Buy the bag whose comfort rating sits at or below the coldest temperature you expect to sleep in, then subtract another 10 °F for wind, damp, altitude, and a cold pad. The number printed in the product name is usually the limit rating, which describes the coldest temperature a warm sleeper can survive a night without shivering — not the temperature you will sleep well at. For most three-season camping between 30 °F and 50 °F, a bag with a 20 °F comfort rating and 650-fill down covers the widest range of conditions at a reasonable weight.
What the three temperature ratings actually mean
Modern bags sold in Europe follow EN ISO 23537-1 (which replaced EN 13537 in 2016); US brands often test to ASTM F1720. Both standards produce the same three numbers, measured on a heated manikin wearing a base layer and socks.
| Rating | Who it is measured for | What it tells you | How to use it |
|---|---|---|---|
| Comfort | Standard woman, relaxed posture | The temperature at which an average sleeper stays warm without curling up | Buy against this number. It is the only one that describes sleeping, not surviving. |
| Limit | Standard man, curled posture | Coldest temperature before shivering begins | Treat as a hard floor, not a target. Roughly 10–15 °F below comfort. |
| Extreme | Standard woman, risk of hypothermia | Survival boundary for a short period | Ignore for purchase decisions. A bag marketed on its extreme rating is telling you it has nothing better to say. |
Two bags with the same limit rating can differ by 15 °F in comfort. That gap is where most bad purchases happen: a “0 °F bag” with a 15 °F comfort rating is a three-season bag, not a winter bag.
Why the comfort rating is the only number to buy against
Sleep quality degrades before safety does. Shivering starts at the limit rating, but most people stop sleeping well several degrees above it — cold feet, a cold lower back, and repeated waking. If you sleep cold, are over 40, or run low on body fat, add 10 °F of margin to the comfort rating. If you sleep hot and wear a warm base layer, the comfort rating is already conservative.
The practical rule:
- Expected low 30 °F or above: comfort rating 30 °F is enough, 20 °F gives margin.
- Expected low 20–30 °F: comfort rating 20 °F, plus a pad with R-value 3 or higher.
- Expected low 0–20 °F: comfort rating 0–10 °F, down fill, full draft collar, R-value 5 pad.
- Below 0 °F: no single bag solves this; you need a layered system and a rated pad.
Down versus synthetic: the wet-cold tradeoff
Down wins on warmth per ounce and packed size. Synthetic wins when the bag gets wet, because it retains roughly 60–70% of its insulation when soaked, while down clumps and loses most of its loft until it dries. That single difference decides the choice more often than price.
| Down | Synthetic | |
|---|---|---|
| Warmth per ounce | Best (fill power 650–900) | About 60–70% of comparable down |
| Packed volume | Small, compresses hard | Bulky, holds shape |
| Performance when wet | Collapses until dry | Retains 60–70% of loft |
| Lifespan | 10+ years with care, loses loft slowly | 5–7 years before fibers flatten |
| Typical 3-season price | $200–400 | $80–180 |
| Best for | Dry cold, backpacking, weight limits | Humid climates, canoe trips, kids, loaner bags |
If your trips involve sustained rain, a boat, or a tent that condensates heavily, synthetic is the lower-risk choice even at the weight penalty. If you backpack in dry mountain air, down pays for itself in every mile.
Fill power: what 650 versus 800 actually buys
Fill power measures how much space one ounce of down occupies, in cubic inches. Higher fill power means more loft per ounce, which means less weight and smaller packed size for the same warmth — not more warmth by itself.
| Fill power | Loft per ounce | Practical effect | Price band (3-season) |
|---|---|---|---|
| 550–600 | Low | Heavier, cheaper, durable; fine for car camping | $150–250 |
| 650–700 | Moderate | Best value for most campers; good weight without premium pricing | $220–350 |
| 800–850 | High | Noticeably smaller pack, better for long carries | $350–550 |
| 900+ | Very high | Marginal gains per dollar; ultralight and expedition use | $550–800 |
A 20 °F bag at 650 fill typically weighs 2 lb 8 oz to 3 lb; the same rating at 850 fill lands near 1 lb 12 oz. That 12-ounce difference is worth carrying if you hike, and irrelevant if you drive to the site.
How your sleeping pad changes the bag’s rating
A sleeping bag insulates above you; the pad insulates below. Compressed insulation under your body provides almost nothing, so the pad is half the system. Putting a 20 °F bag on a summer pad gives you a 40 °F night.
| Pad R-value | Ground temperature | Season | Notes |
|---|---|---|---|
| 1.0–1.9 | 50 °F+ | Summer | Foam or thin air pad; cold sleeper will feel it below 50 °F |
| 2.0–3.9 | 20–50 °F | Three-season | Most air pads with some foam; the practical default |
| 4.0–5.9 | 0–20 °F | Winter | Insulated pad or two pads stacked |
| 6.0+ | Below 0 °F | Expedition | High-R pad plus closed-cell layer |
R-values add when you stack pads. A summer air pad (R-1.5) over a closed-cell foam pad (R-2.0) gives R-3.5 for very little money and packs flat against the back of a pack.
Shape, draft collar, and where heat actually escapes
Mummy bags are warmer per ounce because there is less dead air to heat. Rectangular bags are more comfortable and better for car camping, but they lose heat at the corners and along the zipper. Three details matter more than shape:
- Draft collar: a tube around the neck. Without it, warm air spills out every time you turn over — worth 5–10 °F.
- Draft tube behind the zipper: stops cold air leaking through the stitch line.
- Hood: a shaped hood you can cinch keeps the head warm; a flat top does not.
If two bags share a comfort rating and one has a collar and a hood, buy that one. The difference shows up at 3 a.m., not in the spec sheet.
Matching the bag to the trip
| Trip type | Expected low | Bag spec | Weight target | Price band |
|---|---|---|---|---|
| Summer car camping | 50–60 °F | Synthetic, 40 °F comfort, rectangular | 3–4 lb | $60–120 |
| Three-season backpacking | 25–40 °F | Down 650, 20 °F comfort, mummy | 2–3 lb | $220–350 |
| Cold-weather camping | 10–25 °F | Down 800, 0–10 °F comfort, draft collar | 3–4 lb | $350–550 |
| Winter / snow | Below 10 °F | Down 800+, 0 °F or lower, expedition cut | 4–5 lb | $500–800 |
| Humid or wet climate | 35–50 °F | Synthetic, 30–35 °F comfort | 3–4 lb | $100–180 |
Three ways people get this wrong
1. Buying the limit rating as if it were comfort. A bag sold as “0 °F” with a 15 °F comfort rating is a three-season bag. Read the tag, not the product name.
2. Upgrading the bag instead of the pad. If you sleep cold at 35 °F on an R-1.5 pad, a warmer bag adds weight and cost; an R-4 pad fixes the actual problem for less money.
3. Storing the bag compressed. Down left in a stuff sack loses loft permanently. Store it loose in a cotton bag or hanging, and it will hold its rating for a decade.
Care that preserves the rating
Wash down bags once every 15–20 nights with a down-specific detergent, dry them on low with clean tennis balls until completely dry (a damp bag clumps and smells), and never dry-clean them — solvents strip the natural oils. Synthetic bags tolerate machine washing better but lose loft faster with each wash; wash them every 10–15 nights.
Quick answers
Can I use a 20 °F bag at 40 °F? Yes. You will be warm; open the zipper and use the bag as a quilt to avoid overheating.
Is a liner worth it? A silk or fleece liner adds roughly 5–10 °F and keeps the bag clean. It is cheaper than a warmer bag and adds almost no pack volume.
Down or synthetic for a first bag? Synthetic, unless you know you will only camp in dry conditions. It tolerates damp, costs less, and survives being stored badly while you learn.
If your bag still runs cold after matching the pad and adding a liner, the problem is usually the sleep system as a whole rather than the bag — see the sleep system troubleshooting guide for the diagnostic order. For the rest of the kit, start with the camping gear and packing guide; if you are still choosing a tent, the tent camping guide covers shelter first, and the safety guide covers the cold-weather failure modes that matter most.
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