How to compare sleeping bags: the specs that matter and the numbers that mislead

Spec sheets make sleeping bags look easy to compare: a temperature, a weight, a fill power and a pack size. But brands don't all use those numbers the same way. Two bags with "0 °C" in their name can be 5 °C apart, and two bags with exactly the same lab rating can differ in weight by almost 1.4 kg. This guide shows how to compare sleeping bags like for like, using the spec sheets of the UK bags we track.

Still deciding what kind of bag you need? Start with how to choose a sleeping bag, then come back here to compare the shortlist.

How to compare sleeping bags in 6 steps

1. Line up the comfort ratings. Compare the lab-tested comfort figure on every bag, never the number in the name.
2. Check the rating is lab-tested. ISO 23537 and EN 13537 figures can be compared with each other. A brand's own figures can't.
3. Compare weight only between bags with the same comfort rating. A warmer bag needs more fill, so it will weigh more.
4. Look at fill weight, not just fill power. The amount of down sets the warmth. Its fill power mostly affects weight and pack size.
5. Convert pack sizes to the same unit. Some brands give litres, some give centimetres, some give the size of the stuff sack.
6. Check what each weight includes. Size and stuff sacks change the figure, so compare the same size on the same basis.

Trap 1: the number in the name

Among the bags we compare with a temperature in their name, all seven from Decathlon are named after their comfort rating. Every one from Rab and Sea to Summit is named after its limit rating, the temperature at which a man curled up is just not cold. So a Decathlon bag and a Rab bag with the same number in their names are not equally warm.

Trap 2: ratings that aren't lab-tested

The fair way to compare warmth is a lab test. Under EN 13537, replaced by ISO 23537-1 (current edition 2022), a heated manikin is tested in a climate-controlled chamber. Every bag then gets the same three figures: comfort, for a standard woman lying relaxed; limit, for a standard man curled up; and extreme, a survival-only figure you shouldn't plan around. Because the method is the same for every brand, those figures can be lined up side by side. Other numbers can't.

Brand figures aren't necessarily wrong. Mountain Equipment says its Good Night's Sleep temperature is the coldest it expects the bag to be comfortable for an experienced user, and it lets you swap to a warmer bag within 30 days, paying the difference, if it isn't warm enough. But you can't put a brand figure next to an ISO figure and call it a fair comparison.

Compare weight at the same warmth

Weight only means something next to a rating. A summer bag will always beat a winter bag on the scales. So group bags by comfort rating first, then compare. Even among bags with exactly the same lab-tested comfort rating, the spread is huge:

Fill type explains most of the gap. At a comfort rating of 0 °C, the three down bags we compare weigh 820 g to 930 g. The five synthetic bags weigh 1.32 kg to 2.2 kg. If weight matters, compare down with down and synthetic with synthetic, then decide whether the saving is worth the price.

Fill power vs fill weight

Fill power measures down quality: how much space an ounce of down fills, in cubic inches. Higher fill power means more loft from each gram, so the bag can be lighter and pack smaller. Fill weight is how much down is in the bag, and that sets how warm it is. Rab's own range shows it clearly:

The Mythic has the best down of the three but the least of it, so it's the least warm. Higher fill power on its own doesn't make a bag warmer. Compare fill weight between bags with similar fill power, and use the lab rating to settle it.

Check how the fill power was measured

Fill power is measured in two main ways, and the US method gives a higher number for the same down. Ultralight Outdoor Gear says 800 fill power by the EU method is about 850 by the US method. PHD says its own test reads about 4% lower than the US federal method. So an 850 bag isn't necessarily better down than an 800 bag unless both use the same method. Note that "European goose down" on a spec sheet tells you where the down comes from, not how it was measured.

Two more down details are worth comparing. Among the bags we compare that name the bird, every one at 800 fill power or higher uses goose down, and every duck down bag is 750 or lower. Some makers also give a down-to-feather ratio, such as the 90/10 duck down in the Helium 400. A higher share of down means less feather.

Synthetic bags have no fill power. Makers sometimes give the weight of the insulation sheet instead, in grams per square metre (gsm). The Quechua Arpenaz 0 °C uses 350 g/m² wadding, for example. That only compares fairly between bags using the same fibre, so the lab rating matters even more here.

Compare pack sizes in litres

Brands quote packed size as a cylinder in centimetres or as a volume in litres, and some quote it squeezed in a compression sack. To compare centimetres with litres, use π × (diameter ÷ 2)² × length ÷ 1,000:

Check whether each figure is for the bag's own stuff sack or for a compression sack, which can squeeze most bags smaller. Compare like with like.

Check what the weight includes

• Size. Most quoted weights are for the regular length. The Helium 400 is 820 g in regular and 870 g in long.
• Sacks. Some weights leave out the stuff and storage sacks. Ultralight Outdoor Gear lists the Sea to Summit Spark -18 °C at 1,118 g excluding its storage bags, so a bag weighed with its sack will look heavier on paper.

What the extra money buys

Once the ratings match, price mostly buys lower weight and a smaller pack, not more warmth. These five bags are all lab-rated comfortable to 0 °C. The cheapest weighs almost three times as much as the lightest. Prices are the lowest we found on 7 November 2026:

Five bags rated comfortable to 0 °C, compared

Bag	Best for	Warmth	Weight	Best price
Mountain Equipment Helium 400	Lightest	Comfort 0 °C	820 g	£240 at Trekitt
Rab Neutrino -6 °C	Goose down	Comfort 0 °C	870 g	£375.95 at Cotswold Outdoor
Decathlon Simond MT900 0 °C Down	Cheapest down	Comfort 0 °C	930 g	£169.99 at Decathlon
Decathlon Simond MT500 0 °C	Synthetic	Comfort 0 °C	1.45 kg	£79.99 at Decathlon
Decathlon Quechua Arpenaz 0 °C	Cheapest	Comfort 0 °C	2.2 kg	£59.99 at Decathlon

For car camping, the extra weight costs you nothing. For backpacking, a lighter, smaller bag is what you're paying for. The down vs synthetic section of our buying guide covers the other trade-offs, such as damp and washing.

Our comparison tables do the lining up for you: every bag shows its comfort rating, weight, pack size and the lowest UK price we found. Compare down sleeping bags, synthetic sleeping bags or winter sleeping bags, sorted by warmth, weight or price.

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