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Mountainsmith Zerk 40 Review

With lots of external carry options and thoughtful features, this pack is designed for those who move fast and light
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Price:  $220 List | $219.95 at Backcountry
Pros:  Comfortable shoulder straps, great feature set, not great for heavy loads
Cons:  Designed for a specific use, relatively small capacity
Manufacturer:   Mountainsmith
By Jane Jackson ⋅ Senior Review Editor  ⋅  May 1, 2020
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#8 of 14
  • Weight-to-Volume Ratio - 35% 7
  • Comfort to Carry - 25% 8
  • Features - 20% 7
  • Adaptability - 10% 8
  • Durability - 10% 6

Our Verdict

The Mountainsmith Zerk 40 is designed for long, burly hikes and hundreds and hundreds of trail miles. With a plethora of pockets and external carry options, including a bear canister strap that goes over the top, it's a versatile, adaptable pack. It can be loaded down with lots of weight or stripped to carry a minimal kit, depending on the mission. This pack is not for the slow and steady and is ideal for those who want to cover lots of ground, fast. A minimalist design provides little support for heavy loads, so you'll want to make sure your kit is dialed before setting off with this high-tech pack.

Compare to Similar Products

Our Analysis and Test Results

With a lightweight design and extremely comfortable shoulder straps, the Zerk gets fairly high scores across the board in our side-by-side comparisons. Though it scores highly in our weight to volume ratio, it still has a relatively small capacity, which should be taken into consideration. The Zerk boasts a classic feature set with multiple external storage options, which rounds out this lightweight, minimalist pack.

Performance Comparison

Cruising under spring skies in the Zerk 40.
Cruising under spring skies in the Zerk 40.

Weight to Volume

With a measured weight of just over a pound and a half (27.5 ounces, to be exact), the Zerk 40 is a lightweight-gear-carrying machine. We found the main body to comfortably carry 38 liters of gear while still being able to close the roll-top. Additionally, the external mesh pockets provide approximately six liters extra carrying capacity, rounding out the pack's total capacity at 44 liters. That said, the pack was not incredibly comfortable once loaded down with 25 pounds of gear, so its positive weight to volume ratio means it has the capability of carrying a large volume of lighter gear.

Weight Bottom Line:
Total Weight with all modular components = 27.5 oz
Pack striped of components = 21.4 oz
Lid = na

The mesh pocket adds a fair bit of extra volume to the Zerk.
The mesh pocket adds a fair bit of extra volume to the Zerk.

OGL Measured Volume Bottom Line:
Total Volume = 44 L
Main Bag = 38 L
Pockets = 6 L
Lid = na

Load Carrying Comfort

The shoulder straps are where most of the weight rests when the Zerk 40 is loaded up. These straps are some of the most uniquely designed shoulder straps we've tested; they are reminiscent of a running vest, with four mesh pockets placed high so that water and snacks are easily accessible. The straps are comfortable and well-padded, which is a good thing since the hip belt provides almost no support or cushion at all. This design means that the pack shines with a 20-pound load and starts to feel uncomfortable with anything heavier.

The mesh shoulder straps are thick and well-padded  and still provide exceptional ventilation.
The mesh shoulder straps are thick and well-padded, and still provide exceptional ventilation.


The shoulder straps on the Zerk are unique and designed for quick access to snacks and water. The rest of the features on this pack are equally well-thought-out and intentional. A large mesh pocket on the back of the pack provides extra storage, while the two mesh side pockets are double-layered, allowing for organization and separation of water bottles and extra gear you'll want to access quickly. We appreciated this feature, as we tend to stuff all sorts of gear in our side pockets throughout the day, and it is nice to keep things organized.

The dual side pockets allow for extra organization  which we appreciated.
The dual side pockets allow for extra organization, which we appreciated.

The roll-top closure is simple and creates a nice water-resistant seal. Though the pack doesn't ft a bear canister, it comes with an extra strap designed specifically to hold a canister on the top of the pack if necessary.

The large pockets on the shoulder straps are perfect for holding phones or snacks.
The large pockets on the shoulder straps are perfect for holding phones or snacks.


With its roll-top closure system, removable foam pad, and removable hip belt, the Zerk 40 is a highly adaptable pack. The roll-top allows it to compress to a daypack size that's comfortable, and the removable foam pad makes it easy to cut weight and bulk for shorter missions. The hip belt is removable, but this is almost negligible since its weight is so low; however, the fact that the feature exists allows extra adaptability and versatility.


The pack's main body is made from durable nylon with a TPU coating, making the Zerk a highly durable pack overall. There is, however, a lot of mesh exposed on the pack, which runs the risk of tearing after miles and miles on the trail. We didn't run into any issues with the mesh tearing, but we could foresee this happening over time. It's a good reminder to take care when tossing your pack down in exhaustion at the end of a long day.

This material used on the back panel felt hot and lacked exceptional durability.
This material used on the back panel felt hot and lacked exceptional durability.


Compared to many of the other high-tech packs in this review, the Zerk 40 comes at a fairly reasonable price. Though its carrying capacity is smaller than some, its feature set and durable design rival some of the top models out there.

Unique features like the running vest-style shoulder straps make the Zerk an intriguing product.
Unique features like the running vest-style shoulder straps make the Zerk an intriguing product.

It's fairly specific in terms of use — designed specifically for fastpacking, which makes it lose some value because it lacks the versatility of other packs.


If you've done the research, put in the trail miles, and committed to the ultralight way, the Zerk could be an obvious next step in your light-and-fast quiver. Long-distance sufferfests, traverses, and ultra-fast thru-hikes are this pack's dream. The Zerk is designed for a specific use, and it does its job very well; we are fans of the pack's feature set, overall design, and performance. It carries comfortably with a light-to-medium load and allows for versatility in terms of configuration, depending on your ultralight mission de jour.

Jane Jackson