Sleeping Bag Temperature Calculator
Enter the lowest temperature forecast at your campsite for the night
Cold sleepers need a warmer bag; warm sleepers can use a less warm bag
Women use the EN/ISO Comfort rating; men use the Lower Limit rating
Enter Your Trip Conditions
Fill in the overnight low temperature and your camping conditions to get a personalized sleeping bag temperature rating recommendation.
How to Use This Calculator
Enter Your Overnight Low Temperature
Start by entering the coldest temperature forecast for your campsite on any planned night of your trip. Use the unit toggle to switch between Fahrenheit and Celsius. If you are unsure, check a mountain weather service or contact the land management agency — always use the lowest expected temperature, not the average.
Set Your Personal Profile and Basic Conditions
Select your sleeper warmth type (cold, average, or warm sleeper) and gender — these determine whether your recommendation targets the EN/ISO Comfort or Lower Limit rating. Then choose your shelter type (tent, bivy, hammock, or open camp) and humidity level. These three factors create the largest adjustments to your required bag rating.
Use Advanced Options for Precision
Click 'Advanced Options' to enter your campsite elevation, clothing layers you plan to wear inside the bag, any sleeping bag liner you own, your sleeping pad's R-value, and a safety buffer. Each field has an information note explaining its effect. The altitude input automatically applies the -3°F per 1,000 ft adjustment. Set a conservative or extra-safe buffer for beginners, high-altitude trips, or conditions you are unfamiliar with.
Review Your Results and Check Your Current Bag
The results show your required bag rating, the EN/ISO Comfort and Lower Limit targets, season classification, pad adequacy status, and a visual temperature zone bar. If you already own a bag, expand 'My Bag Comparator' in the inputs, enter your bag's current Lower Limit rating, and the sleep system assessment will tell you whether your existing bag plus liner and clothing is sufficient — and by exactly how much margin.
Frequently Asked Questions
What is the difference between the EN/ISO Comfort and Lower Limit ratings?
Both ratings come from the same ISO 23537-1:2022 standard but are tested on different reference subjects. The Comfort rating is the lowest temperature at which a standard female (5'3", 132 lbs, 25 years old, relaxed supine position) can sleep a full night without shivering. The Lower Limit is the lowest temperature at which a standard male (5'8", 154 lbs, 25 years old, curled position) can sleep a full night without shivering. The Comfort rating is approximately 8–12°F (5°C) colder-rated than the Lower Limit for the same bag — meaning the Comfort-rated bag keeps you warmer. Women and cold sleepers should shop using the Comfort rating; men and warm sleepers can use the Lower Limit as their primary reference.
Why does my bag feel colder than its rated temperature?
Several common factors cause a bag to perform colder than its label rating. The ISO standard test uses a brand-new, properly lofted bag on an R-4.85 pad with specific test clothing in a climate-controlled room — conditions rarely matched in the field. An old bag with compressed or clumped insulation loses significant warmth. A sleeping pad with insufficient R-value lets cold seep up from the ground, which is responsible for up to 30% of heat loss. Wet or humid conditions degrade down insulation dramatically. Not wearing appropriate base layers, being fatigued, under-hydrated, or having eaten too little before sleeping also reduces metabolic heat output. This calculator accounts for the environmental factors; make sure your gear is properly lofted and dry.
How much warmth does a sleeping bag liner actually add?
A sleeping bag liner adds meaningful warmth depending on material. A thin silk or polyester liner adds approximately 5°F (3°C) of warmth. A standard cotton or mid-weight synthetic liner adds around 10°F (5°C). A fleece or thermal liner can add 15°F (8°C) or more. Some specialty thermal liners marketed as 'overbags' can add 20–25°F (11–14°C). Liners also protect bag insulation from body oils and moisture, extending the life of an expensive bag. In wet conditions, note that a liner adds warmth but does not fix the problem of down insulation losing effectiveness — consider the bag material, not just the liner.
What sleeping pad R-value do I need for different seasons?
R-value measures a sleeping pad's resistance to heat conduction into the ground. For summer camping above 35°F (2°C), even a basic foam pad at R-1.0 to R-2.0 is adequate. For three-season camping (15–35°F / -9 to 2°C), a minimum R-3.0 to R-4.0 insulated inflatable pad is recommended. For winter camping (0–15°F / -18 to -9°C), R-5.0 is the accepted minimum standard. For expedition or extreme cold below 0°F (-18°C), R-7.0 or higher is required, and many winter campers use two pads stacked for combined R-values. An inadequate pad is one of the most overlooked causes of a cold night — every 1-unit R-value deficit reduces effective warmth by approximately 5–10°F.
Should I choose down or synthetic insulation for wet conditions?
This is a critical gear decision that the calculator's insulation warning flags for wet conditions. Down insulation provides superior warmth-to-weight ratio when dry but loses approximately 70–80% of its insulating effectiveness when wet because the down clusters clump together and stop trapping warm air. Synthetic insulation (such as PrimaLoft or Climashield) retains approximately 80% of its warmth when wet because the synthetic fibers maintain their structure even when soaked. For reliably wet environments — coastal ranges, the Pacific Northwest, rainy season trips — a synthetic fill or water-resistant down (WR-treated down) bag is strongly recommended over untreated down. Water-resistant down performs significantly better than untreated down in humid conditions but still degrades faster than synthetic when fully saturated.
What does the Extreme rating mean and when can I use it?
The Extreme rating on an EN/ISO certified sleeping bag represents the temperature at which a standard female can survive for six hours in a curled position without freezing to death — but with significant risk of hypothermia and frostbite. This is strictly a survival threshold, not a comfortable or even tolerable sleeping temperature. You should never plan to camp at a temperature near the Extreme rating. The industry-wide safety guideline is to treat the Extreme rating as an emergency reference only and never use it for trip planning. For actual safe sleep, always plan around the Comfort or Lower Limit rating, then apply an additional 5–10°F safety buffer as this calculator recommends.