Mountainsmith Zerk Ultralight 25L Backpack – Black | Lightweight Trail Pack for Thru-Hiking

Meet Zerk: Mountainsmith Ultralight 25L Backpack

The Zerk is built for thru-hiking and fast-paced days on the trail, delivering a minimal footprint without compromising essential storage. It weighs about 1.7 pounds when empty and features a main compartment that holds up to 23 liters of gear, inviting you to move lighter and stay prepared for changing terrain.

From the first mile, you’ll notice a compact silhouette, intuitive organization, and roomy pockets that keep pieces within reach as you travel across varied terrain. The design emphasizes practical access and a balanced load for longer climbs or quick day trips alike.

Lightweight, but capable

With a total 25-liter footprint, Zerk balances weight and capacity for a range of adventures. It supports comfortable carry up to roughly 20 pounds of gear, depending on your setup, while maintaining a slim profile on narrow switchbacks and rocky paths.

• Long main compartment holds a substantial amount of gear without feeling bulky.
• Large side water bottle pockets with angled openings simplify reloading while moving.
• Expansive front mesh pocket is ideal for bulky items like a rain shell or maps.
• Large mesh side stash pockets keep small items easily accessible.

Stretch your comfort, not your pack weight

• Removable 1.5-inch waistbelt for a personalized, stable fit.
• Torso range adjustable from 16 to 19 inches for diverse body heights.
• Removable interior frame sheet (EVA Atilon) adds rigidity and promotes airflow against the back.

Durable, practical design you can rely on

Co-designed with The Real Hiking Viking, the Zerk is built for rugged trails and variable conditions. Details like a lid closure with SR buckles, reflective screen printing for low-light visibility, and cord loops for optional bungee configuration help you stay confident and prepared on the move.

Real-life use cases

Whether planning a multi-day trek, a through-hike, or a quick overnight, Zerk adapts to your pace. Its organized storage, weight-conscious design, and comfort features support flexibility on damp mornings, bright afternoons, and everything in between. The goal is a lighter carry that still delivers dependable practicality on the trail.