Gear review Sierra Designs Flex Capacitor 40 60L

Long Term Gear Review: Sierra Designs Flex Capacitor 40-60L

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The Sierra Designs Flex Capacitor 40-60L has been my main backpacking pack since late 2019; it replaced my Osprey Aura 65L. I am only just now writing this gear review because the pack has been working so well for me that I couldn’t initially think of anything critical to say. I knew that eventually after using it enough, any eccentricities would become more apparent. It’s been incredibly durable and I still haven’t found much wrong with it, though it is missing a few features that might be important to you depending on your priorities.

The pack I have is the “prior year” model here; you can still buy this version. The more recent model has a few changes which I’ll discuss below alongside the pros and cons of the old model. Sierra Designs has not commissioned this gear review nor did they gift the pack to me. I’ve never been given free gear for this website.

Andrew Skurka had some involvement in the original design of the Sierra Designs Flex Capacitor (which has been modified a few times since then). Skurka has a bit of a cult following in the backpacking world; this may either entice you or repel you.


The flexible capacity

What makes the Sierra Designs Flex Capacitor 40-60L unique is obviously its ability to be adjusted to different capacities! The four gusset straps and two compression straps are the main selling point.

The Flex Capacitor can expand to hold 60 liters, or the straps can be tightened enough to reduce the capacity to 40 liters. This is especially handy if, after hauling your tent and sleeping bag to camp and setting things up, you want to venture back out for a short day hike from camp. Now that you’ve removed the heavier gear from your pack, you can tighten it down to 40L and continue using it as a day pack. This versatility also allows you to use the same pack for different types and lengths of trips, instead of needing to buy a variety of backpacks in different sizes.

Note the Sierra Designs Flex Capacitor 40-60L backpack does not have an expandable “neck” above the main compartment, which might be what you’re imagining. For reference, Deuter defines the neck as “the tube of fabric on top of the pack that can be either cinched down or stuffed all the way to the brim” (this feature is standard in most Deuter packs). A neck can add another 10L of capacity when fully expanded, though this capacity is entirely vertical, making the backpack taller. By contrast, the straps on the outside of the Sierra Designs Flex Capacitor can allow the body of the backpack to expand outwards to become girthier. 

Sierra Designs explains that the “adjustable volume is more than just compression of a larger pack. With our Flex packs, the entire pack grows/shrinks in size and volume, instead of just being squished down. This allows you to adjust the size of the pack to your load to ensure a stable carry, rather than rely on compression to maintain shape. And if needed, there’s always the additional compression straps, too.”


Pros and cons of slow travel
Hauling the Sierra Designs Flex Capacitor up the Tour du Mont Blanc ladder section. Looks like I have my rain jacket secured on the outside under the gusset straps.

The Sierra Designs Flex Capacitor 40-60L weighs about 2lbs 10oz, which makes it competitive in the ultralight category. For comparison, Hyperlite Mountain Gear 55L packs are about 2lbs 3oz. The most popular non-ultralight brand of backpacks is probably Osprey. The Osprey Ariel Plus 60L Women’s pack and Osprey Aether 60 Men’s pack both weigh about 5.6lbs. My old Osprey Aura 65L weighed 4.6lbs.

When you can have such a lightweight backpack that still offers the same capabilities as a heavier pack, there are so few reasons to pick something heavier. I’m flabbergasted by how often people choose a multi-day trekking backpack that weighs 5lbs+ when empty. Some people might say it’s about comfort, but I’ve written at length here and here about how I never found my Osprey pack to be particularly comfortable.

The newer model of the Flex Capacitor is heavier by a few ounces at 2lbs 12oz for small/medium torso, or 3lb 2oz for medium/large torso.

Comfortable fit

The first few times I hiked with the Sierra Designs Flex Capacitor, it was like coming out of an abusive relationship and realizing there are packs out there that will treat me better. The Flex Capacitor doesn’t bruise my hip bones like my old pack did. I’m guessing this is not just because of the lower weight of the pack itself, but also the moldable, comfortable hip fins and what Sierra Designs calls the “Y-Flex DAC suspension”:

“A central aluminum hub and 3 separate stays allow our Y-Flex DAC suspension to comfortably carry any load. The Y-Flex adds lightweight support to the Flex Capacitor, and lets the lightweight bag hit above its weight class with comfortable carry.”

In English, this means that they’re using a lightweight Y-shaped pole as a frame for the pack, providing both rigidity and flexion. It’s made of the same material they use for their tent poles. 

The Flex Capacitor has cushioning where the pack touches your back, which keeps the load away from your body and allows some airflow. Pretty much every backpack brand is going to have this. There are two scapular pads and one lumbar pad.

Many external pockets accessible from the front

I love a good pocket. I want sizable pockets on both hip fins, and they need to be large enough to fit an iPhone. It was always frustrating that I couldn’t completely get my phone into the hipbelt pocket on my old Osprey pack, and one of the only real drawbacks about my Deuter day pack is that just one of the hip fins has a pocket, weirdly.

The Sierra Designs Flex Capacitor 40-60L has pockets on both hip fins, and they are large enough for a phone. It also has pockets on each shoulder strap in the chest area. Their marketing refers to these as extra “hydration pockets” but I usually put sunglasses or a bug net in there. These would also be a good place to keep bear spray. The shoulder straps have various loops from which you can clip carabiners or a Garmin Inreach.

Each side of the pack has a lower mesh pocket for holding large water bottles. I am able to fit my one liter Nalgene bottles on each side and retrieve them without taking the pack off. In my experience they are secure enough so that the bottles don’t fall out when I’m leaning different directions; with my old pack I once had a water bottle slip out and start rolling towards a cliff, so I’m always paranoid about that now. There is not a strap to tighten the water bottle pockets, however. One commenter did say they find the water bottle pockets too loose.

There is a hydration bladder pocket on the inside of the pack too, if you’re into that. I’m not a fan of hydration reservoirs. It can be removed from the inside and attached to the outside as a mesh “shove it” pocket, which I’ve never bothered to try but could be nice for storing wet clothes on the outside of the pack. On the newer Flex Capacitor model, the hydration sleeve doubles as a removable summit sack. I think this is a great addition, because even though the main backpack can already be adjusted down from 60L to 40L for a short hike, 40L is still a bit much for that purpose.

Sierra Designs offers a limited lifetime warranty on the Flex Capacitor 40-60L.


Top loader

A top loader backpack has one opening at the top to shove gear inside. If you need to retrieve a piece of gear that’s sunk to the bottom of the pack, you have to dig your arm way down to the bottom to fish for it, or start pulling everything out. A front loader can open from the front, usually with a zipper, so you can easily access anything at the bottom, middle, or top of the pack without rearranging things. The Sierra Designs Flex Capacitor 40-60L is a top loader only, not a front loader.

I’ve visited thru-hiker forums where people swear they prefer a top loader backpack because these folks are just so elite at packing things smartly that they never need to retrieve any gear from the bottom of the pack. I sort of call bullshit on that. We all know we’re supposed to pack the heavy things we won’t need until nighttime towards the bottom, such as the tent, sleeping bag, and unneeded clothes, and pack lighter items at the top where they’re reachable. But the trail is never predictable, is it? That’s the fun of hiking; every day is wide open to possibility and you’re not sure what you might encounter. There are bound to be situations where you randomly need that piece of gear you didn’t expect to. It’s also bound to happen that things you packed towards the top somehow shift and sink lower in the pack throughout the day.

Ultralight packs are almost always going to be top loaders though, so there’s not much chance of getting around this without sacrificing weight. Which feature you prioritize just comes down to personal preference and there’s no right or wrong with top vs. front loaders.

Very few compartments

Sierra Designs Flex Capacitor 40 60L lid
The lid simply zips closed; it’s not the typical “brain”

While the outside of the pack has a good amount of accessible pockets on the hip belt and shoulder straps, the body of the pack itself only has the one zipper pocket on the head of the pack. This lid pocket will do the job if you’re just trying to store a handful of small items, but the Sierra Designs Flex Capacitor does not have a typical “brain” if you’re imagining a removable lid that holds a lot of volume. That said, Skurka made a great point in his product demonstration video that a brain can be cumbersome to unbuckle, especially if a pack also has a neck underneath that needs to be uncinched and unrolled, then recinched, re-rolled, and rebuckled every time. I have used packs like that and it gets annoying. The Flex Capacitor lid is unfussy and I love how quick it is to get into the pack.

The Flex Capacitor 40-60L does not have any outside mesh pockets on the very back**, unless you count the hydration pocket that can be used as a shove-it sack. There are not any organizational compartments on the inside or any hidden zippered pockets underneath the lid.

These are all tradeoffs for weight. A lot of ultralight top loaders are really just one long tube.

**The newer model does now have mesh pockets built into the outside! They’ve clearly listened to feedback and adjusted accordingly. You can see how it works in this video.

Weavers Needle Superstition Mountains Arizona
So many straps! Ignore the weirdly distributed pack job I did that day

On the old model that I have, the gusset straps could sometimes serve the purpose of a mesh pocket, or be used creatively for other things. It was easy to secure sandals, trekking poles, or wet clothes under the many outside straps. Still, a real mesh pocket is more protective and secure if you want to store something like a map or small trinkets like a compass, and if you didn’t think there was enough room in the zippered lid pocket.

On a related note, another change with the newer model is that there are fewer straps and they are now covered by fabric; I’m curious why they did this instead of letting the straps be open to the elements and available to hook carabiners onto, and to secure things underneath. Maybe customers reported that the straps caught on things or tore through the stitching. I never had a problem with that, but I suppose the fabric on the new model will protect the straps and also keep them in place. Or, maybe these changes were just necessary to make way for the new outer mesh pockets.

Doesn’t come with a pack cover

I’ve only ever owned one backpack that came with a rain cover included, the Deuter Trail 28SL women’s day pack, so I don’t feel very spoiled in this realm. Having to buy a pack cover separately wouldn’t stop me from purchasing an otherwise impressive backpack (although maybe reviewers should all lie and say it would, so brands will start feeling pressured to include them). 

60L may be too small, 40L may be too large

There’s really no reason for a day pack to hold more than 32L at the most, and even that can be overkill. In my opinion, 25-28L is a sweet spot for day packs. On the other end of the spectrum, 65L is a standard multi-day trekking backpack capacity. 

On my recent guided Yellowstone group trip, we used even larger 80L+10SL expedition backpacks because we needed to carry extra group gear (fancier food options for everyone, cooking utensils, a tarp for us to socialize under when it rained, etc). I brought my Flex Capacitor along in case I could use it, but it was clearly not going to be enough. That said, I can’t think of too many reasons to carry a large 80L+ pack for a solo trip, where I live off protein bars and tortillas, unless it’s a really long trek without much resupply opportunity or perhaps a canyoneering trip where climbing gear is needed.

60L has been enough for me on ~10 day (or less) solo backpacking trips where I had many opportunities to resupply. Thru-hikers that manage to get their base weight really low would probably find 60L to be too much, and would be happy the Flex Capacitor can cinch down to 40L. 

Otherwise, I think 40L is kind of a weird middle ground for wilderness trips. I have seen some travelers vote that a 40L backpack is the perfect size for plane travel and “backpacking” in the sense of “backpacking Europe”, aka taking trains and staying in hostels. I am nomadic myself but I can’t really attest to the Flex Capacitor’s usefulness in this regard because I still haul my camping gear when I travel. When I traveled around Europe with the Sierra Designs Flex Capacitor for a few months, I simultaneously carried my normal town clothes, laptop, and my hiking and camping gear because I was doing some trekking in between city hopping. For this reason, the full 60L capacity wasn’t enough and I ended up carrying a small day pack around with me too. If I was only doing the digital nomad thing and not trying to incorporate hiking trips, maybe 40L would be a suitable capacity.

I will continue to use the Sierra Designs Flex Capacitor 40-60L prior year model on future trips and have no reason to make a change. That said, if I am ever in the market for a new backpack, there are a few features I would pay attention to. I might like a backpack with an adjustable torso length, a front loader design, and an outer mesh pocket on the back. Is there anything you prioritize in a hiking backpack that isn’t represented in the Flex Capacitor?


Sierra Designs Flex Capacitor 40-60L for backpacking
Sierra Designs Flex Capacitor 40-60L (prior year)
Garmin InReach Mini
Garmin InReach Mini
Nalgene On The Fly hiking water bottle
Nalgene Water Bottle
Bear Spray
Counter Assault Bear Spray
BearVault Canister
BearVault BV450 Food Container
Sierra Designs Flex Capacitor 40L 60L Butterscotch
Sierra Designs Flex Capacitor 40-60L (current model)

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Hiking Backpack Review Sierra Designs Flex Capacitor 40L 60L


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