Sleeping bag temperature ratings explained

Q: What do sleeping bag temperature ratings actually mean?

A bag tested to ISO 23537 carries three numbers. Comfort is the temperature at which a standard cold-tending sleeper rests comfortably lying relaxed on their back — this is the number to buy by. Limit is where a warm-tending sleeper, curled up, can still get through the night without waking from cold. Extreme is a survival figure only: roughly six hours without hypothermia, with frostbite and misery entirely on the table. The big number on the packaging is usually limit, not comfort.

By Marius Schöpplein · updated July 2026

The headline number on a sleeping bag is usually its limit rating, not the temperature you will sleep comfortably at. Buy by the comfort rating and you will rarely be cold.

Sleeping bag laid out on a mountain meadow at dusk with hills in the background

What do the three numbers on a sleeping bag label mean?

Bags tested to the ISO 23537 standard (the successor of EN 13537) carry three temperatures. Comfort is where a standard cold-tending sleeper rests comfortably, relaxed on their back. Limit is where a warm-tending sleeper curled up can still sleep without waking from cold. Extreme is a survival figure — six hours without dying of hypothermia, not without frostbite or misery. Never plan a trip around the extreme number.

Which number to buy by

If you sleep cold, are slim, are a woman, or simply want to enjoy your nights: buy by comfort. If you sleep notably warm you can shop by limit and save some money and weight. Marketing loves the limit figure because it is 4–6 °C more flattering, so a "0 °C bag" is very often a bag whose comfort rating is around +5 °C. Always find the actual comfort line in the specs.

What the lab test assumes

The ISO test dresses a heated manikin in a base layer and a hat and lies it on an insulated pad. That means the rating already assumes you wear dry base layers to bed and — crucially — that your pad's R-value is up to the temperature. A cold pad invalidates the bag's rating entirely: the system is only as warm as its weakest layer.

Real nights are colder than lab nights

Humidity, wind pressing on the tent, an exhausted or underfed body, and damp down all push your effective rating down. A sensible rule for planning: take the comfort rating and add a 5 °C buffer to the coldest night you expect. It is far easier to vent a too-warm bag than to shiver a too-cold one warmer.

Quilts and unrated bags

Many quilts and cottage-brand bags are self-rated by the maker rather than ISO tested. Good makers state honest comfort-style figures; cheap no-name bags routinely overstate by 10 °C or more. If a bargain bag claims "−10 °C" at a price and weight that seems too good, the number is fantasy.

Down needs to stay dry

Whether a rating survives the week also depends on the fill: down loses loft as it absorbs body moisture night after night, synthetic cares much less. Our down vs. synthetic guide covers that trade-off.

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Sleeping bag temperature rating FAQ

What do sleeping bag temperature ratings actually mean?

A bag tested to ISO 23537 carries three numbers. Comfort is the temperature at which a standard cold-tending sleeper rests comfortably lying relaxed on their back — this is the number to buy by. Limit is where a warm-tending sleeper, curled up, can still get through the night without waking from cold. Extreme is a survival figure only: roughly six hours without hypothermia, with frostbite and misery entirely on the table. The big number on the packaging is usually limit, not comfort.

What is the difference between EN 13537 and ISO 23537?

They are the same test lineage. EN 13537 was the European standard introduced in 2005; ISO 23537 replaced it in 2016 and is now the international version. Both put a heated manikin in a base layer and hat on an insulated pad inside a climate chamber and measure the heat needed to hold a steady skin temperature. A bag quoting EN 13537 is not using a worse test, just an older label — but a bag quoting no standard at all has not been independently tested.

Which temperature rating should women buy by?

Comfort, essentially always. The ISO standard uses two manikin profiles, and the comfort rating is derived from the colder-sleeping one, which the standard models on a woman of average build. That is not a marketing gesture — it reflects a measured average difference in how quickly people lose heat. Buying by the limit rating, which is modelled on a warm-sleeping man, is the single most common reason people end up cold in a bag that was supposedly rated for the night.

What temperature bag do I need for each season?

For European summer camping, a comfort rating around 10 °C covers most nights. For general three-season use from spring to autumn, look for comfort around 0 to 5 °C. For frosty shoulder-season nights, comfort around −5 °C. For real winter and snow, comfort of −10 °C or below. Always compare comfort figures against comfort figures — a bag advertised as a 0 °C bag on its limit rating is really a 5 °C bag.

Why am I cold in a bag rated for the temperature?

Nearly always the pad. The lab test lies the manikin on a well-insulated pad, so the bag’s rating silently assumes your pad’s R-value matches the night — pair a winter bag with a summer pad and you have a summer sleep system. After that, the usual causes are going to bed in damp clothing, going to bed underfed or exhausted, wind pressing the tent fabric against the bag, and down that has quietly lost loft over several humid nights. Real nights run colder than lab nights; plan with a 5 °C buffer on the comfort rating.

Are cheap bags’ temperature ratings honest?

Often not. ISO testing costs money, so budget and no-name brands frequently skip it and print a number chosen by marketing — overstatements of 10 °C or more are routine. If a bag claims −10 °C at a price and weight that seem impossible, the number is fantasy. Look for an explicit comfort figure, a named standard, and a stated fill weight and fill power; a bag that hides all three is telling you something.

Can you make a sleeping bag warmer than its rating?

Yes, by a useful margin and cheaply. A better pad is the biggest single gain. After that: a silk or fleece liner adds roughly 3–5 °C, dry base layers and a hat add several more, and eating properly before bed genuinely matters because the bag only traps heat you produce. Wearing a puffy jacket inside the bag works too, though only up to the point where it compresses the bag’s own loft.

Related: pad R-value (a bag rating assumes you got this right), down vs. synthetic, and quilt vs. sleeping bag.