How to Choose a Down Sleeping Bag for Backpacking | Rating, Fit, and Fill Power

Pick the wrong rating and you’ll freeze or sweat. Pick the wrong fill power and you’ll pay for warmth you don’t need or carry weight you could have saved. The key to choosing a down sleeping bag for backpacking is matching three things — temperature rating, insulation quality, and fit — to the trips you actually take. Here’s the order that works.

Temperature Rating: Start With the Coldest Night

Your bag’s rating should be based on the coldest overnight low you honestly expect on a trip, not the day’s forecast. REI’s advice is to choose a bag rated a little bit lower than typical lows. Feathered Friends takes it further: pick a bag rated about 10°F (5°C) below the coldest temperature you expect — and 20°F (10°C) below if you sleep cold. Other buying guides suggest 10–15°F colder than the lowest expected temperature.

For most three-season backpacking trips, that lands you in the most common range: bags rated 20–30°F. A 20°F bag covers most spring, summer, and fall trips for average sleepers. If you’re a cold sleeper or heading into shoulder seasons, step down a rating rather than layering up.

One trap: comparing ratings across brands. A “20°F” bag from one company may not feel the same as from another. The comfort rating is usually the more realistic number for how warm you’ll actually sleep, so check which rating a manufacturer is quoting before you buy.

Down vs. Synthetic: Why Down Wins for Backpacking

Down is the right insulation for backpacking because it delivers the best warmth-to-weight ratio and compresses far smaller than synthetic. That’s why REI calls the down vs. synthetic decision the main insulation choice for backpacking bags — and why down is the standard for trips where you carry everything on your back.

The trade-off is moisture. Down performs at its best in dry conditions, and wet down loses its loft and its warmth. If you backpack in a wet climate, you’ll need to protect the bag with a dry sack and take extra care keeping it dry. Synthetic bags handle damp conditions better but cost you weight and packed size.

The quality of the down matters as much as the choice itself. Look for a bag with 100% duck or goose down. Fill power measures loft per ounce — the higher the number, the more warmth you get for the weight.

Fill Power: Match It to Your Trips

Quality down starts at 600 fill power, but most backpackers should aim higher. Here’s how the tiers break down:

For most people, 700–750 fill is the sweet spot — it delivers the best balance of price and performance without the premium of higher fill powers. Step up to 800–850 if you’re counting ounces on long trails, and reserve 900+ for serious alpine or winter work where every gram counts.

Compare weights only among bags with the same temperature rating. A 40°F bag will always weigh less than a 0°F bag regardless of fill quality, so a raw weight number means nothing without the rating attached. For three-season backpacking, a common target is 2 lb or less.

Fit, Shape, and Sleep System Compatibility

A bag that doesn’t fit creates cold spots. Feathered Friends recommends about 10 in (25 cm) of shoulder room beyond your own measurement. Measure around your shoulders with a soft tape (arms at rest), add 10 in, and compare that to the bag’s shoulder dimensions. If you’re between lengths, choose the longer one. Mummy bags usually don’t offer width options, so add a few inches if you sleep on your stomach or move a lot.

Shape follows priority. If weight and packed size are the top concern, a down mummy bag is the best choice. Look for features that hold warmth in: a draft collar, hood drawstrings, and a DWR shell treatment for light moisture resistance. Many tested backpacking bags also include a stash pocket and a compression sack.

Your sleeping pad’s R-value is part of the sleep system, and REI notes it’s a key component of perceived warmth. A warm bag on a thin pad with low R-value will still feel cold; match the pad’s insulation to the conditions as carefully as you match the bag’s rating. For a full list of tested models, our roundup of the best down sleeping bags for backpacking breaks down the top options for each rating and budget.