Overview

What this tool is, and who it is for

Sleeping-pad warmth is governed by the ground, not the air, and the only meaningful number is the standardised R-value. This advisor uses the ASTM F3340 measurement standard, the same one most manufacturers now publish against, and walks you from forecast low temperature to a target R-value using a comfort-calibrated mapping. It then identifies whether a single pad will do the job or whether you need to stack two pads, and shows the comfort versus weight and packed-size trade-off at each step. It does not pretend that R-value is the only thing that matters; sleep warmth depends on bag temperature rating, clothing, hydration and shelter, and the limitations panel flags those variables clearly.

Inputs

Change any value to recalculate instantly.

Forecast low at sleeping elevation

°C

Use the coldest night of the trip.

Your sleeper profile

Sleeping bag temperature rating (EN/ISO 23537 lower limit)

°C

Results

Computed from the method described below.

Required total R-value

3.0

Sleep system warmth needed for the conditions entered.

Bag warmth in R-units

3.3

Bag lower limit -7 °C, converted via the standard EN/ISO 23537 method.

Pad target (incl. ground penalty)

0.5

Plus 0.5 R for ground contact and field-conditions loss.

Recommended configuration

Single pad + foam supplement

Closed-cell foam (eg Z Lite Sol) — R 2.0, 410 g, large packed size.

A standard three-season pad covers this requirement. If you sleep cold or the trip has any chance of colder-than-forecast nights, round up to the next pad in the recommendation.

The ground penalty and the bag-to-pad split are approximate. They err on the safe side: a real-world pad delivers somewhat less than the laboratory figure because of compression, moisture and uneven ground.

What you get

A target pad R-value plus a recommended pad type for your sleep system, with the trade-offs explained.

•Computes target pad R-value from forecast temperature, comfort preference and bag rating
•Recommends single, supplemented or stacked pad configurations
•Ranks recommended pad types by weight and packed size
•Explains the comfort versus weight trade-off at each step
•Flags when the request exceeds what a single pad of any R-value can do

Honest limits

Bag temperature ratings vary in their conservatism between manufacturers; the model treats EN/ISO 23537 ratings as the lower comfort limit, which is a generally safe but not universal assumption.
Individual sleep warmth depends on metabolism, hydration, food intake and shelter, none of which this calculator models.
Pad R-value under field conditions can be lower than the laboratory figure because of ground contact, moisture and compression, so treat the recommendation as a minimum, not a maximum.

How to use

Step-by-step user guide

How to operate the calculator in your browser, what to enter, and what to do with the result.

01
Enter the forecast low temperature at your sleeping elevation for the coldest night of the trip.
02
Set your comfort preference — cold sleeper, average, or warm sleeper — and the model adds the appropriate offset.
03
Pick your sleeping bag temperature rating. The calculator subtracts bag warmth from the requirement, leaving the pad with the residual to manage.
04
Read the target pad R-value and the recommendation: single pad, single pad plus a foam supplement, or stack of two pads.
05
Compare the recommended pad types by weight and packed size. The advisor ranks them on the result card.

The method

What the calculator actually does with the numbers you enter

01
R-value requirements are mapped from forecast low temperature using a comfort-calibrated table drawn from outdoor industry guidance and field testing.
02
Sleeping bag warmth is subtracted from the requirement under the assumption that bag ratings are reasonably accurate when used as published; EN/ISO 23537 ratings are treated as the lower limit of comfort.
03
Cold, average and warm sleeper offsets of plus or minus 1 R-value unit are applied based on the user's comfort preference, derived from commonly cited individual variation data.
04
Pad recommendations prioritise R-value efficiency, then weight, then packed size, with stacked configurations returned only when no single pad meets the requirement at a reasonable weight.

References

Published sources behind the formulas

[1]
Sleeping pad R-value testing and revision history
ASTM F3340 (ASTM International) · 2024
[2]
Thermal insulation of sleeping pads (lab and field protocol)
Outdoor Industry Association / R-Value Working Group · 2025
[3]
Cold-weather sleep comfort and pad R-value requirements
Wilderness Medical Society · 2024

FAQ

Frequently asked questions

Short, sourced answers to the questions readers send us most often about this calculator.

Are R-values additive?

Yes, in the linear sense. A pad with R-value 3 stacked on a foam pad with R-value 2 delivers R-value 5 to the system. In practice the benefit is slightly less than linear because of interface effects between the layers, but the additive approximation is good enough for planning.

How accurate are manufacturer R-values?

Since the ASTM F3340 standard was published in 2020 and most major manufacturers adopted it by 2022, published numbers are typically accurate to plus or minus 0.5 R-value units when measured under the standard. Older ratings may not be comparable; if the manufacturer has not updated to the standard, treat the number with caution.

Is a higher R-value always better?

Higher R-value buys warmth, not comfort. There are diminishing comfort returns above R-value 5 in three-season conditions, and the weight and packed-size cost is real. Pick the lowest R-value that meets the requirement for your trip conditions, not the highest the manufacturer makes.

What about inflatable versus foam?

Inflatable pads deliver more R-value per gram and pack smaller, but they fail if punctured. Closed-cell foam pads are nearly indestructible and add insurance, but they are bulky. The planner ranks by R-value efficiency; if your trip involves serious bushwhacking, lean toward including a foam supplement regardless of the headline figure.