Choosing a sleeping bag: temperature ratings, down vs. synthetic, and shape
A sleeping bag is the piece of gear people most often buy wrong, usually by shopping on the big number on the tag. That number means something specific — and probably not what you think.
This guide covers how temperature ratings really work, the down-versus-synthetic decision, and the shape and fit choices that matter as much as insulation.
Temperature ratings: comfort vs. limit
Modern standardized ratings (EN/ISO) come in two numbers. Comfort is the temperature an average cold sleeper stays warm; Limit is where an average warm sleeper just makes it. The marketing number on the bag is usually the limit.
Buy for the comfort rating at the coldest temperature you realistically expect, then subtract a margin if you sleep cold. For most three-season campers that's a 20°F (-6°C) bag — warm enough for shoulder seasons, manageable in summer with the zip open.
- Cold sleepers and women should generally buy 10°F below the expected low
- A 30°F bag at 30°F is a miserable night, not a comfortable one
- Your sleeping pad matters as much as the bag — insulation under you is half the system
Down vs. synthetic
Down is lighter, packs smaller, and lasts decades — the right choice for backpacking and anyone carrying their bag. Its weakness is moisture: wet down loses most of its loft, though modern hydrophobic treatments help.
Synthetic insulation keeps working when wet, dries faster, and costs much less. It's the sensible choice for car camping, humid climates, kids, and first bags. The penalty is weight and bulk — a synthetic 20°F bag is roughly twice the packed size of an equivalent down bag.
Shape: mummy, rectangular, and the honest trade-off
Mummy bags are warmest per pound because there's less air for your body to heat — but some people simply can't sleep in them. Rectangular bags let you sprawl and zip together as doubles, at the cost of warmth and weight. Semi-rectangular splits the difference.
The best bag is the one you sleep well in. If you're a side-sleeper who hates confinement, a slightly heavier roomy bag will outperform a technically superior mummy you lie awake in.
Fit and details that matter
A bag should fit close without compressing the insulation — excess length and width are dead air you pay to heat all night. Check shoulder and hip girth, not just length.
Details worth having: a draft collar at the neck (the single biggest warmth feature after insulation), a snag-resistant zip you can operate at 3am, and a hood that cinches. Store the bag loose in a large sack at home, never long-term in its compression sack.
Last updated September 16, 2026. Questions or a correction? Get in touch.