A comfort rating is a laboratory value: makers who test to EN 13537, or to its successor ISO 23537, measure the bag on a heated mannequin under controlled conditions and publish the result. It is an indication for comparing bags, not a promise about a night outdoors.
What the test involves
The measurement is made on a heated mannequin, not on people. The mannequin is dressed in a defined way, laid on a defined mat, and placed in a chamber where the temperature is controlled, and the heat it has to supply to hold its surface temperature is what the figures are calculated from.
That design is what makes the numbers comparable between bags. It is also what makes them unlike your night: nothing in the chamber is tired, wet, hungry or lying on a root.
The figures the standard produces
Bags tested to the standard carry a comfort figure and a limit figure. A third figure sometimes appears at the bottom of the label; it is not a temperature to plan a night around, and this site does not rank on it.
The comfort figure is the one to compare between bags, and it is the one most makers lead with for that reason.
Why the standard has to be named
The whole value of the number is that it was produced by a defined procedure. A bag quoting a bare temperature with no standard beside it may have been rated in-house by any method at all, and it cannot be lined up against a bag quoting EN 13537 or ISO 23537.
When you compare two listings, the first thing to find is not the number but the standard beside it. Where a maker does not name one, that is worth recording as a gap.
What sits outside the test
Everything that is not the bag. The R-value of the mat under you carries a large part of the answer, and cold from underneath is what people usually mistake for a bag being under-rated. What you wear, whether the bag is the right length for you — an oversized bag is more air to warm — and whether the shoulder girth lets it close all change the outcome.
The standard is not attempting to model any of that. It is holding everything constant so that two bags can be compared.
How to use the figures sensibly
Compare comfort figures between bags that both name a standard. Read the fill weight as well as the fill power, since that is the grams of insulation actually present. Read the length and shoulder girth as fit rather than as warmth, and read the mat’s R-value in the same breath.
Then follow the manufacturer’s own instructions on how the bag is intended to be used, stored and cared for. The maker publishes guidance alongside the rating, and that guidance is part of the specification rather than an afterthought.
The one sentence to take away
The comfort rating is a repeatable laboratory measurement that lets you rank bags against each other. It is not a statement about a particular night, a particular person, or a particular pitch, and no rating on any bag is.
The manufacturer's own instructions come first, and this page does not stand in for them. Every weight, packed length, hydrostatic head, denier, micron rating, lumen figure and temperature on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A weight is reported as the maker counts it, and is never restated as another kind of weight, because the two are not the same figure. And the statement repeated rather than assumed: a comfort, limit or extreme temperature is a laboratory figure measured to EN 13537 or ISO 23537 on an instrumented manikin, not a promise about you. How cold a night you can spend is answered by the bag's own instructions, by what you sleep on and wear, and by conditions nothing here can know. Nothing on this site is advice about hypothermia.
What people ask before buying
Is ISO 23537 different from EN 13537?
ISO 23537 is the successor standard, and makers who test to it publish the same family of figures. What matters when comparing two bags is that both name a standard: a bare number with no standard beside it was not produced the same way.
Why do two bags with the same comfort figure feel different?
The figure describes the bag under test conditions, not the night. Fit, the mat underneath, what you wear, food, tiredness and the pitch all sit outside the test. That is why makers publish the figure alongside their own guidance on how the bag is intended to be used.
Last reviewed 6 September 2026