What this calculator does#
It takes the number printed on your bag, converts it to a comfort-equivalent figure, then adds and subtracts the things the laboratory test does not know about: your pad, your shelter, what you sleep in, whether you ate, and how you personally sleep. The result is a temperature, a margin against your forecast low, and a ranked list of the cheapest ways to gain the next degree.
How the maths works#
Step 1: normalise the label. ISO 23537 reports three temperatures from one manikin run. If your bag quotes a comfort rating we use it directly. If it quotes only a limit rating we add 6 degrees C, which is the typical gap the standard reports between its standard man and standard woman. If no standard is quoted, we add 8 degrees C and flag the result as an estimate.
Step 2: price the pad. The required R-value at a given temperature is interpolated from these anchor points, which follow the usual season guidance: R-1.2 at 20 degrees C, R-2.0 at 10, R-3.4 at 0, R-5.0 at minus 10, R-6.5 at minus 20 and R-8.0 at minus 30. If your pad is short by more than 0.15, we apply a penalty of 2.2 degrees C per R-value of shortfall. If it exceeds the requirement we credit 0.7 degrees per R-value, capped at 2, because conduction losses are already small by then.
Step 3: apply the system adjustments. Shelter, liner, sleep clothing, metabolism, food, wind and accumulated damp, each as a fixed shift in degrees C. These are the same figures published in our adjustment table.
Step 4: compare with the forecast. Comfort temperature minus the forecast low is your margin. We call 3 degrees or more comfortable, minus 1 to 3 marginal, and anything below that too cold.
What it cannot tell you#
Human thermal comfort varies by several degrees between individuals of the same size, and that variation is larger than most of the adjustments here. This model is reliable for relative questions (am I 2 degrees short or 8, and which change closes the gap most cheaply) and only indicative for absolute ones.
It also assumes the bag is in good condition. A bag that has never been washed, or that has been stored compressed for years, has lost loft that no adjustment here accounts for. If your result says comfortable and your experience says otherwise, the likely explanations are a tired bag, a damp bag, or a night colder than the forecast because cold air pooled where you pitched.
Worked example#
A bag labelled minus 6 degrees C limit, on an R-3.2 pad, in a double wall tent, with a hat and a base layer, for an average sleeper who ate a hot dinner, on a forecast low of minus 3.
- Limit minus 6 becomes comfort 0
- At minus 3 degrees C the pad requirement is about R-3.9, so an R-3.2 pad is 0.7 short, worth about +1.5 degrees of felt cold
- Hot meal before bed, minus 1
- Everything else neutral
Comfort temperature: about 0.5 degrees C, against a forecast low of minus 3. That is a cold night, and the cheapest fix is not a new bag: it is a 350 g closed cell foam pad underneath, which more than closes the gap.
Frequently asked questions#
Can I add the R-values of two sleeping pads?#
Yes, closely enough for planning. Stacked pads add their R-values, which is why a foam pad under an air pad is the standard way to extend a three-season sleep system into freezing conditions. Put the foam pad underneath, where it also protects the air pad from the ground.
Why does the calculator penalise a bag rated by the manufacturer only?#
Because that number has no defined test behind it. Comparing bags that publish both an in-house figure and an ISO figure, the marketing number is consistently the colder one. Adding 8 degrees C brings unrated bags roughly into line with comfort ratings, but the uncertainty is much wider than for a tested bag.
Does sleeping in more clothing always help?#
Up to the point where it compresses the bag insulation or makes you sweat. A hat, dry socks and an insulated jacket are worth about 2 degrees C. Bulky trousers inside a snug mummy bag can compress the fill and be worth less than the same clothing in a wider bag.
Should I trust the forecast low?#
Not on clear, still nights. Cold air drains downhill and pools in hollows and valley floors, so a campsite low can sit several degrees below the regional forecast. If you are pitching low and the sky is clear, subtract a few degrees before you use the number here.
Does a vapour barrier liner really add 4 degrees?#
In deep cold, used correctly, yes, because it stops evaporative loss from the skin and keeps body moisture out of the fill. It is uncomfortable in mild conditions and it is a specialist technique rather than a general recommendation.
How do I use this for a quilt?#
Quilts are almost never tested to ISO 23537, so treat the label as a manufacturer figure and select the unrated option. Then be honest about drafts: a quilt that is not sealed to the pad loses more than the calculator assumes.
Standards, sources and further reading
- ISO 23537-1, Requirements for sleeping bags, which defines the comfort, limit and extreme outputs and the standard mattress the test is run on.
- ASTM F3340, the guarded hot plate method that makes pad R-values comparable between brands.
- Our own adjustment table, published in full below and on why you sleep cold.
How this page is made. Every number here is either a published standard, a physical constant, or arithmetic we show in full so you can check it. Read our evaluation method and editorial standards, or tell us we got something wrong.
Last reviewed and updated 9 September 2026.