Interactive tool
Sleep System Temperature Calculator
Combine sleeping bag comfort rating, pad R-value, liner, and worn clothing to estimate your real-world lowest safe sleep temperature — with the formula and ASTM/EN sources published.
Use the EN/ISO Comfort rating, not the Limit rating.
ASTM F3340 tested value.
The formula
estimate = bag_comfort_F + pad_shortfall_penalty − liner − clothing
The pad penalty compares your pad's R-value to the R-value you actually need at that temperature (R 2 for above-freezing, R 3 for near-freezing, R 4 for 20°F, R 5 for winter). Each 1.0 R-value shortfall raises the safe-low by ~4°F because heat loss to the ground increases roughly linearly with 1/R.
Sources
- EN/ISO 23537 sleeping bag temperature rating standard.
- ASTM F3340 standard test method for pad R-value.
- Therm-a-Rest, NEMO, and Big Agnes published R-value seasonality guides.
- OutdoorGearLab & Backpacking Light layering thermal experiments (2018–2023).
Last reviewed: 2026-07-25.
Frequently asked questions
What does 'comfort rating' mean on a sleeping bag?+
The EN/ISO 23537 Comfort rating is the lowest temperature at which a 'standard woman' (25 y/o, 60kg, 160cm) can sleep comfortably in a relaxed position. It's the honest number to use for planning — the 'Limit' rating is a curled-up survival number that most people should not rely on.
How does pad R-value factor in?+
A sleeping pad's R-value measures resistance to conductive heat loss into the ground. ASTM F3340 is the standard test method (Therm-a-Rest, NEMO, and Big Agnes all publish to it). Below R 3.0 you'll lose warmth in freezing conditions no matter how good your bag is; R 5+ is winter territory.
Can layering clothes really lower my sleep temp?+
Yes — a merino base layer, dry socks, and a puffy jacket inside the bag can add roughly 8–12°F (4–7°C) of comfort. This tool uses the conservative +8°F number.
What are the tool's limits?+
This is a planning estimator, not a guarantee. Wind exposure, wet gear, hydration, calorie intake, and individual cold tolerance can shift real-world results by 10°F or more. Always leave margin — especially for shoulder-season and winter trips.