Big Agnes Pluton UL 40°F

Our take We’ve carried freeze-dried dinners that weigh more than this summer sack. The gram-saving recipe has three parts: high-quality down (850-fill) paired with the lightest fabric that will hold it ( 7-denier nylon; be gentle) and horizontal baffles that use the least possible material to prevent the down from moving around and creating cold spots. It even has a hood (albeit a thin one; wear a beanie at the temp rating). And the whole package packs up smaller than a 1-liter bottle. Caveat: The Pluton is built for weight savings, not comfort—larger testers described the 56-inch shoulder circumference as “coffin-tight.”

The details Despite the ultralight pedigree, the Pluton has a full-length, two-way zip. One tester fully unzipped the bag on a 65°F night and used it as a quilt. Gripe: The lower zipper head does not have an anti-snag slider—it jams.

Trail cred “I’m a cold sleeper, and I was surprised this bag kept me warm on several 40°F nights in California’s Emigrant Wilderness,” says one tester. “I slept through the night without feeling any cold spots.”