Sleeping Bag Temperature Ratings: What the Numbers Really Mean

A sleeping bag’s temperature rating is the most quoted number in camping gear and one of the most commonly misunderstood. The confusion is not your fault: most bags carry three temperature figures, they are defined by an international standard that is rarely explained in shops, and none of them describes the temperature at which you will be comfortable.

Here is what the numbers actually mean.

1. Three numbers, three different sleepers

The international standard for sleeping bag ratings is ISO 23537, which replaced the earlier European standard EN 13537. It defines three figures, measured with a heated mannequin wearing a base layer and socks:

Comfort — the temperature at which a standard female sleeper can maintain a comfortable night’s sleep in a relaxed posture. The standard models this sleeper as cooler than average.

Limit — the equivalent figure for a standard male sleeper in a curled posture. Because the posture is curled and the model is warmer-running, this number is always lower than comfort.

Extreme — a survival figure. It describes the temperature at which a sleeper is at serious risk of hypothermia over a short exposure. It is not a sleeping temperature and should not be used for planning.

The practical translation: if you run cold, or you are a woman, plan around the comfort number. If you run warm, the limit number is a reasonable guide. Never plan around extreme.

2. Two bags, same rating, different warmth

The standard test is a laboratory procedure on a mannequin, and it can only model so much. Real-world warmth depends on factors the test cannot capture:

• How you sleep. Side sleepers move more and create drafts. Back sleepers stay still but compress the insulation underneath them.
• How you eat and hydrate. A meal before bed and a full water bottle give your body fuel to burn. Dehydration makes you cold.
• Whether you went to bed cold. If you get into a bag already chilled, it takes a long time to warm up. A short walk or a hot drink before bed helps more than a thicker bag.
• Drafts. A hood that does not seal, or a draft collar that does not close, lets warm air escape all night. This is a bigger deal than several degrees of rating.

Two bags with the identical laboratory number can feel very different in a tent.

3. The line everyone forgets: your pad

This is the part that catches people out most often.

Under your body, the bottom of the sleeping bag is compressed by your weight. Compressed insulation holds almost no air, and air is what insulates. So from below, your bag provides almost nothing. Your sleeping pad does the whole job.

That is why pad insulation is measured separately, using R-value (thermal resistance) under ASTM F3340. A higher R-value means more resistance to heat loss into the ground.

The practical consequence: if you buy a bag rated for a cold trip but sleep on a thin summer pad, you will be cold, and you will blame the bag. Cold ground pulls heat out of you continuously, and no bag can compensate for it.

Rough guidance on matching pad to conditions:

• Summer, warm ground: low R-value is often enough.
• Three-season: a mid-range R-value is the common target.
• Winter or frozen ground: a high R-value, often achieved by stacking two pads.

If you need to choose where to spend extra money, the pad is usually the better investment — it also works under any bag you own.

4. Fill: down and synthetic, in practice

Down gives the most warmth per gram and packs down smallest. Its performance depends on fill power (how much space a given weight of down occupies) — higher fill power means more loft for the weight. Down loses loft when wet and dries slowly.

Synthetic fill keeps insulating when damp and dries faster. It is bulkier and heavier for the same warmth, and it loses loft over years of use faster than well-cared-for down.

For damp, humid climates and for anyone who will not reliably keep a bag dry, synthetic removes a genuine risk. For cold, dry conditions and for weight-conscious trips, down wins.

5. Add a margin, then verify it

A reasonable planning method:

1. Find the expected overnight low for where and when you are camping — not the town forecast, the actual site.
2. Choose a bag whose comfort rating is at or below that low.
3. Add about 5°C of margin if you sleep cold, if you will be tired, or if you have been underfed or dehydrated.
4. Check your pad’s R-value matches the ground conditions.
5. Take a lighter bag and a warm layer for the first trip, so you can add insulation rather than lie awake.

Once you have used a bag in real conditions, you will know your own offset from the label. That personal number is more reliable than any rating.

6. Care that preserves the rating

A bag’s rating only holds while the insulation still lofts. Three habits protect it:

• Store loose. Never store a bag in its stuff sack. Compression over weeks permanently reduces loft.
• Wash rarely and correctly. Use a proper down or synthetic wash, rinse thoroughly, and dry on low heat with clean tennis balls to break up clumps. Never dry-clean down.
• Dry it before storage. Even a slightly damp bag stored away will mildew.

Treat a good bag well and it will hold its rating for many seasons. Abuse it and the label becomes fiction.

The short version

• Plan around comfort, not limit, and never around extreme.
• Your pad decides whether the bag’s rating means anything from below.
• Down for cold and dry; synthetic for damp and forgiving.
• Add roughly 5°C of margin, and store the bag loose and dry.