Sleeping bag temperature ratings: what the number means

Every bag in this catalogue is listed by its ISO 23537 comfort rating. Across 87 bags from 25 brands they run from −28 °C to +15 °C, with a median of −4 °C. That one figure narrows a shortlist faster than anything else on the spec sheet, which is why the finder asks for the lowest night temperature before it asks anything else.

It is also the most misread number in outdoor kit. A comfort rating is not the coldest night the bag will keep you alive in, and it is not a temperature at which everyone sleeps well. It is the output of a defined laboratory test. Knowing what that test does tells you exactly how much of it to trust, and in which direction to err.

ISO 23537 is the standard behind almost every European temperature label. It does not produce one number. It produces several from the same test run, and the manufacturer chooses which one to print largest on the hang tag.

The standard defines those reference bodies: the woman as 25 years old, 60 kg and 160 cm, the man as 25 years old, 70 kg and 173 cm. You are almost certainly neither. Comfort is the conservative figure of the set, which is why it is the honest one to shop on, and why a bag advertised on its limit or extreme rating is quietly several degrees colder than it first looks.

Two limits on the standard are worth knowing before you lean on it. Testing is voluntary, so a brand can publish a figure it worked out in-house and label it however it likes. And ISO 23537 states that it is not intended for bags made for extreme expedition use, for children, or for military use. Treat the very coldest end of this pool as less comparable than the middle of it.

The test runs a heated manikin in a climatic chamber. The manikin wears a long-sleeved base layer and socks, lies on an insulating mat in still air at a controlled humidity, and is warmed until the heat going in matches the heat going out. The air temperature at which that balance settles is the rating. Five of those conditions are assumptions about your night.

• Still air. Wind strips warmth straight through a shell, and a pitch behind a wall is not the same night as a pitch on an exposed shoulder. Your shelter is part of your temperature rating — see choosing a tent .
• Dry kit. Loft is the entire insulation, and damp fill does not loft. Two nights of condensation can cost you more than the difference between two bags on the shelf.
• A mat underneath. The rating is measured with insulation under the manikin. It has never been a figure for the bag alone.
• A rested, fed body. The manikin is a constant heat source. You are not, at the end of a fourteen-hour day on half the food you needed.
• One night. Nothing in the test speaks to the fourth night out, when sleep debt and quiet dehydration have already taken the edge off your own thermostat.

None of that makes the number useless. It makes it a fair comparison between bags under identical conditions, which is precisely what it was designed to be, and nothing more. Read it as a ranking you can trust and an absolute you cannot.

The finder asks whether you sleep cold or warm because the spread between two people in the same bag is large and entirely real. Body size, build, resting metabolic rate, circulation, age, and how recently you ate all move where your comfortable temperature sits. The structure of the standard concedes the point: comfort and limit differ because they are defined for different sleepers in different postures, not because the bag changed.

You do not need to quantify your own offset. You need to know which side of the line you are on, and then apply it in one direction only. If you are the person who puts a jumper on indoors in September, take the colder bag. If you throw the duvet off in January you can shop closer to the number, but buy a bag you can vent properly: overheating in a sealed bag makes you sweat, and damp insulation is cold insulation by morning.

Underneath you, the fill of the bag is crushed flat by your own weight. Compressed down or compressed polyester has almost no loft and therefore almost no insulating value, so the layer between you and the ground is doing close to nothing. The mat does that job instead. This is why ISO 23537 puts one under the manikin: the rating already assumes a pad, and it was never measured on bare ground.

The consequence is blunt. Pair a −10 °C bag with a summer mat and you will be cold at −10 °C, and you will blame the bag. Sleeping pad R-values explains which number suits which night; the pad pool here runs from R 0.5 to R 8.5 with a median of 3.4.

So set the budget for the two together. Bags here run from €35 to €1,150 with a median of €400, and pads from €30 to €280 with a median of €130. A colder-rated bag is expensive. Another point or two of R-value usually is not, and on a cold night it is the better spend.

1. 1

   Start with the real low, not the regional average

   Take the coldest night you actually expect, then look at where you will pitch. Valley floors, clearings and anywhere beside water collect cold air, and a clear sky costs you several degrees against an overcast one. Plan for the pitch, not the forecast for the nearest town.

2. 2

   Subtract for yourself

   If you sleep cold, take 4 °C off the figure you just wrote down. That is a rule of thumb rather than a measurement, but it is the direction experienced buyers err in. Tell the finder you sleep cold and it applies the same margin for you.

3. 3

   Subtract again for the trip

   Take another 5 °C off for anything that stops you drying out: several nights in a row, persistent rain, snow, or a shelter that condenses onto the foot of the bag. A bag that starts a trip dry and ends it damp has quietly moved several degrees the wrong way.

4. 4

   Buy the bag whose comfort rating meets that figure

   Its comfort rating, not its limit and never its extreme. If two bags straddle your number, take the colder one and learn to vent it: open the zip, push an arm out, loosen the hood and the draught collar. You can always shed warmth you carried. You cannot add insulation you left at home.

5. 5

   Test it somewhere you can walk away from

   A garden, or a campsite with a car in it, at something near the temperature you are planning for, tells you more than any label. Do it while the bag can still go back, and before the trip where being wrong matters.