Sleeping bag temperature ratings, explained honestly
A 30-degree bag does not mean comfortable at 30 degrees. What the numbers mean, and how to actually stay warm.
Comfort rating vs. limit rating
Most bags carry two numbers. The comfort rating is the temperature at which a person sleeping in a relaxed position is expected to stay comfortable. The lower limit is where a person curled up is expected to survive the night without hypothermia — not to sleep well.
Marketing usually leads with the lower number. As a working rule, add 10 to 15 degrees to a bag's headline rating to get a temperature you would actually enjoy.
The pad matters more than the bag
Insulation only works when it is lofted. The moment you lie down, the fill underneath you compresses to nothing and stops insulating almost entirely.
This is why a cheap bag on a good pad outperforms an expensive bag on bare ground every time. The ground pulls heat out of you far faster than air does. If you are choosing where to spend, the pad comes first.
Down vs. synthetic, in plain terms
Down is lighter, packs smaller, and lasts longer if you look after it. It also loses most of its insulating ability when wet and takes a long time to dry.
Synthetic fill is heavier and bulkier for the same warmth, but keeps insulating when damp and dries fast. For car camping in a humid climate, or for anyone new to camping, synthetic is the more forgiving choice.
Shape changes the numbers
A mummy bag is warmer than a rectangular bag of the same fill because there is less air inside for your body to heat. A hood matters more than most people expect — a large share of heat loss happens through your head and neck.
Rectangular bags unzip flat into a quilt, which is genuinely useful on warm nights. If most of your camping is summer weekends, that flexibility can be worth more than the extra warmth.
Five habits worth more than a warmer bag
Eat something before bed. Digestion generates heat, and going to sleep hungry in the cold is a common mistake.
Change into dry clothes, including socks. Damp base layers wick heat away all night.
Vent before you get cold. Sweating inside a bag makes the insulation damp and you colder later.
Use a liner. A simple fleece or silk liner adds real warmth and keeps the bag clean.
Put tomorrow's clothes in the bag with you so you are not putting on cold layers in the morning.