Thru-hiking backpacks typically sit in the 50–65L range, which is smaller than many hikers expect given trail lengths that can run 2,000-plus miles. The reason is resupply frequency: thru-hikers restock food every three to five days at a trail town, post office or resupply point, so the pack is never carrying more than a short segment's worth of food regardless of how long the overall trail is.
What does drive capacity on a thru-hike is a combination of base gear bulk and the longest stretch between resupply points — some sections of long trails run six or more days without a town, which pushes toward the higher end of the 50–65L range. Ultralight thru-hikers running a low base weight can comfortably fit a standard resupply segment into 40–50L, while hikers carrying bulkier three-season gear tend toward 55–65L.
Durability matters as much as capacity for a thru-hike pack, since the fabric and stitching need to survive months of daily use rather than a handful of weekend trips. Many thru-hikers choose packs built with tougher fabrics or reinforced high-wear zones — hipbelt attachment points, the bottom panel, and zipper pulls — over an equally sized pack optimized purely for minimum weight, since a failure hundreds of miles from a gear shop is a bigger problem than carrying a few extra ounces.
Weight loss over the course of a long trail is another factor some thru-hikers plan around: food consumption drops base weight day to day, and many hikers also swap gear or mail home unnecessary items partway through, which can make a pack that felt right-sized at the trailhead feel slightly large by the final weeks.