Zerk 40 UL Backpack

$229.95

UPDATED FOR 2026! Co-designed with The Real Hiking Viking, the ZERK 40L, part of our ZERK UL Series, has been torture tested & painstakingly developed thru 10's of thousands of thru-hiking miles; keeping you going faster, longer and more efficiently on your next ultralight adventure. Now with increased modularity, new UL materials and additional convenience on the trail.

Details
Tech Specs

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

FEATURES

Roll top closure with opposing SR buckles for self closing roll top or for side panel attachment with removable straps
Extra wide shoulder straps with dual adjustable sternum straps
4 Large mesh front pockets on shoulder straps (2 per side) accommodate 500ml bottles, bear spray, cell phones, gps units, etc
Airflow EVA foam padded shoulder straps
Modular side compression straps (removable) for securing load throughout entire pack or discarding entirely
Customizable and removable front panel bungee system
Removable 1.5” waistbelt with Gatekeeper attachment hardware. Works with accessory Zerk 3L Loki Lumbar (Sold Separately)
Large main compartment holds up to 40 liters of gear with extra 4L storage in roll top area
Expansive front stretch mesh pocket made with ultra durable Spundura (w/ UHMWPE grid) for hauling bulky gear
Reflective bungee and select webbing hits for increased visibility
Removable interior frame sheet (EVA / Atilon)
Dual side water bottle pockets made with ultra durable Spundura (w/ UHMWPE grid) stretch mesh and angled openings for easy access
Adjustable over-the- top V-shaped compression strap with SR buckle closure
Hydration bladder sleeve with hose port (Bladder Not Included)
Compatible w/ Zerk 7L Top-lid (Sold Separately) for extra carrying capacity
Bright yellow lining promotes visibility

MATERIALS

210d Robic® Nylon X-Grid Extreema (UHMWPE) R/S UTS*
100d Robic® Nylon X-Grid Extreema (UHMWPE) R/S UTS*
Spundura Stretch Mesh (w/ UHMWPE grid)*
150d High Density Poly Liner*
YKK® Zipper
EVA Foam

*There are no intentionally added PFAS in this item

CALIFORNIA WARNING: Cancer and Reproductive Harm | www.P65Warnings.ca.gov

DIMENSIONS

12” x 20” x 9” (L x H x D)

VOLUME

2441 in³ / 40 L
2685 in³ / 44 L (Extended Roll Top)

CAPACITY

Waistbelt Size Range: 28"-48"
Torso Range: 16"-21"
Weight Load: Suggested up to 30 lbs, but can accommodate 50% more (ie., 45 lbs) for your next sufferfest

WEIGHT

2 lbs 2 oz / 0.97 kg
1 lb 14 oz / 0.86 kg (Stripped)

Unique and feature rich pack

I bought this pack when it was on sale, and i am happy I did, because it is a very nice backpack with a lot of features I was looking for and more. First off the 40L, you can definitely fit more than that, the side pockets are BIG ive thrown my big wayer bottle there and more no issues. The small mesh pockets on the outside are great for smaller stuff and I use them a lot more than I thought, the daisy chain on the front and back, just clip your random things there and are out of the way. The dual side pockets are 👌 they expand for big things and smaller items they dont pop out. The front mesh is big enough big items and clothing. I appreciate that the top can roll down and clip to the sides OR can clip to itself, so you can use the buckle straps to hold other items, when clipped to each other, tempted to clip a skateboard 🛹 😉
Feedback for the next version, add some closure buttons/clips/velcro for the roll top to keeps the lips closed.
Add a bottom pocket for trash, same material from the front stash pocket, add another one on the bottom for trash or other items that can be packed there
But overall im happy with this backpack, there are other more luxury brands but this one just feels fancy in itself

Seems Fine After One Hike

A bit clunky when trying to snap the various straps and there aren’t straps I’ve haven’t figured out what they are for. Might be a bit too big for my purposes. On the positive side, I carried 18 pounds on my first hike and had no discomfort in my shoulders or back. Also could reach my drinks and snacks with no issues. I’ll give another test hike or two in the coming days with a 9kg load and use more of the capacity to see if we still are comfortable and happy after 15 miles.