Mountainsmith 2021 Zerk 40 Backpack Review

If it ain’t broke, only fix it a little. This is clearly what the team at Mountainsmith was thinking when they came out with 2021’s iteration of their super popular pack, the Zerk 40. The 2021 Zerk 40 Backpack is, as the name suggests, a 40-liter backpacking pack made with speed and efficiency in mind. Originally released in 2019 and made in collaboration with The Real Hiking Viking, the 2021 model makes only a few small improvements to the pack that put Mountainsmith on the map for ultralight pack options. While not the absolute lightest pack on the market, it’s hard to beat the efficient usability of the Zerk 40, with its many easily accessible pockets and running vest-inspired straps. The Zerk 40 is a hot pack for a reason.

mountainsmith 2021 zerk 40

The Mountainsmith 2021 Zerk 40.

Mountainsmith 2021 Zerk 40 Backpack At-a-Glance

  • MSRP: $219.95
  • Capacity: 40 liters / 30 pounds
  • Weight: 27 ounces 
  • Weight w/ All Accessories: 29 ounces
  • Exterior Pockets: 9
  • Torso Range: 16-19 inches
  • Waistbelt Max: 48 inches

Circumstance of Review

I’ve taken this version of the Zerk 40 out across my typical Southeast stomping grounds: Central Alabama, East Tennessee, and Western North Carolina, including sections of the Appalachian Trail.  This has totaled a bit over 100 miles so far, including rain and heavy sweat in the Southeastern Spring and Summer. In addition to those miles, I hiked the Northern 1,300 miles of the PCT with the original version in 2019.

The 2021 Zerk 40 making its way across a bridge in Tennessee Wilderness.

The 2021 iteration has held up as expected so far (favorably). It isn’t totally waterproof, but doesn’t absorb much water either and repels it respectably. The carry doesn’t hurt my shoulders on mid-teen-mile days over moderate terrain. The included detachable waistbelt can help ease the load a little, but really functions more to keep the pack snugly against the body.


  • Nine exterior pockets: 2 XL water bottle pockets with 2 external side pockets (total of 2 pockets on each side), 4 large mesh shoulder strap pockets (again, 2 per side), and 1 large front mesh pocket
  • 1.5-inch removable waistbelt
  • Roll-top closure with side compression SR buckles
  • Reflective screen printing and cord loops for low light visibility
  • Extra-wide shoulder straps with adjustable sternum strap (includes safety whistle)
  • Removable top bear canister straps (with SR buckle closure)
  • Removable front panel bungee system

Fellow hikers will hopefully be so distracted by how efficiently you carry everything that they will forget to realize you don’t know what to do with your hands when you pose for pictures.

The 2021 Zerk 40 Highlights

Oodles of Pockets

As previously stated, this pack is designed for the backpacker craving efficiency with a relatively light load. It really is difficult to imagine a more efficient pocket setup. The four shoulder strap pockets make knick-knack organization a breeze. My setup is phone (either side easily fits my old iPhone 8+) and snacks on one side, more snacks and wrappers on the other. And, if you have even more little trinkets you need to stash, there are exterior pockets on the sides of the already large and stretchy water bottle pockets. You could definitely fit two Smartwater bottles, a filter, and an empty water bag on just one side. Paired with any fanny pack, there is virtually infinite room for any hiker’s entire arsenal of little odds and ends.

Mountainsmith 2021 Zerk 40


Efficient Carry

Pockets to drool over may be the top selling point, but the shoulder straps are right there on the podium. They’re wide and beefy, in the absolute best way. The shoulder straps make this pack feel like you’re strapped into a parachute or jetpack or any other cool thing you imagined on your back as a kid (and of course they’re great at distributing weight, too). Also, if you’re going waistbelt-free, this pack is wonderfully simple to release from. It barely takes two seconds to transition from full send mode to pack off and resting on a log. It keeps a hiker locked in when it’s time to go, and pops off as easily as it gets when it’s time to stop. Even taking this thing off is efficient.


Overall, I think the greatest feature of this pack is its simplicity. The pockets and straps are the highlights, and other than that, well, everything is pretty simple. The roll-top closure is straightforward (attaches to self or buckles on the sides for maximum synching). All the pockets, as wonderfully efficient as they are, are also basic: good stretchy material with plenty of space in spots that are easy to access.

Even the accessories are all designed with simplicity in mind, with the mere fact that they’re all accessories. On many lightweight packs, there are series of add-ons that cause the price to fluctuate based on what a hiker does or doesn’t want included. The Zerk 40 takes out the multitude of options and leaves the user with 3 basic features to toggle with: the hip belt, the outer bungee, and the bear canister strap (the rigid frame sheet can also technically be removed, bringing that number up to 4).

Mountainsmith 2021 Zerk 40

Accessories clockwise: bear canister straps, front bungee attachment, and removable waistbelt (all Mountainsmith’s terminology).

The Upgrades

As I mentioned in the beginning, the upgrades on this pack are rather minor. They also don’t seem to come with any noticeable drawbacks (the price hasn’t even changed). 

The only adjustments that Mountainsmith claims to have made are a tweakage of the materials used. The main body of the pack has been upgraded to 210D Extreema nylon. Basically, this means a stronger and more durable pack, with the added bonus of being made with a recycled fabric! The other material updated is the 100D Nylon x Span 840D Stretch Mesh, which translates to a more durable and tear-resistant mesh that doesn’t come with a weight penalty.

Original Zerk 40 logo deteriorating on the PCT.

Updated 2021 Zerk 40 logo!

I noticed one other subtle change made to this pack: the logo. The original Zerk 40 had a beautiful red screen print of “Zerk 40” printed on the front mesh pocket. It was a sharp look… while it lasted. The letters peeled off my pack in the first few hundred miles of usage, which meant if the user didn’t peel them off preliminarily, they were doomed to litter the backcountry. In its place, there is a little tag with the pack’s name. Good lookin’ out, Mountainsmith.

Mountainsmith 2021 Zerk 40 Pros & Cons


  • Simple – This pack is awesomely straightforward. No fancy bells and whistles (other than the actual whistle), just all the basics anyone would want, right where they’d want them.
  • Highly efficient – One of the beauties of this pack is its accessibility on the go. 8 out of the 9 pockets are easily accessible while wearing it. The shoulder straps are beefy and distribute weight well. Even the roll-top closure is just what you need and nothing more.
  • Pockets, pockets, pockets – All the pockets on this pack are a huge part of what makes it simple and efficient. Despite the high number of them, they’re all really quite basic: roomy, stretchy, and easy to reach.
  • Accessories – The 3 accessories that come with this pack are all handy add-ons that the user may or may not need all the time. They’re simple to add or subtract, adding to the pack’s adaptability to a variety of trips.

Two Smartwater Bottles (1.5L and 1L, respectively) nestled next to a Sawyer Squeeze all on the same side of the pack.


  • Hip belt doesn’t do much – While it’s cool to have the option to easily tack on a waistbelt, without a solid frame, it doesn’t make too much of a difference in load distribution. I was able to notice a bit of weight off my shoulders while using it, but not anything substantial. More than anything this feature is helpful to keep the pack snug against the body while in go mode.
  • Hard to obtain – The first iteration of the pack went quick and it looks like this one has done the same. It’s only been out a few months at the time of this post (halfway through 2021) and Mountainsmith is already sold out.
  • Not insanely light – Sure, this is a bit of a reach for a con, though I think it’s still worth noting. For someone looking to really cut some weight on their setup, this pack is a bit on the heavier side. I’d put it more in the mid-ultralight category.

Final Thoughts

The 2021 Zerk 40 Backpack is a beautifully efficient backpack, plain and simple. For a beltless pack, it carries about as comfortably as it gets, and the pocket count is truly a dream. I’d say the base weight sweet spot of this pack is anywhere from 8-15 pounds. Anything much lower would probably want a daintier pack, and anything much heavier is going to require a true hip belt. This is a seriously sweet pack that I think will only see a rise in its already growing popularity.

Shop the Mountainsmith 2021 Zerk 40
Mountainsmith 2021 Zerk 40

One more flex with the big shoulder strap pockets: iPhone 8+ on the left, Sony RX 100VI on the right.

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The 2021 Zerk 40 was donated for purpose of review.

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Comments 2

  • Scott : Feb 22nd

    I’ve heard that the shoulder straps are relatively far apart and would be rough to use for someone with a slimmer frame. Do you believe this to be true?


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