A sleeping bag comes down to four decisions: how warm, what insulation, what shape, and how light. Get the temperature rating right first, because everything else is a trade-off you make only after the bag is warm enough for where you sleep. Here is how each choice works, grounded in the specs across the 173 bags and quilts we track.
Temperature is the one spec you cannot compromise on, and it is also the one most often misread. Lab-tested bags carry an EN 13537 / ISO 23537 rating with two numbers: a Comfort rating (the temperature a cold sleeper stays comfortable) and a Lower Limit (where a warm sleeper is still okay, curled up). Many lightweight and cottage bags skip the lab test and publish a single self-assigned number that follows the limit convention, so it runs optimistic.
The practical rule: shop on the Comfort rating if you sleep cold or the number is lab-tested, and treat a maker-assigned rating as roughly a limit, meaning you want a bag rated about 10°F colder than the coldest night you expect. The bags we track span a comfort range of roughly -30°F to 56°F, so there is a bag for anything from a summer hut trip to deep winter.
This is a preference, not a quality gap, and it is why we never score fill type. Down wins on warmth-to-weight and packs smaller, so it dominates the ultralight end. Synthetic keeps more of its warmth when damp, dries faster, and costs less, which makes it the safer pick for wet climates and budgets. If you backpack in dry conditions and count grams, down is usually worth the money. If you paddle, camp in persistent damp, or want to spend less, synthetic earns its place.
See the head-to-head on down vs. synthetic bags for the ranked trade-off.
A traditional mummy bag fully encloses you, including a hood, which traps the most warmth. A quilt drops the hood and the crushed-flat insulation under your back (which does little anyway) to save weight, and it drapes over you and the pad. A bivy is a minimalist shell, not a real insulation layer. Quilts are the ultralight favorite for three-season use; bags are warmer and more forgiving of drafts in the cold. Compare the two on quilt vs. sleeping bag.
Once a bag clears the warmth you need, the remaining specs are all sliding scales where lighter and smaller is better. For down bags, fill power (the loft of the down, from about 550 to 1000 in our data) tells you how much warmth you get per gram: a higher number is lighter for the same warmth, not warmer. It is worth paying up for on a bag you carry far, and less important for car camping. Match the bag to how you travel, then let the numbers break ties.
For three-season backpacking, most people want a comfort rating around 20°F to 32°F, which covers spring-through-fall nights above freezing with a little margin. Size colder if you sleep cold or camp higher. For a maker-assigned rating (not lab-tested), plan for it to run about 10°F optimistic.
Both come from the EN/ISO lab test. Comfort is the temperature a cold sleeper stays comfortable; Lower Limit is where a warm sleeper is still okay but curled up. Cold sleepers and most beginners should shop on the comfort number.
If your trips are mostly dry and you want the lightest, most packable bag, down. If you expect damp conditions or want to spend less, synthetic, which keeps more warmth when wet and dries faster. Neither is "better," they are suited to different conditions.
A three-season down bag or quilt typically lands between about 1.2 and 2.5 lb depending on rating and fill power. Lighter is better once the bag is warm enough for your trip, so use weight to break ties, not to lead the decision.
This guide explains published standards and industry specs, illustrated with figures drawn from the 500-plus products in our database. It is educational reference, not a substitute for matching gear to your own conditions. We take no sponsorships and run no paid placements.