Sierra Designs Flex Capacitor Review (Flexible but Limited)

Sierra Designs Flex Capacitor Review

The Sierra Designs Flex Capacitor is a well-made pack by a company that has been making quality packs for a long time, but it misses the mark for me. I fall in between the two sizes, and I miss the large back pocket that most packs have. 

This pack is different than most other packs in this category. If you are looking for one pack to do it all, and it happens to fit, the Flex Capacitor might be for you.

Sierra Designs Flex Capacitor
  • Expandable capacity
  • Decent back ventilation
  • Durable construction
  • Water bottle pockets on the shoulder straps
  • Compression straps require some fiddling
  • No large pocket on the back
  • Limited sizes
  • Uncomfortable
  • Expands horizontally, not vertically
View on Amazon View on Sierra Designs
02/19/2024 04:16 am GMT

My introduction to backpacking began almost 30 years ago when my dad had the bright idea to hike into Independence Lake in south central Idaho, carrying all of our gear in a large wooden crate. We made it about 100 yards before we gave up.

Since then, I have refined my technique and my gear to meet my needs and try to make it past the 100-yard mark.  

What I have learned is that lightweight backpacking is not for everyone. If you plan to carry outlandish loads, please buy an outlandish backpack.

If, however, you are planning on carrying only what you need and are ready to shed some weight off your current system, a lightweight backpack is a great place to start.

I was excited when I received the Sierra Designs Flex Capacitor for review. The Flex Capacitor was designed by one of my backpacking heroes Andrew Skurka.

The Flex Capacitor also gave me the excuse I was looking for to head off to the high alpine lakes of the Sawtooth Range of Central Idaho. The Sawtooths didn’t disappoint but this pack did. 

Check out our article on all the Best Ultralight Backpacks.

Sierra Designs Flex Capacitor 40-60 liter Hands-on Review

One of the Flex Capacitors selling points is the expandable volume, and as the name states, this pack has a volume of 40 to 60 liters. I usually do not need that much capacity, but I can see the advantages of having one pack that does both long and short trips.

The Flex Capacitor is designed to be comfortable while carrying loads up to 35 pounds to 40 pounds. This is more weight than lighter packs like the Osprey Levity can handle. On one trip, I carried a 35 pounds pack and it was ok, but I found that the pack did better below 30 pounds.

The Flex Capacitor weighs in at 2 pounds 10 ounces making it an average pack in this category. The model that I received was black with red and grey accents. I thought that it was a nice-looking pack.

I prefer a smaller pack to help me make better discissions on what to pack and I could imagine that the extra capacity of this pack might lead me to the temptation to pack more than I need.

This pack comes in three different capacitates; the 25-40 liter version, the 40-60 liter version, and the 60-75 liter version. Out of the three, I feel that the 40-60 liter is the most versatile pack, so that is the pack that I chose to review.

Fit-wise, the Flex Capacitor only comes in 2 sizes: an S/M and an M/L. I am 5’ 10” and weigh in at 170 pounds. Ok, ok, 180 pounds if I’m being honest.

I found that the M/L size that I tried was too big for me. I did not get a chance to try the S/M but I’m afraid it would be too small. Being in-between sizes resulted in a pack that did not fit me very well, and I wished that Sierra Designs would make at least three sizes like the other packs in this category.

To help with sizing, Sierra Designs does sell two different sizes of waist belts. The M/L comes with the M/L belt, and that is the belt that I used. I was constantly tightening up the waist belt and might have needed the S/M.


Outdoor Empire Score: 5/5

Testing sierra designs flex capacitor 40's capacity
Overloaded for an overnight trip. With the pack closed, there is a flat shelf to place items. In this case, it is my daughter’s backpack that she was “too tired to carry.”

The pack is made from a combination of 100D Nylon-Poly ripstop with some high-use areas reinforced with 420 nylon oxford material. There is no mention of any waterproof coating being placed. I did not get a chance to test this out, but it would be prudent to have a pack cover available if it looks like rain.

The build quality of this pack is top-notch, as should be expected from Sierra Designs. After my testing, I found no evidence of any wear.

Expandable Capacity

The expandable capacity is a big selling point for this pack and is what differentiates it from other packs, so we will spend a little time talking about it.

The expandable nature of the pack comes from horizontal gussets that six compression straps can tighten. It should be noted that all of the expansion happens, just like me as I got older, horizontally. There is no vertical expansion. 

I usually don’t need 60 liters of volume on my regular trips. So to test this pack, I took it on my annual family backpacking trip to Halverson Lake. This was an excellent trip for this pack because I ended up carrying all of my gear plus the gear for my 6-year-old daughter. This pushed the 60-liter capacity to its limit.

While on the trip, I found that the compression straps worked ok, but I did have to fiddle with them on occasion. 

Just like my stature, I wish the pack would expand vertically and horizontally. I found that as the volume expanded, it drew the weight of the pack further away from my center of gravity, making me slightly unbalanced. 

Main Pack

The main pack is basically a giant sack with no dividers and only one access through the top. The top is closed via a U-shaped zipper. There is a top pocket that is small and only has room for the essentials. Not having dividers makes organizing gear a little tricky but with a little practice and forethought, it can be done.

One of the major disadvantages of this design is the lack of a large stretch mesh pocket on the back. Other packs in this category, like the Gregory Focal 48 and Gossamer Gear Gorilla 50, have a large mesh pocket on the back that helps organize the gear.

I found that I really missed this pocket since I use it to keep items that I want handy and to organize my load. With the Flex Capacitor, everything had to be inside the main pack. 

Hip Pockets

There are two hip pockets sewn into the hip belt. The pockets are big enough to fit my phone and snacks. I sometimes forgot that the pockets were there and I didn’t always use them as well as I could have. That being said, these pockets were a great size and did not get in the way. 

Side Pockets

Sierra designs flex capacitor 40 side pocket
The side pockets, while small, held everything that I wanted them to. | Photo Credit: Mark Gurney

The Flex Capacitor has a side pocket on each side of the pack. These pockets were smaller than the side pockets found on other packs. For example, the pockets on the Hyperlite Mountain Gear Junction 2400 are almost twice as big.

I thought I would not like them since they were so small, but I found that they worked fine and I had no problems with them. I could easily put a 1-liter water bottle on each side with enough extra space for a Tenkara fly rod or tent poles.

The pockets were also low enough on the pack to be able to get at my water bottles easily without taking my pack off or feeling like I needed to dislocate my shoulder.

Water Bladder Sleeve

I don’t usually use a water bladder sleeve while backpacking but the Flex Capacitor does accommodate one. Because I opted out of the water bladder, I did not get the chance to use this feature, but I was happy to note that the sleeve was removable. I appreciate that I can take it out and forget about it.

Water Bottle Holders

Sierra designs flex capacitor 40 water bottle holders
I love the water bottle holders on the shoulder straps. These allowed easy access to my hydration of choice on that day. | Photo Credit: Mark Gurney

One of the things that I appreciate about this pack is the built-in water bottle holders on the shoulder straps. They fit 16-ounce water bottles for easy access to water. I usually keep one filled with plain water and the other with an electrolyte drink like Tailwind.

The water bottle holders worked great and kept my fluids easy to access, allowing me to stay hydrated. They are made of lightweight mesh and only add a small amount of weight.

Any time that you add weight, it should be balanced against the benefit that you get, and I found that the minuscule amount of extra weight was definitely worth it to have my water bottles handy. 

I wish that more companies would add these.

Suspension and Weight Distribution

Outdoor Empire Score: 3.5/5

Sierra designs flex capacitor 40 foam for ventilation
The back panel has three molded foam pieces that give it some separation from the back and allows for ventilation.

The suspension system of a backpack is how the pack supports the load and helps distribute the weight between the shoulders and hips. Suspension systems can range from complex and heavy to simple and light.

I would classify the suspension system of the Flex Capacitor as more on the complex side with all of the adjustments you would find on a bulkier pack yet it remains lightweight.

The Flex Capacitor has a Y-shaped internal frame that helps structure the back panel. Sewn onto the nylon pack panel are three shaped pieces of foam that keep the rear panel off of the back and provide a decent amount of separation and ventilation.

It provides more ventilation than packs like the Hyperlite Mountain Gear Junction 2400 and the Gossamer Gear Gorilla but less than the Gregory Focal 48 and the Osprey Levity 45.

The Flex Capacitor includes load lifter straps to help pull the load closer to the body and fine-tune the fit. These are important because the load is carried further back on this pack, especially when it is filled to capacity.

The hip belt on the pack is removable and comes in two different sizes, as stated earlier. I have found that removable hip belts do not hold weight on the hips as well as integrated hip belts. There is usually too much play in them.

I found this to be the case here as well. I also found myself constantly tightening the belt on my trip to keep it tight enough to take some of the load off my shoulders.

The ability to keep the weight off your shoulders becomes more critical as the weight increases.

At 20 pounds, I find that I am fine with most of the weight on my shoulders, and the hip belt is just there to provide stability and keep the pack from swinging around too much. As the weight gets closer to 30 pounds, it becomes more important to distribute that weight.

The suspension on this pack was adequate. It actually did a better job than the Hyperlite Junction 2400, but the Flex Capacitor is designed to carry bigger loads.

When loaded light on a solo trip it was fine, but when I loaded the pack up with my stuff and my daughter’s gear, it struggled. Part of the problem could be that the pack didn’t quite fit me.


Outdoor Empire Score: 3/5

Hiker with sierra designs flex capacitor 40 up the alpine
I was excited to get up into the high alpine. Maybe a little too excited. | Photo Credit: TJ Eardly

Comfort is subjective and different for everyone, but I struggled to get this pack to fit me. I was constantly adjusting both the shoulder and hip straps but could never seem to get the fit dialed in. The pack was never horribly uncomfortable, just slightly annoying.

When the pack capacity was under 50 liters, the pack swing was manageable. However, as the pack got closer to 60 liters, the weight was carried further away from my center of gravity and the pack started to swing back and forth a little.

Versatility and Accessories

Outdoor Empire Score: 4/5

The Sierra Design Flex Capacitor gains its versatility through its ability to expand from 40 liters to 60 liters. That is a large capacity range and represents a wide range of possible trips this pack can handle.

From an overnight trip to trips longer than a week, this pack can expand and contract to cover them all. It makes this pack one of the more versatile packs in this category.

Other than a replaceable hip belt and a pack cover, Sierra Designs does not make any other accessories for this pack. The placement of the water bottle holders on the shoulder straps makes it difficult to place any other accessories there. To be fair, I would rather have the water bottle holders than anything else. 

I usually carry a large camera on backpacking trips and strap it onto the shoulder straps so it hangs on my chest. The only place to hang it off were two vertical straps made from an elastic material. These elastic straps made my camera bounce every time I took a step.


Outdoor Empire Score: 4/5

The Flex Capacitor retails for $219.95, which is about average for a pack in this category. This works out to an average of $4.58 per liter. 


Outdoor Empire Score: 3.7/5

The Flex Capacitor is a well-made pack by a company that has been making quality packs for a long time, but this pack misses the mark for me. Having only two sizes makes me feel like Goldilocks. This one is too big and the other is too small.

I missed the large back pocket found on other packs. Also, I prefer a pack that expands horizontally and vertically rather than just horizontally like the Flex Capacitor.

Sierra Designs Flex Capacitor
  • Expandable capacity
  • Decent back ventilation
  • Durable construction
  • Water bottle pockets on the shoulder straps
  • Compression straps require some fiddling
  • No large pocket on the back
  • Limited sizes
  • Uncomfortable
  • Expands horizontally, not vertically
View on Amazon View on Sierra Designs
02/19/2024 04:16 am GMT


This pack is for the person on a budget who wants one pack that will cover them for various trips of different lengths, from overnight to 10 days. It would also be suitable for someone with lightweight gear that might be a little bulkier.

The Flex Capacitor is also a pack that does better with a pack weight under 30 pounds but it can be used, on occasion, up to 40 pounds. 

This pack might also be beneficial for the father of small children or the Scout Leader who needs to overpack to help augment ill-prepared kids on one trip, but then the following weekend they are on their own and need to go light and fast.

If this category includes you and this pack interests you, I would highly recommend trying it on before you buy it. Of course, I would suggest that for all packs.

For those that want a specialized tool for a specialized task and are ok owning multiple packs, this pack is probably not for you. This pack is also not for the person who only does one trip type and thus only needs a specific pack.

The Hyperlite Mountain Gear Junction 2400 and the Osprey Levity 45 are lighter but lack the capacity for longer trips. Both packs also come in larger sizes but then they are harder to manage for the quick overnight trip. 

Flex Capacitor  Recommended For Not Ideal For
Flexibility One pack to do it all Those who prefer specialized gear
Trip Length 2-10 days >10 days
Base Weight 10 to 15 pounds >15 pounds <10 pounds
Total Weight 25 to 35 pounds < 20 pounds >40 pounds
Miles per day 15 to 25 miles per day >30 miles per day