather than fighting the cut.
The pad matters as much as the bag
The insulation under you is compressed flat by your body weight and does almost nothing. Ground conduction is the main heat loss in a tent, and the sleeping pad's R-value is what stops it. A 0°F bag on a summer pad will leave you cold.
Storage kills more bags than use
Never store a bag compressed. Long-term compression permanently damages loft in both down and synthetic. Hang it or keep it in the large mesh storage sack it came with, and use the compression sack only for travel.
How these were picked
I haven't handled these. Here's what I did instead.
Everyone in this category says they tested twenty of these. I haven't tested any, and I say so. What I do instead: read the manufacturer's manual and published specification, compile the numbers that decide the purchase, do the arithmetic in the open, check the live price, and name one product to skip. The scores above are judgments from that research against a published rubric — they are not measurements I took, because I do not have a lab and I am not going to pretend otherwise.
