Sleeping Bag Temperature Ratings Explained
Choosing the right sleeping bag means understanding temperature ratings. The European EN 13537 standard (now ISO 23537) provides three tested ratings — comfort, lower limit, and extreme — that let you compare bags objectively. In the UK, the season rating system (1–5) offers a simpler shorthand. This guide explains both systems and helps you pick the right bag for UK conditions.
EN/ISO 13537 Temperature Ratings
| Rating | Test Subject | What It Means |
|---|---|---|
| Comfort | Standard female (25 yrs, 60 kg) | Temperature at which a “cold sleeper” can sleep comfortably in a relaxed position all night |
| Lower Limit | Standard male (25 yrs, 70 kg) | Lowest temperature at which a “warm sleeper” can sleep for 8 hours without waking from cold |
| Extreme | Standard female (25 yrs, 60 kg) | Survival only — risk of hypothermia. Never plan to use this rating |
Always base your choice on the comfort rating, not the lower limit or extreme. If you tend to sleep cold, or you camp in the UK where damp conditions reduce insulation effectiveness, add a further 5°C margin.
Season Ratings for UK Camping
| Season Rating | Comfort Range | UK Months | Conditions | Best For |
|---|---|---|---|---|
| 1 Season | +5°C and above | Jun–Aug | Warm summer nights only | Festival camping, summer lowland sites |
| 2 Season | 0°C to +5°C | May–Sep | Late spring through early autumn | General summer camping, DofE expeditions |
| 3 Season | −5°C to 0°C | Mar–Nov | Spring, summer, autumn | Most popular choice for UK backpacking |
| 4 Season | −10°C to −5°C | All year | UK winter, high camps | Winter mountain camping, Scottish Highlands |
| 5 Season | −15°C and below | Extreme winter | Severe cold, alpine, expedition | High-altitude mountaineering, Arctic expeditions |
Factors That Affect Warmth
| Factor | Effect |
|---|---|
| Sleeping mat R-value | A good mat (R3+) prevents ground cold; a poor mat can drop effective bag temp by 5–10°C |
| Shelter type | A tent adds 2–3°C vs bivvying; a double-skin tent adds a further degree or two |
| Bag liner | Silk liners add 1–3°C; thermal liners can add up to 10°C |
| Humidity and dampness | Wet down loses most of its insulation; synthetic retains warmth better when damp |
| Fatigue and food intake | Exhaustion and insufficient calories reduce your body's heat production |
| Bag fit | Too large wastes heat; too tight compresses insulation. Aim for a snug fit with room to move |
Plan Your Sleep System
Use our shelter and sleep calculators to build the right sleep system for your trip:
- Shelter & Sleep Calculators — sleeping bag, mat, and tent sizing tools for your camping setup
EN 13537 / ISO 23537 ratings are tested on a standardised thermal manikin. Individual perception of warmth varies significantly. These ratings should be treated as a guide — always err on the warm side for UK conditions where damp and wind are common.