A sleeping bag liner is a lightweight, removable sheet-like insert that goes inside your sleeping bag. It works by adding an extra insulating air layer between your body and the bag, trapping more body heat. Liners are typically made from silk, cotton, merino wool, fleece, or synthetic materials. Beyond warmth, they protect your sleeping bag from sweat, oils, and dirt, significantly extending the bag's lifespan and reducing the need for frequent washing (which can degrade insulation over time).

These estimates are based on manufacturer claims, outdoor gear lab testing, and community-reported experiences. Actual results vary depending on individual metabolism, what you're wearing, humidity levels, wind exposure, sleeping pad R-value, and how well the liner fits inside your bag. Think of the boost as a general guideline rather than an exact science. On average, users report that a quality fleece liner comfortably extends a bag's range by 10–15°F in real-world conditions.

The EN 13537 / ISO 23537 standard defines three temperature ratings for sleeping bags:

Comfort Rating: The temperature at which a "standard woman" can sleep comfortably in a relaxed position. This is the rating most relevant to recreational campers.
Lower Limit Rating: The temperature at which a "standard man" can sleep for 8 hours in a curled position without waking. This is often the number marketed on men's bags.
Extreme Rating: The minimum survival temperature (with risk of hypothermia). Never rely on this for trip planning.

Our estimator uses the Comfort Rating as the baseline because it's more conservative and realistic for most users.

While you can physically layer two liners, it's generally not recommended. The second liner adds minimal insulation due to compression of the inner layers and reduced air circulation. More importantly, two liners can restrict movement, create uncomfortable bunching, and make it harder to get in and out of your bag. For significant warmth upgrades, consider a single thicker insulated liner or upgrading your sleeping bag instead. Some ultralight hikers carry a silk liner for warm nights and layer it with a fleece liner for cold nights, but the combined boost is typically less than the sum of each liner's individual rating.

Silk: Ultralight (3–5 oz), packs tiny, modest warmth boost (+3–5°F). Best for minimalist backpackers and summer trips. Dries quickly. Less durable than other options.

Merino Wool: Moderate weight (6–9 oz), good warmth (+8–12°F), excellent moisture-wicking and natural odor resistance. Best for multi-day trips where you can't wash easily. Feels luxuriously soft.

Fleece: Heavier (10–16 oz) but offers the most comfort-oriented warmth (+10–15°F). Feels like a cozy blanket. Best for car camping, shoulder-season trips, or anyone who prioritizes comfort over weight.

Cotton: Budget-friendly but heavy, slow to dry, and loses insulation when wet. Best reserved for indoor use or warm-weather car camping.

Yes, a liner does change the interior feel. Most quality liners are cut with a mummy shape to match sleeping bag contours, minimizing bunching. Silk and thin synthetic liners add almost no noticeable bulk. Fleece liners, while warmer, do reduce interior space slightly and can make a snug bag feel tighter. If you have a very fitted mummy bag, consider sizing up or choosing a stretch-knit liner. Some users actually prefer the soft, sheet-like feel of a liner against their skin compared to the slippery nylon of a sleeping bag interior.

Most liners are machine washable on a gentle cycle with mild detergent. Avoid fabric softeners — they coat fibers and reduce breathability. Silk liners should be washed in cold water and air-dried (never put silk in a dryer). Merino wool liners require wool-specific detergent and should be laid flat to dry. Fleece and synthetic liners are the most durable and can usually handle warm water and low-heat tumble drying. Always check the manufacturer's care label. A major benefit of using a liner is that you'll need to wash your actual sleeping bag far less often — spot-cleaning the liner is much easier.