Sleep system builder
The calculator needs JavaScript. The worked example, sum and figures below show exactly how it works, so you can do it yourself. A guide, not advice.
Short answer
Enter the forecast low, your bag's comfort rating and the R-value figures from your mat labels. The tool compares the bag rating with the forecast and your chosen cold-sleeper margin, and compares mat values against an optional figure you provide. It does not predict personal comfort or convert mat R-value to temperature.
Worked example
| You enter | Value |
|---|---|
| Forecast overnight low | 5 °C |
| I tend to feel cold at night | Yes |
| Extra margin if you feel cold | 3 °C |
| Sleeping bag comfort rating | 2 °C |
| Main mat R-value | 3.5 |
| Additional mat R-value | 1 |
The entered comfort rating is at or below your comparison target.
| Result | Value |
|---|---|
| Sleeping bag comparison | The entered comfort rating is at or below your comparison target. |
| Comfort rating to compare against | 2.0 °C or lower (comfort rating) |
| Combined entered mat R-value | 4.5 |
| Mat comparison | Enter a comparison figure if you have one; this tool does not map mat R-value to temperature. |
Show the working
- Bag comparison target: 5 °C − 3 °C personal margin = 2.0 °C
- Compare the bag comfort rating 2 °C with the target 2.0 °C
- Entered mat figures: 3.5 + 1 = 4.5
How it works
- Bag comparison target = forecast low, with your editable margin subtracted if you tend to feel cold
- Compare the bag's comfort rating (not its limit rating) with that target
- Add entered mat R-values only when the ratings use a comparable basis
This compares figures you enter; it is not a personal comfort guarantee. Check the bag's comfort rating, not its limit rating.
Mat R-value has no universal conversion to a night-time temperature. Consider the full system and your own sleep comfort.
Our figures (adjustable)
These are our planning allowances and, once checked, typical figures with their sources. They aren't facts about your kit: change any of them in the calculator.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Extra margin if you feel cold | 3 °C | Our assumption Not yet checked against the source | Our choice: change it to suit your trip How much warmer a bag to look for if you tend to sleep cold. Our assumption, not a standard figure. |
What to buy
Shopping links will appear here once our retailer partners are confirmed. We don't rank kit by testing it: see our spec-based kit guides.
FAQs
Which sleeping bag rating should I enter?
Use the comfort rating from the bag label. The lower limit is not the comfort rating for most people.
Does a higher mat R-value guarantee a warm night?
No. It is one part of the sleep system. Ground conditions, the tent, fit, sleeping bag and personal comfort all matter.
Guides that use it
How to stay warm camping
How to stay warm camping: match your sleeping bag and mat to the forecast, keep sleepwear dry, and never bring a flame heater into a tent.