Understanding Sleeping Bag Temperature Ratings

A sleeping bag's temperature rating is an important guide when choosing a bag, but it is not a guarantee that every person will feel warm at the stated temperature. Temperature ratings are affected by the way a bag is tested, the sleeping system used, weather conditions and individual differences between sleepers.

How are sleeping bags tested?

For adult recreational sleeping bags, the current international standard is ISO 23537-1:2022. It specifies requirements and test methods for assessing the thermal performance of sleeping bags designed for limit temperatures of -20°C and higher. The standard is intended to provide a consistent way of assessing sleeping bag performance, making tested products easier to compare.

Laboratory testing uses controlled conditions, including a heated test manikin inside the sleeping bag and an insulated sleeping surface underneath it. The test measures how effectively the sleeping bag retains heat under those controlled conditions. Standardised testing helps reduce differences between manufacturers' testing methods, but it cannot reproduce every condition a camper will encounter outdoors.

The ISO standard applies to adult recreational sleeping bags. It does not cover children's sleeping bags, military-specific sleeping bags or extreme-climate expedition sleeping bags.

What do Comfort and Lower Limit mean?

Where a sleeping bag has been tested to the ISO standard, you may see separate Comfort and Lower Limit temperatures.

Comfort rating
The Comfort rating represents the lower temperature at which a standardised sleeper who is more sensitive to cold can be expected to remain comfortable.

Lower Limit rating
The Lower Limit represents a lower temperature at which a standardised sleeper who is less sensitive to cold can be expected to remain comfortable.

In simple terms, someone who normally sleeps cold should pay particular attention to the Comfort rating, while a person who normally sleeps warm may find the Lower Limit useful as a reference.

You may also encounter an Extreme rating on some sleeping bags. This is not a recommended camping temperature or a temperature at which normal comfort should be expected. It represents a much more severe cold-exposure reference and should not be used as the primary figure when selecting a sleeping bag.

What if a bag simply says "-5°C"?

Not every sleeping bag carries an ISO temperature rating.

If a manufacturer simply states that a sleeping bag is rated to -5°C, without identifying the figure as an ISO Comfort or Lower Limit rating, it should not automatically be assumed that the figure comes from ISO testing. It may be the manufacturer's own rating or estimate. Such ratings can still be useful, but they should not automatically be treated as directly comparable with an ISO-tested sleeping bag from another manufacturer.

This is particularly important when comparing camping sleeping bags from different brands. Look for the specific terminology used by the manufacturer rather than assuming that all temperature ratings have been determined using the same method.

Your sleeping bag is only part of the sleep system

Your sleeping bag does not work in isolation. The sleeping mat underneath you is particularly important, because it provides insulation between your body and the ground.

Your warmth can also be affected by:

• Your individual metabolism and whether you generally sleep hot or cold
• The clothing worn inside the sleeping bag
• The insulation and R-value of your sleeping mat
• Wind and weather conditions
• Humidity and moisture
• How well the sleeping bag fits
• Your food and hydration levels
• Your physical condition and fatigue
• Whether the sleeping bag's insulation is dry and able to loft properly

Consequently, the temperature printed on a sleeping bag should be treated as a useful guide rather than a promise of personal comfort.

Choosing the right temperature rating

When selecting a sleeping bag, consider the coldest conditions you realistically expect to encounter, rather than the average temperature of your trip.

If you tend to sleep cold, choosing a bag with a more conservative temperature rating provides additional margin. If conditions are warmer than expected, a sleeping bag can generally be opened for ventilation, whereas being inadequately insulated on a cold night is much harder to solve.

For bags with ISO ratings, use the Comfort rating as the more conservative reference if you are a cold sleeper. For bags carrying only a manufacturer's temperature rating, follow the manufacturer's stated rating but remember that it may not be directly comparable with an ISO-tested rating.

The key takeaway

A sleeping bag temperature rating is a standardised measurement where ISO testing has been used, but it is not a guarantee of how warm you personally will feel.

When comparing sleeping bags, first establish whether the stated temperature is an ISO Comfort rating, ISO Lower Limit rating, another standardised rating, or simply a manufacturer's temperature claim. Then consider your sleeping mat, clothing, expected conditions and whether you normally sleep hot or cold.