GUIDE · gear choice
A sleeping bag may have “20°F,” “-5°C,” or another impressive number in its name. That number does not necessarily mean “I will sleep comfortably at this temperature.” Standardized test specifications commonly publish separate Comfort and Lower Limit values, while real-world comfort also depends on the sleeping pad, conditions, and the sleeper.
A better question is: what is the lowest overnight temperature I realistically expect, and what does my whole sleep system look like?
Start by separating Comfort from Lower LimitCurrent specifications for ISO-tested bags publish the values separately. A bag marketed around “30°F,” for example, can have a Tested Comfort of 38°F and a Tested Lower Limit of 29°F. That is not an error: the values describe different reference thresholds.Comfort is the more conservative first reference, especially if you tend to feel cold at night.Lower Limit should not automatically be read as “comfortable for everyone.”If a product page shows only one large marketing number, look for the full technical specification before comparing models.Two bags with similar names can have different Tested Comfort values, so “the lower number is always better” is not a sufficient comparison rule.
The sleeping bag is only part of the systemPublished descriptions of ISO testing include a base layer, socks, and an insulated sleeping surface. The practical consequence is important: the sleeping bag and the insulation beneath you work as a system.A sleeping bag with an appropriate tested temperature reference.A sleeping pad or mattress with insulation suited to the conditions.Dry sleep clothing.Shelter appropriate to the weather and terrain.The label on the bag alone does not describe every condition in which you will sleep.
Start with the overnight low, not the daytime forecastCheck the expected overnight low for the place and elevation where you will sleep, and refresh the forecast shortly before departure. Avoid choosing exactly “on the line” simply because two numbers happen to match. Humidity, wind, insulation from the ground, personal cold sensitivity, and forecast uncertainty can all change real-world comfort.Keeping some margin is sensible, but there is no proven universal rule such as “always add exactly 10°C.”
Example: why one number is not enoughImagine an expected overnight low around 3°C. Model A is marketed with a prominent number around 0°C, but its full specification shows a Comfort value noticeably above 0°C. Model B has a similar marketing name, but publishes a Comfort value closer to the expected overnight low and a separate, lower Lower Limit.The names make the bags look similar. The complete tested values reveal which is the more relevant starting point for the comfort level you seek. It is still not a guarantee: your sleeping pad, clothing, shelter and personal response to cold remain part of the decision.
What to check on a specific product pagewhether standardized/ISO temperature testing is stated;whether separate Comfort and Lower Limit values are published;bag size and shape;insulation type and amount;weight and packed volume if you will carry the equipment;manufacturer use and care guidance.
“Women's” and “men's” ratings are not a sufficient explanationRetail descriptions sometimes explain Comfort through a “cold sleeper” and Lower Limit through a “warm sleeper.” Brands also differ in which value they emphasize in the product name. Reading the actual tested values is more useful than assuming one temperature automatically applies to all women or all men.
A quick decision in five questions1. What is the realistic overnight low?Check the location and refresh the forecast before departure.2. What is the Tested Comfort value?It is a better first comfort reference than one marketing number.3. How well are you insulated from the ground?The bag and sleeping pad work together as a system.4. Do you normally feel cold at night?If so, plan more conservatively.5. Is there a sensible margin?On a more demanding trip, margin is part of planning rather than merely extra weight.
Sources and information boundariesCurrent product specifications and REI's temperature-rating guide checked on September 9, 2026 show separate ISO Tested Comfort and Tested Lower Limit values and illustrate why a product's marketing name is not enough for a decision.REI Expert Advice — Sleeping Bag Temperature RatingsREI — Big Agnes Greystone 30REI — Exped Terra 30REI Co-op Magma 30REI — Marmot Bantamweight 15Boundary: this is a guide to understanding temperature ratings. It is not a guarantee of personal comfort and does not define survival or medical-safety thresholds.
