Review: Mountainsmith Zerk 40 Ultralight Backpack

Ultralight Backpack
Mountainsmith Zerk 40
$220, 40 liters/2,440 c.i., 1 lb. 13 oz./822g (including removable accessories)
One unisex size, fits torsos 16-19 inches

Within the rather exclusive category of ultralight backpacks weighing two pounds or less, one sees similarities, most commonly and conspicuously a frameless, roll-top design with large external pockets. The Mountainsmith Zerk 40 takes that template and juices it with some smart details and add-ons, tougher materials, and a touch of modularity while keeping it significantly under two pounds. That suited my needs quite well trekking hut to hut for six days on Iceland’s Laugavegur and Fimmvörðuháls trails—and I think much about the Zerk will also appeal to many ultralighters and thru-hikers.  

On our six-day hike on the 33-mile/54k Laugavegur and 15.5-mile/25k Fimmvörðuháls, I carried a maximum weight of around 35 pounds/15.9k, which included all of my own food (you cook your own meals in the huts), water, sleeping bag, and extra clothing, my DSLR camera and two lenses, as well as incidentals like toiletries and an iPad. It also functioned well fully compressed as a daypack on two half-day hikes of the peaks Blahnukur and Brennisteinsalda in Iceland’s Fjallabak Nature Preserve.

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The Mountainsmith Zerk 40 ultralight backpack.
The Mountainsmith Zerk 40 ultralight backpack.

A simple, frameless, ultralight pack, it resembles others in this category with its roll-top access to the main compartment—with a top and two side compression straps that securely compress the top of the pack, augmented by Z-compression side straps to further squeeze the pack when unfilled, helping to keep it quite stable while hiking even steep trails.

Like similar ultralight packs, it has large external pockets: nine on the Zerk—more than some other packs, though the Zerk’s include four pairs of overlapping pockets that naturally affect one another’s total capacity. The front stretch mesh pocket has substantial space: I stuffed my sandals for the huts and river crossings as well as, at times, my rain jacket and pants, warm gloves, and other small items into it without reaching its capacity. The angled side pockets—an inner and outer pocket on each side, easily reached while wearing the pack—hold a liter bottle with space to spare. I kept a liter bottle on one side, while using the other side pair of pockets to organize light gloves, low gaiters, and a beanie.

The Zerk’s distinctive, EVA foam, wide shoulder straps improve comfort by dispersing weight better than narrower straps. Taking a page from trail running-hydration vests, each has a two-compartment pocket with adequate space for a phone, flexible water bottles, and energy snacks and similarly small items. I also found those pockets useful for holding a second camera lens, to have quick access to it. (I used one of those pockets for that, with snacks in the others.)

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The Mountainsmith Zerk 40 ultralight backpack.
The Mountainsmith Zerk 40.

The pack comes with some useful features: a bungee cord that you easily install in your preferred configuration through five or all seven front gear loops, with enough stretch that, if you actually fill the front pocket, you can still tuck a jacket under the bungee; and a removable accessory webbing strap that can be run through four small gear loops for attaching a large piece of gear atop the pack, like a tent or bear canister—the latter required in some parks, including Yosemite, Grand Teton and the Teton Crest Trail, and on the John Muir Trail and PCT throughout the High Sierra.

As with any ultralight pack, the Zerk’s frameless design, removable foam back pad, 1.5-inch-wide, removable webbing belt, and lack of external load-control straps that you’d see on a heavier pack prioritizes minimizing weight over support and comfort with heavier loads. As expected, I found the Zerk comfortable carrying up to around 30 pounds (as Mountainsmith states as its max weight). When I had around 35 pounds inside, I felt the weight more heavily on my shoulders.

Still, when properly loaded and kept within its intended max weight, the Zerk 40 was comfortable hiking for hours. And the one unisex size, which fits torsos from 16 to 19 inches/40.6 to 48.3cm, suited my 18-inch torso just fine, as did the belt, with a huge fit range for waists 28 to 48 inches/71 to 122cm. That said, it’s not likely a good fit for a much bigger person (I’m 150+ pounds with a 38-inch chest and 30-inch waist) or someone with a small torso and/or narrow shoulders. The adjustable sternum strap features a safety whistle.

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The Mountainsmith Zerk 40 ultralight backpack.
The Mountainsmith Zerk 40.

The main compartment has a wide mouth and interior that makes loading and unloading an easy chore—although, as with any roll-top pack that lacks a lid, accessing the main compartment is somewhat tedious, demanding that and you keep items you’ll need to access quickly and routinely while hiking in exterior pockets. Consistent with its minimalist design, the Zerk lacks a bladder sleeve, with only a click for hanging a bladder.

The Mountainsmith Zerk 40 ultralight backpack.
The Mountainsmith Zerk 40.

At 40 liters/2,440 cubic inches with the collar fully extended—and 32 liters/1,952 cubic inches with the roll-top fully compressed, more the size of a large daypack—the main compartment is not as spacious as some packs weighing two pounds or less. On our six-day hut trek in Iceland, I filled it despite not carrying a tent, air mattress, or kitchen gear. This is an ultralight pack for a very efficient packer outfitted with ultralight, compact gear, not someone who brings many comfort items.

Still, it has the capacity for ultralight backpacking and thru-hiking up to about five days between resupplies, and the roll-top offers good extension for overfilling the Zerk—although that can compromise comfort by making it top-heavy, especially with a tent or bear canister on top, but largely only if your load exceeds about 30 pounds. Plus, it makes for an ideal hut pack or large daypack.

While not waterproof, the 210-denier Extreema recycled nylon pack fabric proved very tough while helping keep the pack’s overall weight very low, and it demonstrated good water resistance in the many rain showers we hiked through in Iceland: I packed everything in stuff sacks that are largely waterproof in rain (but not made for full immersion) and did not use a rain cover and the interior never got wet. Any roll-top closure naturally provides a high degree of water resistance in rain when hiking, anyway.

The 100-denier x Span 840-denier stretch mesh used on six of the Zerk’s exterior pockets—two on the side, four on the shoulder straps—shows impressive durability, appearing unfazed by dropping the pack against abrasive volcanic rocks on the Laugavegur and Fimmvörðuháls trails.

Lastly but most importantly: With all of this, the Zerk 40 falls on the lighter end of the weight continuum of ultralight backpacks at one pound, 13 ounces (822g) with the belt, front bungee, and accessory strap attached and two ounces lighter without them. (I weighed it at 1 lb. 12 oz./794g with the belt and bungee, not the accessory strap.)

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Mountainsmith Zerk 40


The Verdict

While it has less capacity than other ultralight packs of similar design and weight, the minimalist, featherweight Mountainsmith Zerk 40 stands out for its simple yet innovative, modular, roll-top design and abundant external pocket space, which will appeal to committed ultralighters backpacking with up to about 30 pounds.



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NOTE: I tested gear for Backpacker Magazine for 20 years. At The Big Outside, I review only what I consider the best outdoor gear and apparel. See categorized menus of all gear reviews at The Big Outside.

—Michael Lanza


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