Sleeping bag temperature rating explained: comfort vs limit

A sleeping bag rating is a guide to a test, not a promise of how you will feel. Use the comfort number for your coldest night, then add a margin if you sleep cold.

The label may show two or three numbers. Here is what they usually mean, and how to use them.

What the numbers mean

Many makers test bags to a common standard. It gives a few figures:

• Comfort. The lowest temperature at which a person is expected to sleep without being cold.
• Limit. A colder number. At this level, a person may sleep curled up but will feel cold. It is not a number to plan around.
• Extreme. The coldest figure. A warning, not a target.

Not all bags are tested. Some labels use only a single number and no standard. Read the maker's notes to see what the number means.

What goes wrong first. People buy by the limit number. A bag with a limit of 0 degrees may feel cold at 5. Fix: use the comfort number.

A rule to pick the bag

Pick a bag whose comfort rating is at or below the lowest temperature in the forecast. If you tend to sleep cold, go a step lower. If you are a child, a small adult or someone who gets cold feet, also go lower.

Worked example

Say the forecast low is 5 degrees C. A bag with a comfort rating of 5 will be fine for many people. A bag with a limit of 5 and a comfort of 10 will probably feel cold. Pick the first one, or add a liner and a warm layer.

What the rating does not cover

The rating assumes you have a good pad under you, dry clothes and a bag that is fluffed up. It also assumes you are fed and rested. Things that change the real warmth:

• The pad. A thin pad lets the cold in from below. See sleeping pad R-value for cold camping.
• Wind and damp. A bag in a wet tent is colder.
• Your own body. Some people sleep warm, some cold.
• Fit. A bag with empty space is colder. Kids need a kids bag.
• Food and hydration. A fed, hydrated body makes heat.

Down or synthetic

Down is light and packs small. It loses warmth when it gets wet. Synthetic fill is heavier and bulkier, and it keeps more warmth when damp. For wet cold, many campers choose synthetic. For dry cold and for carrying a pack, down is common. Check the maker's notes for care.

Shape and features that help

• A hood you can cinch.
• A draft tube along the zip, to block cold air.
• A mummy shape for cold, which has less empty air to heat.
• A zip that does not snag.

Where the answer flips

For summer car camping, a rectangular bag with a high rating is fine and more comfortable. For winter or high ground, a mummy bag with a low comfort number is better. If you camp in many seasons, two bags can be cheaper than one that tries to do all.

For a full cold-night plan, read how to stay warm in a tent in winter.