The Mountainsmith Zerk 40 is an ultralight backpack designed with durability in mind. It features unique running vest-style straps that distribute weight across your torso. Additionally, the Zerk 40 has 9 exterior pockets allowing for easy access to snacks, devices, etc. - ideal for hikers that like to stay on the move without having to stop and remove their pack to access something.
Mountainsmith Zerk 40
✅ Very durable fabric
✅ Running vest-style straps are unique and comfortable
✅ Lots of easily accessible pockets
❌ A bit on the heavy side for an ultralight frameless pack
❌ Hip belt attachment loops dig into the sides. Easy fix - cut them off.
- Weight: 1lb 13oz with accessories, 1lb 11oz without accessories
- Capacity: 40-liter main compartment (32 liters with roll-top fully compressed)
- Dimensions: 18.5" x 11.75" x 5.75"
- Materials: 210D EXTREEMA Nylon (body), Nylon 100D x Span 840D (mesh pockets)
- Recommend max carry capacity: 30 pounds
- Recommended torso length: 16” - 19”
- Accessories: Removable bungee cord and bear canister straps
- Pockets: 4 shoulder pockets, 2 side pockets, 2 water bottle pockets, 1 large front pocket
- Features: Removable 1.5” hip belt, Roll top closure, and side compression straps
The Mountainsmith Zerk 40 is one of my favorite backpacks. I’ve put lots of miles on mine over the last couple of years, including several thru-hikes. I’ve found it to be a reliable, well-built, and comfortable pack. It is a great option for thru-hikers looking for a durable backpack with running vest-style straps and lots of accessible pockets.
If looking for similar options, you may want to compare with the Nashville Pack Cutaway (perhaps a better strap design, but more expensive) and the newly released Gossamer Gear Kumo Fastpack (much less durable fabric). If you decide on the Zerk 40, I hope you’ll enjoy using it as much as I have!
For other ultralight backpack reviews, read our post on the best ultralight backpacks.
Performance Test Results
How We Tested:
I’ve tested the Zerk 40 on multiple trails (Colorado Trail, Continental Divide Trail, Wonderland Trail) and have put well over 3,000 miles on it. This includes testing in challenging environments such as the heat of the Chihuahuan Desert, to the rugged terrain and inclement weather of the Colorado Rockies at over 13,000 feet elevation.
One of the other main selling points of the Zerk 40 is the pack’s durability. It was co-designed by “The Real Hiking Viking,” a well-known thru-hiker and social media influencer. If you follow him on Instagram, you’ll know it makes sense that this pack is focused on being as tough as he is.
In my testing, I’ve found the pack to indeed be very durable. After several thousand miles my pack is still in great shape. Sure, the mesh pockets are starting to wear thin in areas and have a few small holes, but this is to be expected with any pack.
Otherwise, the pack body itself is in excellent shape with few signs of wear and tear. I have no doubt that I’ll be able to get a couple thousand more miles out of this pack, something that I can’t say for some others I’ve purchased.
I tested the Zerk 40 and after several thousand kilometers, it’s still in excellent condition.
The durability is thanks in large part to the fabric used. The body features 210d Extreema Nylon, while the pockets feature 100d Nylon x Span 840d stretch mesh. The only downside is that this makes the pack a bit heavier than many other ultralight packs. If you are someone who counts every single ounce, you’ll need to decide if this extra weight is worth the added durability.
Honestly, for a thru-hike like the Pacific Crest Trail you’d be fine with a less durable pack. However, if you plan on hiking some more rugged trails or routes, then the durability is well worth the extra weight in my opinion.
The Zerk 40 weighs 1 lb., 13 oz with accessories and 1 lb., 11 oz without accessories.
I’ve also found that the main compartment of the Zerk 40 is a bit wider than many packs. This is something I like as it allows me to store items such as my food bag sideways within the pack. The roll-top closure is easy to open and close, and side compression straps can help cinch down a big load.
The pack has a 40-liter main compartment, 32-liter with a roll-top fully compressed.
Design & Features
The Zerk 40 comes with a few accessories including a removable elastic bungee cord as well as bear canister attachment straps that allow you to conveniently attach a BV500 (or similar) to the top of the pack.
Perhaps the most unique thing about the Zerk 40 is the ultra-running-inspired shoulder straps. These are something that aren’t commonly found on ultralight backpacks designed for thru-hikers. This running vest-inspired design allows some of the weight that would normally be on your shoulders to be distributed across your chest instead.
In use, I’ve found this to work well, although I feel like it could be even better and distribute more weight across the torso if the straps were a tad bit longer or included a second sternum strap. However, I am 6’4” with a 20” torso and this puts me at the upper end of the recommended size for this pack, so perhaps most users will feel the straps are the perfect length.
The running vest-style straps also allow for my favorite feature - lots of pockets! This pack is perfect for those who love having a variety of pockets readily accessible thus limiting the need to stop and remove the pack to access gear. You’ll find 4 pockets on the shoulder straps, perfect for storing your phone and lots of snacks.
On the outside of the water bottle pockets, you’ll find two small pockets that are ideal for stuffing wrappers and trash. Speaking of the water bottle pockets, they can both fit two Smartwater bottles and can be easily accessed while hiking. And of course, the large mesh pouch on the back of the pack is a great place to stash larger items you may want to quickly access, such as a rain jacket or fleece.
The back pocket is stretchy and expands to fit items well. Really the only thing lacking is a Pa’lante style bottom pocket, but this is something I really didn’t miss due to the abundance of other pockets.
This pack is perfect for those who prefer easily accessible pockets, reducing the need to stop and remove the bag to access gears.
So, what don’t I like about the Zerk 40? Well, not much. Really my only complaint has to do with the removable hip belt. The attachment points where you clip the belt onto the pack are quite stiff material and I found that these dig into my sides a bit as I hike.
If you don’t plan on ever using the hip belt (and let’s be honest, this type of hip belt isn’t going to transfer a ton of weight to your hips) it’s an easy fix - simply cut off the attachment points. It’s also possible that, because of my height, these attachment points rubbed me in a way they wouldn’t a shorter hiker.
The only other complaint (which really isn’t a fault of the Zerk 40) is that the pack digs into my shoulders a bit and isn’t overly comfortable when carrying a heavy load, such as 7 days' worth of food. But this is something that will happen with any ultralight frameless pack when pushing the limit of the recommended carry capacity.