How the ISO sleeping bag test works.
ISO 23537-1:2022 is the current international standard for the thermal performance, dimensions and mass of adult leisure sleeping bags designed for limit temperatures of −20°C and above.
A heated thermal manikin lies inside the closed sleeping bag in a controlled room. The test measures the insulation needed to maintain the manikin’s prescribed surface temperature, then applies a physiological model to calculate the published temperature ratings.
Calibrated manikin
The manikin, calculation method and posture are defined and calibrated against reference sleeping bags.
Specified foam mat
The test bag lies on an insulating foam mattress over an artificial ground rather than directly on a cold floor.
Air and temperature
Room temperature, air movement and humidity conditions are prescribed so products can be compared consistently.
Three measurements
Three tests are averaged. If one sample is reused, it must be pre-treated again and that choice recorded.
The bag is tumbled gently, conditioned for at least 12 hours and tested with its zip closed and hood adjusted under specified rules. This is a serious laboratory procedure—but it remains a steady-state model of a motionless sleeper.
Comfort, limit and extreme are different decisions.
A lower temperature means a warmer bag. When the forecast says “comfort rating about 8°C or lower”, 5°C, 0°C or −5°C all provide progressively more nominal warmth; 10°C does not meet that target.
A standard improves comparison; it does not remove every difference.
The current standard tightly controls the procedure and requires manikin calibration, limits on deviation from reference bags and repeatability. It would therefore be misleading to describe an ISO result as merely subjective or to assume laboratories can freely choose a favourable method.
There is still legitimate uncertainty. Earlier inter-laboratory work used in developing the standard found that different compliant manikins could produce up to about a 10% spread in measured thermal resistance—equivalent in its example to roughly 3°C at a 0°C lower-limit rating. The standard was revised specifically to improve inter-laboratory variability and repeatability.
Bag construction can also challenge a single test method. ISO itself notes that bags with deliberately non-uniform insulation pose calibration or testing issues. A laboratory number should therefore be read as a strong common benchmark, not precision to the nearest degree for every body and every design.
There is no good basis for saying one particular brand has “gamed” a result without its test report and evidence. The useful consumer response is to compare the same ISO field, inspect the design and allow a personal margin.
Fill weight adds context—but only like for like.
Down: fill power and fill weight work together
Down fill power describes how much loft a given mass of down can produce. Fill weight describes how much down is in the bag. Neither number works alone: cut, baffle height, distribution, hood, draught collar and zip construction determine how efficiently that loft surrounds the sleeper.
Synthetic: useful within a comparable family
Synthetic insulation has no directly interchangeable “fill power” number. Within bags of similar size, shape and construction using the same insulation, a greater fill weight will usually signal more warmth. Across different fibres or designs, grams alone can mislead because different products loft, compress and resist moisture differently.
Total bag weight includes shell fabric, lining, zips, cords and other construction. Look for a separately declared insulation or fill weight, and compare like sizes.
Down versus synthetic in British conditions
Down normally offers better warmth for its weight and packed volume. Synthetic bags generally require more insulation and are heavier and bulkier for equivalent warmth, but they usually retain useful performance better when damp and dry faster. That resilience can be valuable during wet Scottish or Welsh trips, repeated damp nights, or where keeping a sleep system completely dry is uncertain.
The sleeping bag is only part of the system.
- Sleeping mat: the ground compresses the insulation beneath you. A suitable insulating mat is essential; the ISO test itself uses a specified foam mattress.
- Fit and movement: excess internal space takes more energy to warm, while a bag that is too tight can compress insulation. Real sleepers also move and can draw cold air into openings.
- Moisture: damp clothing, condensation and wet insulation reduce comfort. Protect the bag during the walk and ventilate the shelter where conditions permit.
- Food, fatigue and physiology: cold sleepers, exhaustion, illness and insufficient food can all move the comfortable temperature upwards.
- Shelter and pitch: a draughty or damaged shelter, cold pooling and an exposed pitch can make the night feel very different from a laboratory.
Mountain Weather’s number is best treated as a minimum warmth target from the forecast air temperature—not a complete calculation of the sleep system.
A practical comparison checklist.
- Compare the ISO comfort figure, not the marketing name, season or extreme temperature.
- Check the bag length and shape fit you without compressing the insulation.
- Compare fill weight only between similar sizes, shapes and insulation types.
- For down, compare fill power and fill weight together; for synthetic, identify the actual insulation and construction.
- Check hood, draught collar, zip baffle and footbox design rather than judging warmth by fill alone.
- Choose a mat suitable for the ground temperature and conditions.
- Add margin for your own cold tolerance and for damp, fatigue or a multi-night trip.