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Sierra Designs Flex Capacitor (Closeout) Internal Frame Backpack

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The truly innovative Sierra Designs Flex Capacitor backpack has an integrated, gusseted compression system that lets it swiftly expand from a 40 liter to a 60-liter capacity and comfortably carry up to 50 pounds!


The truly innovative Sierra Designs Flex Capacitor backpack has an integrated, gusseted compression system that lets it swiftly expand from a 40 liter to a 60-liter capacity and comfortably carry up to 50 pounds!

Balancing weight and performance, the pack utilizes a Y-shaped DAC Pressfit aluminum support system to ensures vertical rigidity while allowing the top portion of the pack to flex with your movement. The result is maintained load stability without excessive bulk or weight. Raised padding in the lumbar and scapula regions of the backpanel provide support and cushioning while also creating space for airflow. A padded and adjustable hipbelt further enhances balance while load lifter straps and a sliding sternum strap allow you to dial in fit. The pack is available in Small/Medium (best fits 16”-19” torso lengths) and Medium/Large (18”-21”) sizes to make certain that the wearer best benefits from the structural design of the pack.

The top-loading pack employs a U-shaped zipper to offer wide-open access to its interior. By adjusting the horizontal compression system that runs across the face of the pack, you can utilize the full 60-liter capacity or cinch down to secure contents and “shrink” the pack to a more streamlined 40 liters. A stash pocket on the lid, stretch mesh side pockets, and two large hipbelt pockets provide additional storage and organizational options. Other features include a front harness pocket that is perfectly sized for a water bottle (bottle not included), ice axe/trekking poles loops, and lined hydration sleeve (hydration system not included) that can be removed if not needed or if you simply want to shave ounces.

The Sierra Designs Flex Capacitor internal frame pack is available in your choice of sizes.

    Packbag Features:
  • Expandable 40-60L volume (volume varies between sizes)
  • Ultra-lightweight design
  • 2 torso sizes (non-adjustable)
  • U-shaped top access zipper
  • Zippered stash pocket on lid
  • Removable reservoir sleeve
  • Shoulder strap water bottle pocket
  • Stretch mesh side water bottle pockets
  • Two hip belt pockets
  • Compression straps
  • Ice axe/trekking pole loop

  • Suspension Features:
  • Lightweight "Y-FLEX" suspension
  • EVA foam hip belt and shoulder straps
  • Raised lumbar and scapula pads (EVA foam)
  • Load lifter straps
  • Hip belt stabilizer straps
  • Adjustable hip belt
California residents: view proposition 65  Warning

Technical Specs

Best Use
Backpacking, Climbing, Mountaineering
Top Loading
Item Dimensions
Small/Medium: 28.5" x 13" x 10"
Medium/Large: 30" x 13" x 10"
Fits Torso
Small/Medium: 16"-19"
Medium/Large: 18"-21"
Fits Waist
Small/Medium: 29"-32"
Medium/Large: 32"-35"
Frame Material
DAC Pressfit Aluminum
Hydration Compatible
Maximum Weight
50 lbs
Sternum Strap
Yes; Adjustable
Torso Adjustment
Small/Medium: 2400-3400 cu in
Medium/Large: 2550-3650 cu in
Limited Lifetime
Internal Frame Backpack
100D Nylon-Poly Ripstop and 420D Nylon Oxford
Item Weight
Small/Medium: 2 lb, 9.2 oz
Medium/Large: 2 lb, 11.2 oz

16 Reviews

4.38 / 5

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One of the best backpacks for me


Amazingly versatile and comfortable. Love this pack.


I don't know

John K.


strong hiking backpack


even thought it's not updated version, it's still great backpack for hiking.


there's a mesh pocket only one side, not both side



best hiking backpack


I love the size and color, especially great size for hiking. light weight, strong backpack


pretty hard to have it stand up

Forest H.


Sizes are off!


I wanted to love this pack. I wear Medium Sized Packs. 5'10" 166 lbs


I wear Mediums, The M/L is just a tad too large for me. SD cut costs by only offering 2 sizes instead of 3 and the Medium isn't a great fit. It needs a second shoulder strap bottle holder.

Den Sk.


One of the best backpack


I think Sierra Designs Flex Capacitor is one of the best backpack for 5-10 day trekking: comfortable and large belt pockets, well made, amazingly light, bag on the front strap is great place for my sunglasses or smartphone.


no raincover

Valeriia S.


My favorite backpack


This backpack is very comfortable for me. Amazingly light, worked well for me. For the weight it's actually very strong and durable material. Storage on the hip belt is great. I can recommend this pack!


I don`t now

Michael P.


Versatile Pack


Decided it was time for a new pack and just loved the simplicity of this design. At just over 2.5 pounds this is a very light pack but also seems to be very durable. I bought the pack back in the spring but wanted to use it a bit before writing a review. After some use, I can say that I am very satisfied with this pack and would purchase it again. The pack is super comfortable and has incredible airflow (the only thing touching your back are the shoulder pads and the lumbar pad). It is very light, but capable of carrying a heavier load and remains comfortable. I love the large pockets on the hip belt and also like the water bottle pockets on the sides of the pack, they are great for water bottles or using the compression straps they also work great carrying something a little taller like tent poles. My favorite pocket is probably the water bottle pocket on the shoulder strap, I don't use a hydration bladder and having water in an easy to access pocket is great. I also really love the zipper access to the main compartment of the pack, but the zipper does tend to snag. I have not really needed the expandable capacity as the pack has served my needs in the 40 liter configuration, but I love the idea and know that if or when I need more room that this pack will expand up to 60 liters and still manage the load no problem. If you are looking for a 40-60 liter pack this pack should definitely be on your short list, it is light, durable, comfortable and overall a simply well designed pack.


When I first considered this pack, I really wanted a pack that had sleeping pad straps. I almost always carry a closed-cell foam pad as a back-up pad just in case. If you still carry a closed-cell foam pad it is possible to carry it using the compression straps, but including sleeping pad straps would add very little weight and I think there are enough people carrying closed-cell foam pads that they should be included. And while I really love the zipper access to the main compartment, it definitely does snag. And I don't mean sometimes, I mean almost every time, to the point that I simply keep the rain-guard folded up to avoid snagging.

Wei Khoon Ng


My first lightweight backpack


Awesome design.. What Andrew Skurka say on Sierra Designs Gear should be true..


The top lid pocket are too small and shoulder harness straps buckle are not so smooth to pull it.. Wish to have thicker shoulder harness strap , both side have front harness pocket and bigger hip belt pocket..

Mink K


Good choice for long distance hiking


Great ventilation for summer hiking. the stability of shoulder and hipbelt strap is significant.


Sometimes when you open top-lid zipper, it's a little bit chewy

Shannon G.


Great Pack But...


It's expandable!! I love it! This pack can be compressed down to a 40 liter for just a few days on the trail or expanded out to a 60 liter for much longer stays. The large belt pockets are great for those items you need quick access to when your on the move and the water bottle pouch on the shoulder strap is a great touch. It is a simple, straight forward, lightweight pack that carries well and with a few changes could be just about perfect.


#1 Please put a wider storm flap over the zipper! It WILL get caught every time you unzip the pack until you ingrain in your brain to flip it up all the way around before you ever attempt to unzip the pack and when it does get caught it REALLY gets caught! #2 I really wish the shoulder straps had some adjustment giving the ability to increase or decrease the distance between the to arch of the shoulder straps and the waist belt. I measure for a medium and the only option is a M/L. I could get the medium belt to help with excess belt slack, but when carrying a load on an extended trip the pack tends to settle slightly low on my back, with no way to good way to keep it up where it should be. Just a little adjustment ability would go a long way. #3 Would love to see them add external straps to the bottom just in case. Bottom straps are not a problem when you don't need them, but definitely missed when you do need them and they're none existent.

Taylor G.


This could be my only pack


At 2LB 10oz this pack could work as a multi-day bag or for day-hikes. With the Y shaped suspension I will take this for day-hikes vs. my current day bag as it weighs almost the same but this one actually transfers load to hips. Very nice to be able to adjust the capacity on the fly.


External pocket options. ( at least give us lash points to add some) Top pocket should have been sewn in a way that it did not take space from the pack body but sat on top of lid

Chris S.


Great concept, but I question durability


Being able to adjust the pack's capacity is genius. Pockets are large. It fits a large BearVault 500 easily.


The bar tack on the compression strap attaching the buckle looked to be sewn so tightly that it cut the fabric, which began to fray only after one weekend trip. The medium/large hip belt is big; At 150, 5'7", 33 inch waist, I had to order the small hip belt. Note that the smaller belt comes with much smaller pockets, which don't fit large phones or cameras. The pocket in the top lid is hard to access given its slim design when the main body is packed and zipped closed. The removable water bladder pouch will take some getting used to. When the pack isn't fully loaded, the pouch can shift to one side easily, making for an awkward load carry.