Warmth Calculator
Enter the forecast low and your sleep system; the calculator combines EN/ISO rating logic with pad R-value, worn insulation, wind and altitude into one temperature margin.
Result
- Adjusted camp temperature
- -27.2 °F
- Effective system rating
- 26.2 °F
- Pad adjustment
- -4.5 °C
- Clothing adjustment
- +1.5 °C
- Wind adjustment
- -0.2 °C
Planning estimate only - not safety advice.
How the calculation works
The calculator starts from your bag's comfort (or lower-limit) rating and moves it one correction at a time until it describes your actual night:
- Pad: the EN test assumes an R-5 pad. Each point of R below 5 costs about 3 °C; each point above 5 adds about 3 °C back.
- Clothing: worn insulation is credited at 3 °C per clo, up to 3 clo - enough for base layers, a light puffy and a hat.
- Wind: 0.1 °C per km/h of wind on exposed ground, scaled by shelter (tent 25%, bivy 65%, tarp 100%), capped at 8 °C.
- Altitude: 1 °C per 1,000 m above 3,000 m.
- Sleeper: cold sleepers lose 3 °C, warm sleepers gain 2 °C.
The verdict compares the adjusted rating with the forecast low: a margin of 2 °C or more reads as adequate, less than 2 °C as marginal, and anything negative as insufficient. Because the same model powers the sleeping bag temperature calculator, you can also run it in reverse to find the rating a trip needs. For the reasoning behind ratings themselves, read the EN/ISO ratings guide and the sleeping warm checklist.
Planning estimate only - not safety advice.
Frequently asked questions
Does this calculator follow the EN/ISO temperature rating standard?
It follows its logic, not its lab. EN 13537 / ISO 23537 rates bags on a thermal manikin wearing base layers, and the standard maps ratings to a standard woman (comfort) and man (lower limit). This calculator uses your rating numbers the same way, then adds the real-world corrections the lab cannot see: pad R-value, worn clothing, wind exposure and altitude.
My bag says comfort 0 °C. Can I sleep in it at 0 °C?
On the standard test pad with one base layer, marginally. In the field, a worn-out pad, a damp bag, wind or altitude can each cost several degrees. The calculator requires a margin of about 2 °C between the adjusted rating and the forecast low before it calls the system adequate - which is why it will often tell you the "same number" setup is not enough.
How much does a pad really matter?
Roughly 3 °C of bottom-loss per point of R-value in this model. A bag tested on the standard R-5 pad used with an R-2 pad effectively performs about 9 °C worse than its rating. An underrated pad is the single most common reason a "rated for it" bag fails.
Why does wind matter at night if I am inside a tent?
It mostly does not - that is why the calculator discounts wind by 75% for a tent. But under a tarp or in a bivy on exposed ground, moving air strips heat from the bag surface. The correction is capped at 8 °C because still-air nights are the norm in most conditions.
Is the altitude correction real?
It is a planning rule, not physics: systems lose roughly a degree of practical comfort per 1,000 m above 3,000 m, because cold nights, dehydration and poorer sleep compound. Treat it as one more reason to pick the warmer bag.