Guide 4 min
What R-value do I need for my season and altitude?
R4 below 0 °C, R5 below -10 °C: fixed thresholds. Give your season and altitude, we work out your night and the R-value you need.
The R-value is how well your sleeping mat resists the cold rising from the ground. The rule comes down to two thresholds: R ≥ 4 as soon as the night drops below 0 °C, R ≥ 5 once it drops below -10 °C. Above that, R ≥ 2 covers you down to 10 °C, and R ≥ 1 is enough when it's mild.
What's left is working out the night you'll actually spend out there. Your season sets a baseline, your altitude cools it down: take off 6 °C for every 1000 m. A night that starts out mild can drop below zero once you're up high, and it's that figure, not the season on the calendar, that decides your R-value.
Read an R-value as thresholds, not degrees
You'll often see the idea that an R4 "equals" some precise temperature. That's wrong, and it leads you astray. An R-value doesn't convert into degrees: it's a floor. Below 0 °C, you take at least R4; below -10 °C, at least R5. The mat can do better, never less.
Keep the direction of reading in mind: you start from your night, you see which band it falls into, you read off the minimum R. Not the other way round. Trying to translate "R4" into an exact temperature is the wrong calculation.
From your season to your real night, altitude takes off 6 °C per 1000 m
Your season gives you an expected night. Altitude cools it down, mechanically: 6 °C less for every 1000 m. It's the only correction to apply, but it changes the whole band.
Take a night forecast at around 0 °C at the trailhead (a rough figure). At 1500 m, altitude takes off 6 × 1.5 = 9 °C: your real night drops to -9 °C. You stay above the -10 °C threshold, so R ≥ 4. Climb to 1700 m and you're down to -10.2 °C: you cross the threshold, and you need R ≥ 5. Two hundred metres of climb, one band more.
The four R-value bands
Once your real night is worked out, it falls into a single band:
| Real night expected | Minimum R-value |
|---|---|
| Below -10 °C | R ≥ 5 |
| Below 0 °C | R ≥ 4 |
| Down to 10 °C | R ≥ 2 |
| Above 10 °C | R ≥ 1 |
That's all. No grey zone, no average to work out: the night is in one band or the next, and you take the R shown. A mat below the threshold lets the cold of the ground through, even with a good down bag on top.
Too much R is volume carried for nothing
The opposite reflex is out there too: always take the warmest mat, just in case. It costs you. A sleeping mat packs down at 6 litres per kilo: the thicker and more insulating it is, the more it weighs and the more room it takes up in your bag. Aiming for R ≥ 5 for a night at 5 °C means carrying volume you'll never use.
The mat never works alone either: you read it together with your sleeping bag, and we look at the pair when the night drops, in sleeping bag and mat for a cold night in the wild. The right R is the smallest one that covers your real night.
What to remember
- Two thresholds are enough: R ≥ 4 below 0 °C, R ≥ 5 below -10 °C. Otherwise R ≥ 2 down to 10 °C, R ≥ 1 above.
- Work out your real night: start from your season, take off 6 °C per 1000 m of altitude. That figure is what picks the band.
- An R-value is a floor, not a conversion into degrees. The smallest one that covers your night saves you useless volume.
An R-value below the threshold, and the cold of the ground comes through: you spend the night with your teeth chattering despite a good down bag. Give your season and your altitude to the configurator: it works out your real night and hands you the R-value you need, not too much, not too little.
A guide WildKit keeps up to date. Published .