How-To

How to Choose a Sleeping Bag — Temperature Ratings, Down vs Synthetic, and What Actually Matters

March 31, 2026 12 min read
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Choosing a sleeping bag is one of the most important gear decisions you’ll make for backcountry camping. Get it right and you sleep well, pack light, and wake up ready for more miles. Get it wrong and you’re shivering at 2am wondering what went wrong.

This guide covers everything you need to know to choose the right bag.

Temperature Ratings Explained

Sleeping bag temperature ratings are standardized under the EN 13537 / ISO 23537 system. Every bag sold by a reputable manufacturer has been tested in a lab and assigned ratings:

  • Comfort rating — the temperature at which a “standard woman” sleeps comfortably
  • Lower limit — the temperature at which a “standard man” sleeps comfortably
  • Extreme — survival temperature (not comfortable, not recommended for planned use)

When you see a bag rated to “20°F,” that’s typically the lower limit for a standard man. Women generally sleep colder and should size up 10-15°F from the lower limit.

The Golden Rule

Buy a bag rated 10-15°F colder than the coldest temperature you expect to encounter.

Why? Because ratings assume:

  • You’re wearing dry base layers
  • You’re well-fed and hydrated
  • You’re sleeping on an adequately insulated pad
  • You’re not exhausted from a big day

Real backcountry conditions routinely compromise all of these. A 10-15°F buffer covers you when reality doesn’t match lab conditions.

Down vs Synthetic

This is the most common question in sleeping bag selection, and the answer depends on your priorities.

Down

Pros:

  • Best warmth-to-weight ratio available — nothing beats down for packable warmth
  • Compresses smaller than any synthetic
  • Extremely durable — a well-maintained down bag lasts 20+ years

Cons:

  • Loses insulation value when wet
  • Requires more care (washing, drying, storage)
  • Costs more than synthetic equivalent

Best for: Most backcountry use in dry or semi-dry conditions. The vast majority of ultralight backpackers use down.

Synthetic

Pros:

  • Retains insulation value when wet
  • Cheaper
  • Easier to wash and maintain
  • Better for people with down allergies

Cons:

  • Heavier and bulkier than equivalent-warmth down
  • Loses loft faster over time (3-5 years vs 15+ for down)

Best for: Wet environments (Pacific Northwest, coastal areas), kayak camping, high-humidity trips where soaking is likely.

The Hybrid Middle Ground: Treated Down

Brands including Mountain Hardwear (Q.Shield), Western Mountaineering (MicroFine), and Rab (Nikwax Hydrophobic) treat down clusters with DWR coatings that significantly improve wet-weather performance. Treated down doesn’t perform as well as synthetic when soaked, but handles light moisture — fog, condensation, brief rain — far better than untreated down.

For most three-season backcountry use, treated down is the best of both worlds.

Fill Power Explained

Fill power measures down quality — specifically, how many cubic inches one ounce of down occupies when allowed to loft fully. Higher is better.

Fill PowerQualityNotes
500-600BudgetHeavier for warmth, still functional
650-700GoodMost mainstream outdoor brands
750-800ExcellentStandard for quality ultralight bags
850-1000PremiumBest warmth-to-weight, highest price

For ultralight backpacking, target 750-fill and above. The weight and packability difference between 650 and 800 fill is meaningful over multiple trips.

How to Match a Bag to Your Trip

Three-Season Backpacking (most common)

Target: 20°F / -7°C lower limit
Why: Handles everything from summer alpine nights to shoulder-season camping. The most versatile rating to own.

Summer Trips Only

Target: 35-40°F / 2-4°C
Why: Lighter, cheaper, cooler sleeping in warm weather. Not versatile but optimal for one season.

Winter / Alpine

Target: 0°F / -18°C or colder
Why: Serious cold weather requires serious insulation. This is where down fill power really matters — the weight penalty of going from 20°F to 0°F is significant.

Thru-Hiking

Target: 20-30°F / -7 to -1°C
Why: Thru-hikers need to balance warmth across varying seasons and elevations without carrying more than necessary. A 20°F three-season bag with a liner for colder sections is a common setup.

The Sleeping System: Bag + Pad + Layers

Your sleeping bag doesn’t work in isolation. The complete sleeping system determines warmth:

Sleeping pad R-value matters as much as bag temperature rating. Cold ground conducts heat away from your body faster than cold air. A 20°F bag on an R-2 pad in 25°F temperatures will feel colder than the same bag on an R-4 pad. Match your pad’s R-value to your bag’s temperature rating.

Wear dry base layers. A merino wool or synthetic base layer adds 5-10°F of effective warmth inside a sleeping bag. Never sleep in the layers you hiked in — moisture from exertion reduces insulation value.

Eat before bed. Your body generates heat through digestion. A caloric snack before sleeping actively warms you through the night.

Top Picks by Category

Don’t Forget the Sleeping Pad

Your sleeping pad is half your sleep system. A mismatched pad kills a good bag’s warmth.

Accessories That Boost Warmth

What to Avoid

Don’t buy on temperature rating alone. Brand A’s 20°F and Brand B’s 20°F are not necessarily equivalent — look for EN/ISO certified ratings.

Don’t buy a bag too warm for your typical use. Overheating disrupts sleep as much as being cold, and you’re carrying unnecessary weight.

Don’t store compressed. Long-term compression permanently damages down loft. Store in the included mesh sack or a large cotton pillowcase, never the stuff sack. A breathable down storage sack keeps loft intact between trips.

Sleep warm. Hike far.