Add your bag's rating and the low to get an answer
Your answer shows here as soon as both are in.
This is an estimate. Check the National Weather Service forecast (opens in a new tab) before you go.
How this works
- Comfort and limitBags tested to ISO 23537 list a comfort rating (a relaxed sleeper stays warm) and a limit rating (curled up, warm enough for 8 hours). We never use the extreme rating: it’s for survival, not sleep.
- Pad R-valuePads list an R-value from the ASTM F3340 test. Higher is warmer, and stacked pads add up. Bag ratings assume a pad of about R 4.8.
- Wind and shelterThe tool doesn't adjust for wind or shelter yet. Pick a sheltered spot on a windy night.
- What we can't knowThe forecast covers the area around a town, not your campsite, and people and bags vary. A quilt? Enter its rating like a bag’s.
See every rule and its source
How we decide your mark
| You sleep | We compare the low with | Source |
|---|---|---|
| Cold | the comfort rating | Backpacking Light, Standards Watch: Sleeping Bag Temperature Ratings |
| About average | the comfort rating | Our rule of thumb |
| Warm | the limit rating | Backpacking Light, Standards Watch: Sleeping Bag Temperature RatingsOur rule of thumb |
Pad R-value for the night
| Nights down to | At least | Source |
|---|---|---|
| 50°F | R 1 | Our rule of thumb |
| 30°F | R 2 | Our rule of thumb |
| 25°F | R 3 | Our rule of thumb |
| 10°F | R 4 | Our rule of thumb |
| 0°F | R 5 | Our rule of thumb |
| −15°F | R 6 | Our rule of thumb |
| −25°F | R 7 | Our rule of thumb |
| Colder | R 8 | Our rule of thumb |
Wind words
| Forecast wind from | We call it | Source |
|---|---|---|
| 0 mph | calm | National Weather Service, forecast terms |
| 15 mph | breezy | National Weather Service, forecast terms |
| 20 mph | windy | National Weather Service, forecast terms |
One number on the bag: we treat it as the limit rating and don't guess the other one, so the answer stays cautious. A limit below −4°Fis outside the ISO test’s range.
Sources & last checked
- ISO 23537-1:2022, sleeping bag requirements (opens in a new tab)
checked
- Backpacking Light, Standards Watch: Sleeping Bag Temperature Ratings (opens in a new tab)
checked
- NEMO Equipment, the ASTM F3340 R-value standard (opens in a new tab)
checked
- National Weather Service, forecast terms (opens in a new tab)
checked
- Henricks Outdoors rule of thumb (awaiting owner approval) (opens in a new tab)
checked
- National Weather Service API (opens in a new tab)
Overnight lows and wind for the sleep check
Source: ISO 23537-1:2022, sleeping bag requirements, Backpacking Light, Standards Watch: Sleeping Bag Temperature Ratings, NEMO Equipment, the ASTM F3340 R-value standard, National Weather Service, forecast terms, Henricks Outdoors rule of thumb (awaiting owner approval), National Weather Service APIlast checked
Every source we use is on the Credits page.
Related guides
- Choosing the Right Sleeping Bag and Pad for CampingHow to pick a sleeping bag and pad as a system: temperature and ISO/EN ratings, down vs. synthetic insulation, bag shapes, pad types, and R-values, plus why your bag's rating only works when your pad's R-value matches the season.
- First Frost Camping: How to Sleep Warm When Nights Drop Below FreezingThe real rules for sleeping warm in fall camping weather — insulation under you, moisture management, hot water bottles, and why your bag rating lies.
- Winter Camping Essentials for Families: Stay Safe and WarmFamily winter camping basics: choosing an accessible site, four-season tents and cold-rated sleeping bags, the three-layer clothing system, tricks for staying warm like the hot water bottle method, and spotting hypothermia and frostbite early.
About this tool
Will you sleep warm tonight? Enter your sleeping bag’s temperature ratings and your sleeping pad’s R-value, pick the night and the nearest town, and this tool compares your gear with the National Weather Service forecast low. You get a plain answer (likely comfortable, likely chilly, or likely cold) with the reasons behind it and a few simple fixes.
Most bags sold today carry ratings from the ISO 23537 test: a comfort rating and a lower limit. Pads list an R-value from the ASTM F3340 test, and higher numbers are warmer. The page explains what those numbers mean, how your shelter changes the picture, and why a cold sleeper should look at a different number than a warm sleeper.
It’s an estimate, not a promise. Forecasts cover the area around a town, and every campsite is a little different. Your gear stays saved on your device, so next time you can check a new trip in a few seconds.