An Affordable Weekend Sleeping Bag: the Number in the Name Is Not a Temperature
200, 250, 300 — that is grams of fill per square metre, not degrees. The figure that decides whether you sleep is the ISO comfort rating, and the ISO test is run on a well-insulated mat. Buy a £40 bag and a £20 mat before you buy a £60 bag and lie it on an airbed.
The short answer
Decision one: ignore the number in the model name and buy on the comfort figure. Summit 300, Latitude 200, Rayet 250 — those numbers are the fill weight in grams per square metre. They are not temperatures, and they are not comparable between brands. The number you want is the ISO 23537-1 comfort rating, which is the temperature at which a standard sleeping woman is relaxed and warm. Every bag also carries a limit and often an extreme figure, and the spread between them is enormous: the Vango Latitude 200 label reads comfort 2°C, limit −3°C, extreme −20°C. That is a 22-degree range printed on one product. Quote the comfort figure to yourself and forget the other two.
Decision two: the mat decides whether the rating means anything. The ISO test is run with the bag on an insulating mat, and SectionHiker puts the assumed insulation at R 5.38 or better. Decathlon’s popular £19.99 folding foam mat is R 2.2 — well under half — and Decathlon themselves rate it for nights above 5°C. Airbeds mostly publish no R-value at all, and a bag of cold air under you is a heat sink, not insulation. So: £40 of bag plus £20 of mat beats £60 of bag on an airbed, every time. If your budget is fixed, take it out of the bag, not the mat.
What that means in practice for a British weekend. Met Office 1981–2010 averages put the average July minimum at 12°C in England and 11°C in Wales and in the North East and North West. A bag with a comfort figure of 5°C leaves you real margin on an average July night; a 15°C bag leaves you none at all, and averages are not the cold nights.
And one honest limit. Of the six bags we checked, only Decathlon names the standard (ISO 23537-1:2022) on the product page. The others may well be quoting ISO figures — we could not confirm it either way, and we are not going to pretend the numbers are strictly comparable when the paperwork behind two of them is invisible.
Six bags under £60, on the seller's own published figures
| Bag | Price | Weight | Comfort | Limit | Buyer rating |
|---|---|---|---|---|---|
| Quechua Arpenaz 15°C Decathlon | £19.99 | Not published | 15°C | Not published | 4.7/5 from 1,990 |
| Mountain Warehouse Summit 300 | £24.99 (from £64.99) | Not published | 5°C | 0°C | 4.4/5 from 86 |
| Vango Latitude 200 listed discontinued at Trekitt | £30 (from £60) | 1,500 g | 2°C | −3°C (extreme −20°C) | None on the page |
| Forclaz MT500 10°C Decathlon — names ISO 23537-1:2022 | £49.99 | 900 g (L; S 760, M 820, XL 990) | 10°C | 5°C | 4.6/5 from 518 |
| Highlander Rayet 250 | £59.99 | 1,450 g | 6°C | 1°C | No reviews on the page |
| Forclaz MT500 5°C Decathlon — names ISO 23537-1:2022 | £59.99 (from £64.99) | 1,050 g (L) | 5°C | 0°C | 4.6/5 from 874 |
Every figure read off the seller’s own product page on 4 September 2026. Mountain Warehouse publish no weight, no packed size, no fill weight and no standard for the Summit 300; the Rayet 250 is 250 g/m² siliconised hollow fibre, 40% hollow fibre to 60% polyester, packing to 38 x 19 cm; the Latitude 200 packs to 25 x 22 cm, is 205 cm long and takes a user up to 190 cm. Only the two Decathlon bags name a test standard.
What the old version of this page got wrong
This page replaces one we published in September 2025, and we are not going to quietly correct it. The old table listed three bags: a Vango Latitude 200 at £50–65 with a comfort rating of 4°C, a Rab Alpine Trek 300 at £70–90 with 2°C, and a Mountain Warehouse Summit 300 at £35–50 with 2°C. Two of those three comfort figures do not match what the sellers publish.
The Latitude 200 is the one worth flagging by name. Our old page said 4°C. The listing at Trekitt says comfort 2°C. That is a two-degree discrepancy on the single number the whole purchase turns on, and it is exactly the direction that gets someone cold — a bag sold as warmer than it is. We have not been able to reconcile the two figures; we can only tell you which one we have seen on a seller’s page and when. If you are holding an old printout, or a cached copy of the previous page, use 2°C.
The Summit 300 was out by more. We said 2°C; Mountain Warehouse’s own page now says comfort 5°C, limit 0°C. And the old page cited REI and OutdoorGearLab without linking to anything, which is why we could not check any of it against the source. Every figure on this rebuild carries a seller and a date.
What comfort, limit and extreme actually mean
ISO 23537-1 is a laboratory procedure, not an opinion. A heated manikin is dressed in one layer of long underwear, laid on a foam mat, and put in a controlled chamber. The results are scaled to two reference people: a standard man of 25, 1.73 m and 73 kg, and a standard woman of 25, 1.60 m and 60 kg. Rab, Australian Hiker and Nemo all describe it the same way, so this part is not in dispute.
Comfort — the only one to shop on
The temperature at which the standard woman sleeps relaxed. That is a comfortable night, not a survivable one. It is the figure in our table and the figure you should compare between bags.
Limit — a bad night
The standard man, in Rab’s own words, “sleeping in a curled position and fighting against the cold”. Nobody chooses a bag to sleep at its limit. Retailers quote it because it is a lower, more impressive-looking number.
Extreme — not a rating, a warning
Six hours of survival. Rab say “frostbite at these temperatures is very possible”. The Latitude 200’s −20°C extreme figure is not a claim that the bag works at −20°C. It is the temperature at which the bag might keep you alive while injuring you.
And “3 season” means nothing
There is no test behind the season labels. No standard defines a season. If a listing gives you a season and no comfort figure, it has told you nothing measurable.
The mat is the half of the purchase nobody sells you
The single most useful thing we found: the ISO comfort figure assumes an insulating mat underneath. SectionHiker (Philip Werner, April 2023) puts the assumption at R 5.38 or better. Insulation under you is doing at least as much work as the bag over you, because your body weight crushes the fill flat and a compressed layer insulates almost nothing.
Now compare that with what people actually buy. Decathlon’s Forclaz MT500 folding foam mat is £19.99, 480 g, 20 mm thick and R 2.2 — and Decathlon rate it for nights above 5°C. It is a good, popular, well-reviewed mat (4.8/5 from 1,293) and it is still well under half the insulation the bag’s rating was measured with. Airbeds are worse: most publish no R-value at all, and an inflatable full of circulating air will pull heat out of you all night.
So the practical rule. If you are choosing between spending your money on the bag or on the mat, spend it on the mat — or at minimum, do not buy a colder-rated bag and skip the mat to afford it. A 5°C bag on a proper mat will beat a 2°C bag on an airbed on any night you are likely to camp in Britain.
We would like to be more precise than this, and cannot: we have one source’s reading of the R 5.38 assumption, because the ISO document itself is a paid standard we have not read.
Down, synthetic, and why nothing under £60 is down
Every bag in our table is synthetic, and that is not a coincidence. In TGO’s budget round-up (Lucy Wallace, James Roddie, Fiona Russell and John Manning; published 3 June 2026, updated 1 July 2026) the cheapest down bag is £99.99 — the Alpkit Cloud Peak 200, 750 fill-power Downtek, 1,120 g, comfort 8°C, which they call “excellent value for money”. Note that comfort figure: 8°C, for a hundred pounds. Down buys you weight and pack size, not warmth per pound at this end of the market.
For a weekend in a car-camping field, weight is close to irrelevant and damp is a real risk, which is the case synthetic is built for. If you ever do want to know where the money goes above £60, TGO’s other figures are: Robens Morraine III £89.99, comfort 3°C; Vango Mantis 400 at £90; Sea to Summit Boab −1 at £130–140, comfort −1°C; Rab Solar Eco 3 at £180, comfort 0°C. Eighteen times the price of the Arpenaz buys you fifteen degrees.
Liners: a hygiene item that probably buys a degree or three
Sea to Summit claim their Thermolite Reactor Compact Plus adds “up to 11°C”. Nic Hardy, reviewing it for Outdoors Magic in February 2020, found the 384 g liner “cosy and warm during sub-zero winter nights”, which is a real-world impression rather than a measurement.
Here is the thing we could not get past: we found no test standard for liner warmth anywhere. Not ISO, not EN, not a trade body — nothing that would let you compare one manufacturer’s “up to 11°C” with another’s. Until there is one, treat a liner as what it reliably is: a washable layer that keeps the bag clean, adds a little warmth, and does not change the bag’s rating. Do not buy a 10°C bag and a liner and call it a 5°C bag.
Making a cheap synthetic bag last
The storage mistake is the expensive one, and it is the one everybody makes. Mountain Equipment’s own synthetic care guidance says to “avoid storing inside its stuff-sack for long periods” and to store the bag “uncompressed, loose in a large box or pillow case”. The stuff sack is for carrying it, not for the eleven months of the year it lives in a cupboard. Compressed fill loses loft, and loft is warmth.
Washing: only when necessary, at 30°C on a delicates cycle with a technical wash, rinsed twice, tumble-dried on low. Twice-rinsing matters because leftover detergent sits in the fill.
And their honest admission, which is the reason a liner is worth having: “synthetic fillings degrade over time – losing loft and warmth no matter how well they have been cared for.” A cheap synthetic bag is a consumable. Budget for it as one, keep it clean, and store it loose.
What a working sleep system actually costs
| What | Figure | What it means |
|---|---|---|
| The cheap kit | £39.98 | Quechua Arpenaz 15°C at £19.99 plus the Forclaz MT500 foam mat at £19.99. Comfort 15°C is thin for a British night, but it is a complete system rather than half of one. |
| The kit with margin | £79.98 | Forclaz MT500 5°C at £59.99 plus the same £19.99 mat. A comfort rating of 5°C against an average July minimum of 11–12°C is where we would put our own money. |
| Cheap kit over 10 nights | £4.00 a night | Our arithmetic, not anyone’s data: £39.98 divided by ten nights of use, assuming nothing wears out. Roughly a pint. |
| Cheap kit over 30 nights | £2.67 a night | The same £39.98 across three summers of weekends. This is the number that makes buying the mat obviously worth it. |
| The mat on its own | £19.99 → R 2.2 | 480 g, 20 mm, rated by Decathlon for nights above 5°C, 4.8/5 from 1,293 buyers. Still well below the R 5.38 the bag ratings assume. |
| Average UK July minimum | 12°C England, 11°C Wales | Met Office 1981–2010 averages via Current Results; 11°C for both North East and North West England. July only — we found no August table there, and an average is not a cold night. |
Before you buy, check
- What is the comfort figure? Not the model number, not the season label, not the limit. If the listing does not print a comfort temperature, you cannot compare the bag to anything.
- Does the listing name ISO 23537-1? Of the six bags we checked, only Decathlon’s did. That does not make the others wrong — it means you are trusting an unlabelled number.
- What are you sleeping on? Budget for a mat in the same purchase. A £19.99 foam mat at R 2.2 is the floor, not the target.
- Is it long enough? The Latitude 200 is 205 cm and takes a user up to 190 cm; the Summit 300 comes in 200 cm and 215 cm versions. A bag you cannot straighten out in is a cold bag.
- Is the “was” price doing the work? Two of our six were showing less than half their reference price on the day we looked. Compare the price in front of you, not the discount.
- Is the bag discontinued? Trekitt list the Vango Latitude 200 as discontinued. A run-out bargain is fine; a run-out bargain you expect to replace with the same model in two years is not.
- Have you got somewhere to store it loose? If the only home for it is the stuff sack, you are shortening its life from the day you buy it.
Where this came from
- Decathlon UK — Forclaz MT500 10°C Price, weights by size, comfort and limit ratings, buyer rating and sample, and the explicit naming of ISO 23537-1:2022 · checked 4 September 2026
- Decathlon UK — Forclaz MT500 5°C Price, weight, packed volume, the three overlapping 60 g/sqm layers, ratings and the ISO 23537-1:2022 reference · checked 4 September 2026
- Decathlon UK — Quechua Arpenaz 15°C Price, the 15°C comfort figure and the buyer rating with sample size · checked 4 September 2026
- Decathlon UK — Forclaz MT500 folding foam mattress The R-value of 2.2, the price, weight, thickness, the above-5°C guidance and the review count · checked 4 September 2026
- Mountain Warehouse — Summit 300 Price, comfort and limit ratings, length options and buyer rating; also the absence of any published weight, packed size, fill weight or standard · checked 4 September 2026
- Trekitt — Vango Latitude 200 Price, weight, the comfort 2°C / limit −3°C / extreme −20°C set, packed size, fill and length, and the discontinued listing · checked 4 September 2026
- Highlander Outdoor — Rayet 250 Price, weight, comfort and limit ratings, the fill composition and packed size · checked 4 September 2026
- Rab — sleeping bag temperature ratings explained The definitions of comfort, limit and extreme, including the wording quoted here on curled sleeping and on frostbite · checked 4 September 2026
- Australian Hiker — ISO 23537-1:2022 The test method: heated manikin, one layer of long underwear, foam mat, controlled chamber, and the standard man and woman dimensions · checked 4 September 2026
- Nemo — decoding sleeping bag temperature ratings Third corroborating account of the same ISO definitions · checked 4 September 2026
- SectionHiker (Philip Werner) The reading that the ISO rating assumes a sleeping pad of R 5.38 or better — the source of decision two on this page · 19 April 2023
- The Great Outdoors — best budget sleeping bags The down-versus-synthetic price floor and the comparison figures above £60. By Lucy Wallace, James Roddie, Fiona Russell and John Manning · published 3 June 2026, updated 1 July 2026
- Current Results (Met Office 1981–2010 averages) Average July minimum overnight temperatures for England and Wales, and for the North East and North West · checked 4 September 2026
- Outdoors Magic (Nic Hardy) The review of the Sea to Summit Thermolite Reactor Compact Plus liner, and its 384 g weight · 18 February 2020
- Mountain Equipment — synthetic sleeping bag care Storage and washing guidance, and the manufacturer’s own statement that synthetic fill degrades regardless of care · checked 4 September 2026
What we could not check
- We have not read ISO 23537-1 itself. It is a paid document. Everything here about the test method comes from three manufacturers and one outdoor title describing it, and the R 5.38 mat assumption is one source’s reading of it — a load-bearing number on this page that we would rather have from the standard direct.
- We do not know what the 2022 revision changed relative to the older EN 13537. If you are comparing an old bag against a new one, that gap is unquantified here.
- Only Decathlon name a standard. Whether Mountain Warehouse’s and Highlander’s comfort and limit figures are ISO figures is unconfirmed. We are not saying they are not — we are saying nobody told us.
- The Vango Latitude 200 ratings come from Trekitt’s listing and we could not verify them beyond it. This is also the bag on which our own old page and the seller disagree, 4°C against 2°C, and we have not reconciled that.
- No independent lab test of any bag here. Every temperature figure on this page is the seller’s own. Nobody has put these six in a chamber for us.
- There is no sample-sized durability data at this price. Buyer ratings tell you about the first few nights, not the third summer.
- Liner warmth claims are not measurable as far as we can tell — no standard exists for them, so “up to 11°C” cannot be checked or compared.
- We found no August overnight averages in the same source, only July. August bank holiday nights are outside anything quoted here.
- Airbed R-values are mostly unpublished, so our point about airbeds is a physical argument, not a measured comparison against a specific product.
- Every price is from 4 September 2026 and several were promotional. They will have moved.
Prices and specifications change constantly. Every figure here is dated — check the one in front of you before spending.