A perfect marriage of Mountainsmith's Function-First-Design and an ultralight ethos, the all new ZERK 40!

The ZERK 40L, the original cornerstone of our ZERK UL Series, has been tested & developed thru 10's of thousands of thru-hiking miles and co-designed with The Real Hiking Viking (AKA Jabba). When the goal is to hike faster, longer and with more efficiency, consider this award winning pack on your next ultralight adventure. 

For this new addition to the Zerk UL series, we've added Zerk accessory compatibility with our Zerk 3L Loki Lumbar and Zerk 7L Toplid enabling you to add 10 additional liters for a robust and modular 50L pack. We've also included modular compressions straps (x4) and field serviceable bungees; not only increasing your versatility on the trail, but also allowing you to strip the weight when minimalism is key. 

Additional upgrades for 2026:

• Materials upgraded to Robic® Nylon X-Grid Extreema (UHMWPE) Ripstop with UTS coating for both the 210d fabric (bottom panel, backpanel and reinforcement areas) and 100d fabric (main body)
• Ultra durable Spundura stretch mesh pockets with UHMWPE grid in key wear areas (Side bottle pockets and front panel sleeve pocket)
• Additional upper side panel stretch mesh pockets for quick access, on-the-go items (avocados anyone?)
• A new front panel zippered security pocket for improved organization of trail valuables
• Compatibility with our new Zerk 3L Loki Lumbar pack which acts as a chest rig on the ZERK 40 shoulder straps or carry it stand alone with the existing ZERK 40 waistbelt; enabling you to pack the Loki Lumbar as an organizer pouch and then use it as a stand alone lumbar pack for side trips from base camp without carrying an extra waistbelt!
• Compatibility with our new Zerk 7L Toplid accessory, increasing your carrying capacity by an additional 7L
• Changed interior pattern volume to better accommodate some makes of 700 in³ sized bear canisters