How to Choose a Sleeping Bag: Temperature Ratings, Fill Power & More

How temperature ratings work

EN 13537 / ISO 23537 is the international testing standard used by reputable manufacturers. A bag tested to this standard receives three figures: Comfort (temperature at which a cold sleeper — statistically a woman — sleeps comfortably), Lower Limit (temperature at which a warm sleeper — statistically a man — sleeps comfortably), and Extreme (survival rating, not comfort).

The Comfort rating is the number to use for most campers. A bag labeled '0°C' typically carries a Comfort rating of around +5°C and a Lower Limit of 0°C. If you sleep cold, add 5–8°C to the Lower Limit to find your practical comfort threshold.

Always check whether a bag has been independently EN-tested. Bags without testing certification can legally publish any temperature they choose.

Down vs synthetic fill

Down insulation (measured in fill power) compresses smaller, lasts longer, and provides a better warmth-to-weight ratio than synthetic. A 700-fill-power bag weighs less than an equivalent synthetic bag at the same temperature rating. Down's weakness: it collapses when wet and loses nearly all insulating ability. Wet down takes hours to dry in field conditions.

Synthetic insulation retains roughly 70–80% of its insulating value when wet and dries quickly. It is also hypoallergenic and significantly less expensive. The trade-off is weight and bulk — a comparable synthetic bag is 30–40% heavier and compresses less than equivalent down.

Hydrophobic down (Nikwax Hydrophobic, DriDown, etc.) bridges the gap: down clusters treated to resist moisture absorption. Not as resilient as synthetic when fully submerged, but significantly more resistant than untreated down in rain and condensation.

Bag shape: mummy vs semi-rec vs quilt

• Mummy: tapered cut minimizes dead air space and maximizes warmth efficiency. Required for serious cold-weather camping and alpine use. Restrictive for those who move while sleeping.
• Semi-rectangular (semi-rec): slightly more room in the foot box and hip area. Good for shoulder-season camping where the temperature buffer lets you trade efficiency for comfort.
• Rectangular: maximum comfort and usable as a blanket. Warm-weather camping only — dead air space around the body requires more insulation to maintain warmth.
• Quilts: popular with ultralight backpackers. No hood or back panel — requires a sleeping pad with integrated insulation or a top quilt + underquilt system. Saves significant weight but requires practice to use effectively.

How to read fill power

Fill power measures how many cubic inches one ounce of down occupies — higher fill power means more loft per ounce, which means a lighter bag for the same warmth level.

550–650 fill power: entry-level down, heavier, less expensive. Good for occasional use and frontcountry camping.

700–750 fill power: the sweet spot for most backpackers — good warmth-to-weight with reasonable cost.

800–900+ fill power: premium down used in expedition and ultralight bags. Maximum loft, minimum weight, maximum cost.

Fill power alone does not determine warmth — total fill weight matters equally. A bag with 600g of 700-fill down is warmer than one with 400g of 900-fill down.

Choosing your temperature rating

• Summer camping (nights above 10°C): a bag rated to 7–10°C Comfort is comfortable and packs small.
• Three-season (nights 0°C–10°C): a bag rated to 0°C Comfort covers most shoulder-season conditions; add 5°C buffer if you sleep cold.
• Winter and alpine (nights below 0°C): rate to the coldest expected temperature. Down is strongly preferred for weight.
• Always check your destination's historical low temperatures, not averages — a trip can hit a cold night that averages below the mean.