Therm-a-Rest
Therm-a-Rest Parsec 0 °C Down Sleeping Bag (Regular)
Where it fits
- Also works for
- Bikepacking, Backpacking and wild camping
- Not for
- Winter mountain
Why these labels
- Bikepacking: Small enough for bike bags: packs to 3.7 litres
- Backpacking and wild camping: Light to carry: 0.7 kg
- Winter mountain: Comfort rating 5 °C: too cold for winter nights
From the maker's stated use and the published specs.
- Summer wild camp: 8/10 UK, roughly May to September, nights above about 8 °C, sheltered pitches.
- Three-season hills: 4/10 UK hills in spring and autumn. Wind, rain, and nights near freezing. Capped at 4: needs warmth 7+ (comfort 0 °C or colder).
- Winter camp: 4/10 UK winter or high ground. Below freezing, strong wind, possibly snow. Capped at 4: needs warmth 9+ (comfort -5 °C or colder).
- Weight: 9/10 How much it adds to your pack, compared with the whole market.
Comfort 5 °C, limit 0 °C (EN 13537).
Summer and shoulder-season hill camps in the UK, with nights down to about 5 °C
- Comfort
- 5 °C
- Limit
- 0 °C
- Fill
- 800 fill down
- Weight
- 690 g (Regular)
Packed size 3.7 L, 21 cm long and 15 cm across Compare with a 2-litre bottle
Shorter than a 2-litre bottle and about one and a half times as wide. Drawn to scale.
Where to buy
Ultralight Outdoor Gear £189.99 In stock. Free delivery. Regular, in stock on 19 Oct 2026. About half the £394.99 RRP this shop shows for the Regular, so it looks like clearance.
View at shop: Ultralight Outdoor GearAbsolute Snow £394.95 Out of stock. Shop page says currently unavailable.
View at shop: Absolute SnowPrices checked 8 Nov 2026 on each shop's own page; shops can change them. We earn no commission on these links; shops are listed by price. More
Our ratings (out of 10)
1 to 10, compared with the whole market. Each says what it's based on. How we score
Warmth
From maker's figures 5/10How cold a night you can sleep comfortably through. 5: Does the job. Average for wild-camping gear.
Therm-a-Rest's EN 13537 comfort figure is 5 °C (limit 0 °C, extreme -15 °C), in the 0.1 to 5 °C band. The 0 °C in the name is the limit figure, not the comfort figure.
Room inside
Estimate: only 2 reviews about room inside (fewer than the 3 we need, so never above 8) 7/10How much room to move, bend your knees and sleep on your side. Roomier is comfier. 7: Better than most.
One reviewer found it comfortably roomy and never felt constrained; another calls the fit a little roomier than the narrow Hyperion.
Weight
From maker's figures 9/10How much it adds to your pack. 9: Among the best you can buy for this.
Therm-a-Rest gives 0.69 kg (690 g) for the Regular, under 700 g. Reviewers and shops give 685 to 717 g, so it sits on the edge of the 700–999 g band.
Packed size
From maker's figures 9/10How much room it takes in your rucksack. 9: Among the best you can buy for this.
Therm-a-Rest gives only two figures, 21 x 15 cm, so it's a stuff sack and we take the smaller one as the width: 3.1416 x 7.5 x 7.5 x 21 = 3,711 cm³, so 3.7 L, under 4 L.
What reviewers say (out of 10)
Ratings marked * are from reviews and are not part of the trip score. They are reviewers' scores, not our own measurements. How we score
Zip*
Estimate: only 1 review about zip (fewer than the 3 we need, so never above 8) 8/10How smoothly the zip runs without snagging.
1 review on zip, e.g. "The two-way, anti-snag zipper, backed by thick fabric strips on both sides, never gets stuck.".
In short
Good for
- Summer and shoulder-season hill camps in the UK, with nights down to about 5 °C
- Backpackers who want a roomier bag than the Hyperion at a similar weight
Strengths
- 690 g and 3.7 L packed, with 800-fill hydrophobic down
- Roomier cut than the Hyperion: 157 cm girth at the shoulders in the Regular, and a reviewer 'never felt constrained'
- 20D recycled nylon shell is tougher than the 10D fabric on many ultralight bags
- Anti-snag two-way zip and a foot pocket sewn in for warmth
- Straps hold the bag to your mat, so the lighter fill on the underside stays down
Weaknesses
- The 0 °C in the name is the limit rating; the comfort rating is 5 °C, so cold sleepers will want a warmer bag
- Less down underneath (zoned insulation) relies on a good mat and on staying on it
- Only one independent review of this exact bag, so damp performance, comfort and durability are 'Not enough evidence'
- Weight differs between sources, 685 to 717 g
Sources
We haven't tested this ourselves. Every score comes from these specs, tests and reviews.