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Flex Capacitor

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10 Best Lightweight Backpacks of 2018 - CleverHiker

Best Ultralight Backpacks of 2018 - GearJunkie

One of The Best Summer Packs of 2017 - Outside Online

Top Gear of the Year 2016 - GearJunkie

The Flex Capacitor features a unique patent pending gusset system that allows the pack to quickly and easily expand from 40-60 liters with the adjustment of a few straps. Instead of expanding up like other packs, the circumference of flex expands to provide a more stable and comfortable load carry.


Key Features

Packbag Key Features

  • Expandable 40-60L volume (volume varies between sizes)
  • Ultralight weight design
  • 2 torso sizes (non-adjustable)
  • 4 hip belt sizes
  • 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 Key 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


  • Body Fabric: 100D Nylon-Poly Ripstop
  • Secondary Fabric: 420D Nylon Oxford
  • Stay Material: DAC Pressfit Aluminum
  • Number of Stays: 1 "Y-FLEX" stay

Tech Specs and Sizing

TORSO SIZE 16" - 19" 18" - 21"
VOLUME (Liters) 39 - 56L 42 - 60L
VOLUME (Cubic Inches) 2400 - 3400 2550 - 3650
WEIGHT 2 lbs. 9.2 oz. (With S/M hip belt) 2 lbs. 11.2 oz. (With M/L hip belt)
DIMENSIONS (LxWxD) 28.5" x 13 "x 10" 30" x 13" x 10"
MAX COMFORT WEIGHT 35-50 lbs. 35-50 lbs.

WEIGHT (Ounces) 10.2 11 11.5 12.2
WEIGHT (Grams) 290 310 330 350
HIP BELT FIT RANGE* (Inches) <29 29 - 32 32 - 35 >35
HIP BELT FIT RANGE* (Centimeters) 74 74 - 81 81 - 89 89

*This is a hip measurement, not a waist measurement.  Please watch pack fit video to learn how to properly measure.

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    • Sierra Designs Flex Capacitor 40-60 Pack
    • How to pack a backpack: Organization & load distribution
    • How to get measured and pick the perfect Sierra Designs Flex Capacitor backpack, with Andrew Skurka.

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    Product Reviews


    Write A Review

    1. Just got it

      Although I've only used it 1 time so far, I will say instantly I was very impressed with how well it fits and carries a load. It really can bend and flex with you and it really feels like a part of you when it's on. The weight doesn't shift at all. The belt pockets are great in size and the water bottle pouch on the harness works great. It definitely is a MUST see to watch how Andrew Skurka packs this pack. I think everybody is pretty set in their ways in how they pack your standard style backpack with a removable top. This pack can really hold quite a lot but you have to rethink how you do it, and his video was extremely helpful for that. Especially for those who do things like use foam sleeping pads that want them strapped underneath the hood or underneath their backpack, where in this pack you would strap it to the back for example. Works great but just have to be willing to try a new style. In truth once I get used to the differences I can't honestly say I would want to go back to the old style packs, because the features this pack has that all of those don't. If you find yourself really looking to cut weight and go more minimal this pack really caters to that, although not cuben fiber, it's way more durable and has way more features, for what a half pound more - totally worth it. I really am very impressed with this pack and will update this review as I continue to use it, but for the $ this is the best fitting most comfortable pack I've tried and for the price, it really is killer. Again, I cannot stress enough to watch the videos that Sierra designs and Andrew made before you buy this to make sure you get the right fit and understand what it is all about. on Jun 12th 2018

    2. Remarkable versatile pack

      Until they have a pack that carries itself, every pack will be a compromise, but this one comes the closest to balancing light weight, durability, and large capacity as I've seen. Very, very comfortable in return for the extra few ounces, compared to packs with little or no frame systems. Love the spacious hip belt pockets. Like others have said, there should be a provision for strapping something to the bottom - four small attachment loops would only add a gram or two. A great pack - I've already bought a second one to give as a gift! on Jun 12th 2018

    3. Great Design, Great Pack, Great Company

      Just got done with a six day packrafting trip in the desert and it was the ideal pack for the job. I went between 25 lbs (without packraft and water) to 45 lbs (packraft and 10 liters of water) making the compression system invaluable. I was shocked at how comfortable the pack remained at both weights, with great padding and a really robust frame compared to every other ultralight system I have tried. Oversize hip belt pockets and the incredibly convenient shoulder water bottle holder were amazing, and I even really liked the small brain compartment that prevented me from stuffing it with too many random things. I had to scrape through a couple of slot canyons on the trip and was impressed by the durability of the of the material as well. At $180 cheaper and only 4 ounces heavier then a frameless, pocketless, non size flexible Hyperlite, the Flex Capacitor is the way to go.

      Oh and just a P.S. I got the wrong sized hip belt and the return process was fast, friendly, and painless. I am really impressed with the attitude and innovation of Sierra Designs.
      on Mar 21st 2018

    4. Great pack with just three suggested imporvements

      I took this pack on a 6-day, 5-night rim-to-rim backpacking trip in the Grand Canyon in 2017 along with several other multi-day trips in Colorado. I love MOST everything about it.

      However, here are a few suggestions that I feel would improve the pack:

      1) The lumbar pad needs to have a vertical recess in the middle to provide the spine vertebrae some pressure relief. On a long hike, the lumbar support made my vertebrae ache and I was constantly trying to adjust it to provide my back some relief (38 lbs);

      2) This pack has a well located small top-cap zippered compartment. This pocket expands downward into the main compartment. The problem with this design is that the top compartment competes with the main compartment for space when the pack is fully loaded for a long outing. When the main compartment pack is full it can be difficult to close the zipper that joins the upper smaller compartment with the large main compartment. The suggested solution would be to have the top compartment have side walls that expand upward (as opposed to downward with the current design) which would allow for more usable room to accommodate those items that you want ready access too. As much as possible you really want to be able to pack each compartment independently from the other. They should not compete for the same space.

      3) I'm 5'11' and 170 lbs with a 33-inch waist. The pack's main waist belt had to be re-tightened often. The curvature of my waist was just enough that the two cinches, one on each side of the main buck allowed the belt webbing to slowly slip through the plastic cinches. I found myself constantly retightening the waist belt. There either needs to be a true lock-down mechanism that pinches the waistbelt in such a manner that prevents it from loosening or there needs to be a better cinch selected that will not allow the nylon webbing to slowly release through each cinch as you hike along the path.

      Small things but they were annoying enough to comment on. The Flex Capacitor is truly a wonderful, lightweight, backpack. I am scheduled to return to the Grand Canyon in May and I will be using this pack again but with a few modifications and knowledge of packing limitations between the main compartment and the smaller top compartment.
      on Mar 19th 2018

    5. It does what I want

      Cliff Notes
      The Flex Capacitor is a great backpack. It’s light weight and comfortable. The hip belt sizing cart is spot on, trust it. If you’re using modern gear this pack will be great for you. If you have an old Coleman bag or take the kitchen sink with you this bag may not be for you.
      Long Form
      I have never found a backpack I really liked until this one. The backpacks I’ve tried are: Lowe Alpine Centro 45+10 (hip belt was too big), SD Revival 65 (great frame, really bad sectioned hip belt), BD Mercury 65 (Really didn’t like the pivot), REI Flash 65 (I don’t even like talking about it). Needless to say, I’m picky. I’m skinny, so I have problems with hip belts fitting. I have a 29-inch waist and even when hip belt says it goes to 28 inches usually I’m completely maxed out on the tension. I ordered a S/M pack with an XS/SM hip belt because I had been burnt so many times in the past from other manufacturers. SD sizing is true, and I should have gone with the correct size according to the chart. Even with this mistake the pack did great. I had a total pack weight of 25 pounds. I didn’t get the usually hot spots on my hipbones. I like that the pack has the things I want and none of the stuff I don’t. It’s simple, but has stretch side pockets, hip belt pockets, and the shoulder pocket. The shoulder pocket is nice for a half liter of water and moves some weight to the front. It doesn’t have bottom or side access, or a sleeping bag compartment. I don’t like those features and I’m glad they are missing. I have a 17-inch torso and the pack fits fine. It doesn’t need to be adjustable. I was also happy to be able to remove the reservoir sleeve. My concerns about the back is that it is a small 60. It’s a zip top so there is no over loading the thing. The top pocket is small, so you must be selective on what goes in it. I’m going to change the zipper pull on the top pocket to a different color to limit confusion, between it and the main compartment. It’s a steal at twice the price. It’s worth a try.
      on Dec 1st 2017

    6. Great lightweight pack

      I have not been a fan of top loading packs and purchased this pack because of its weight and my experience with other Sierra Design products. I took this pack out for its first trip on the Timberline Trail in late October. We had early snow at some of the lower levels and I was happy to have a lighter pack than I usually carry and pleased with my contents staying dry. I also found the pack to more comfortable than I expected. The only drawback I had with it, was lack of attachment strapping or hoops. Something can be remedied by adding later. Did use the compression straps on the front to hold my foam sleeping pad vertically, and that worked well. If the pack would have been maxed out, that's not an option. Very happy with this pack and will be my main pack from now on. on Nov 18th 2017

    7. An excellent pack, but the bottom material is not hardy enough.

      I used the pack, for the first time, on a eight day cross-country backpacking trip in the Sierra. I have nothing praise for the overall design. My one dislike is the insufficient material at the bottom of the pack. The granite cut numerous holes in the material. I don't think the cloth is strong enough for cross-country hiking. I understand there is a trade off between weight and durability, but the lower section of the pack is durable enough. on Sep 18th 2017

    8. Great pack

      I took this pack out about a month ago for a three day, forty-ish mile trip in Desolation Wilderness. Before I bought this pack, I would have used my Gregory Z55, but I wanted to try something lighter, and so I ended up with this. I have no regrets. I really like having one big internal compartment rather than several-- makes it easier to stay organized (I always forgot which compartment contained what), and it allows you to weatherproof your load from inside the pack (as the guy recommends in the video). The external pockets were all the right size and easy to access, the load carry was super stable, and back ventilation left nothing to be desired. I really like this pack, and I plan to use it for longer trips. on Sep 8th 2017

    9. Great pack, carried on the GR20 in Corsica

      I just returned from a 13 days trekking on what's called Europe's most difficult long distance trail. I've been so happy with this pack in the mountains. I did not fill it complete, so average weight carried was up to 12kg in the morning with 2L waterbladder filled. What is surprising is that with that load I do not feel the weight on my hips or shoulders. Thanks to the lumbar pad doing a lot of the carrying work ! but it is also not felt. My experience is that the load is spread over 3 zones instead of 2 as other packs do . (I was in Spanish Pyrenees last year with a high quality Lowe Alpine backpack and my hips hurt like hell and felt numb for weeks after the trip.) One of the 2 red straps at the bottom i used for holding the bottom of the trekking poles. Other people's remarks about missing straps to attach stuff was not an issue to me : I used the "expansion" straps for that and for sock and clothes drying :)
      on Aug 15th 2017

    10. Awesome pack

      Just bought one of these great packs to start backpacking again. It has been 33 years since I last backpacked, too busy with many other outdoor activities (day hiking, trail running, mountain biking, backcountry skiing, snowshoeing, kayaking) and work has kept me from backpacking adventures. We just had out first trip and I could not be more pleased with this pack. Carried very well, super easy to pack and organize and was the perfect size. Thanks to SD, I am again super excited about backpacking again. I am now excited to buy one of the SD backcountry quilts for a sleep system. on Aug 12th 2017

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