Free Shipping offer is valid in the US lower 48 states only, and is not available for Pro purchases, PO Boxes, or Military APO/FPO addresses. Discount codes are not valid on sale products or Pro purchases. Limit one discount code per order.


Flex Capacitor

$199.95 $149.96
Out Of Stock
Back view of Sierra Designs Flex Capacitor backpack with a small capacity interior load

2017, 2018, and 2019 Budget Pick -

10 Best Lightweight Backpacks of 2018 - CleverHiker

Best Ultralight Backpacks of 2018 - GearJunkie

One of The Best Summer Packs of 2017 - Outside Online

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 lower-back 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.

config_file_url: /content/webrotate360/360_assets/flexCapacitor/flexCapacitor.xml
Award Badge: Yes
    • 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.

    360 Spin

    Click and Drag to Rotate

    Product Reviews


    Write A Review

    1. Post JMT hike review

      I used the Flex Capacitor on the JMT in 2018 (Aug 22- Sep 11). This backpack worked extremely well. I saw 3 other folks on the trail with the same pack. This pack was also checked into 4 flights and it held up well. I had already written a review here under the name Mowgli. You can read that. My views about this bag being excellent in carrying capabilities has not changed post JMT. I still love this bag.

      This is post JMT review.

      The only issue was that was some fraying and a small 2mm slit on the panel on the front of the backpack which gets hidden in the 40 L mode and is exposed in the 60 L mode. This panel has the compression straps running across and gets exposed to abrasion etc when it leans against trees/rocks. While the pack stands on its own most of the time around campsites, it is those times when you are in the middle of the trail and need to take a break for whatever reason and there is not much of a flat area and you need to lean it against a rock or tree. I guess you could lean it with the shoulder strap side.

      One of the other hikers who carried the same bag also had a hole on the front of the bag (non-shoulder strap side). So, it was not just me.

      It will be nice if that panel is made of 210D or 420D nylon or some other durable material. I am planning on making sure there is some other material between bag and tree/rock - maybe use the hiking poles as a buffer.

      Also, while the bag contained everything I wanted to carry during my practice hikes near home, what happened was that the sleeping bag and tent did not compress as well as they were held at home (as at home, I was not re-packing tent/sleeping bag during practice hikes...old dummy sleeping bag which I dont use anymore always stayed inside the bag). So, the sleeping bag and tent took more space and it was a little painful to re-pack everyday at campsite. It will be awesome if the bag can be made 65L or 70L. I think the frame can easily handle these volumes. I had a Bearikade Blazer - one size smaller than expedition - this was above the sleeping bag. I stuffed clothes for night, underwear/socks, electronics, first aid kit, toilet paper etc around the bear canister. The down jacket was also around the bear canister. Tent, stove with pan, rain jacket/pant were above the canister. The brain had my spoon, lamp, rain cover, toilet kit, tent stakes. I had to fight a bit every day to get all of this inside.

      For a week, I tried taking my tent (ZPacks dulex) outside the bag on the side using the two red compression straps. With that, there was enough room inside the bag. But, I wish the bag was at least 65L - maybe one inch wider and 2 inches tall:-)

      Also darker colors please - as the gray does get dirty.

      on Oct 14th 2018

    2. Great Pack

      I bought this as a light weight option which I am pretty happy with. I have used this pack on a few trips now so I have things I like and things I don't like. I have used this pack for a 4 day hike on the Lone Star Hiking Trail where I realized that I didn't like the fact that it does not have a mesh pocket on the back that I have come to rely on for wet and dirty things. This pack does not have. I really started to like the shoulder strap water bottle holder. Very convenient. I also like the fact that it can expand and compress according to your load. I also used this pack on a 3 day hike in Rocky Mountain National Park CDT Loop Flattop Mountain. This pack was super comfortable and carried like a champ, even with a bv500 bear cannister. Which was a blessing with all the altitude we gained and lost. All and all I really like this pack but hate it for not having a big mesh pocket on back. But if you can live without it then you will love this pack. on Oct 8th 2018

    3. One Pack to Rule Them All

      The Flex Capacitor is a remarkable achievement. A simple, Lightweight design that manages to serve as a 40 liter pack, a 60 liter pack and every size in between. The genius lies in the way the pack manages the expansion of said capacity...Horizontally. Which maintains load balance well and I found it simple to loosen up or tighten down.

      Other packs in the market that expand do so vertically, creating a pack that increases in clumsiness the more you expand ever upward above your noggin.

      Thus far I have used it in the Ansel Adams Wilderness hiking the Jackass Lakes trail which is nearly straight up for most of the route to the lakes. The Flex Capacitor was loaded up with about 30 pounds for a three day trout fishing excursion and I must say, I was really impressed with how well the pack handles and distributes weight. Whilst scrambling at times hand and foot up the tree root littered trail sections, I appreciated the way the pack flexes without slipping off my hips. Nice! Weight settled nicely into the small of my back and hips while the adequately padded shoulder straps kept things in check without squeezing the breath outta me. I am pretty crazy about the hip belt pockets...They are big. Much bigger than previous packs I've owned, making it easy to access my Jolly Rancher candies, chap stick and sunscreen stick, iphone was easy to get to for navigation and pics at a moments notice and the zippers don't stick. Cool!

      Speaking of cool, it was July in the Western Sierras when I took this pack for it's maiden voyage and it was damn hot up there. The Flex Capacitor has good ventilation on the backside due to smart placement of the back pads. The side mesh pockets are pretty standard on packs in the genre but the Flex Capacitor's appear to be a bit larger which is appreciated. I have never been a fan of using up precious interior space in my pack to store a water bladder and tube. With this in mind the simple addition of a lightweight bottle holder right on the front of the shoulder pad works beautifully for it's intended purpose - simple, no stop hydration.

      In conclusion, I find the Flex Capacitor to pretty much be a perfect backpack, and trust me I've owned more than a few over 30 years of backpacking in the Sierras. I really can't find anything to bitch about, this pack hits all the right points on a backpackers list in a lightweight, durable, expandable package. Even the color is right. Not too dark so the sun won't bake me and not so light in color that I feel obtrusive in the woods.

      Well Done Sierra Designs and Andrew Skurka. The synergy appears to have worked.

      -Ryan Jones
      on Aug 17th 2018

    4. Awesome load carrying ability with a fantastic hip belt

      I am blown away by this backpack. I own a Zpacks Arc Haul, Seek Outside Divide and I would rank this backpack above those two easily. I have also tried Exos 58 (difficult to get good fit with fixed hip belt) and Gossamer Gear Mariposa (the two bottom side pockets droop with 2L platy bags). The frame on Flex Cap is awesome. It carries very well. The hip belt stays wherever I position it and tighten it - no slippage at all. No pressure on the shoulders at all. I have carried close to 37 lbs with 4.75 liters of water (2L platy bags fit the side pockets well) and a Bearikade Blazer inside and still had room to spare at the top with the M/L bag. You can easily stuff items in various corners of the backpack and fill it up and still have room to spare at the top. You do need to find those spots and stuff it else you may run into space issues.

      It excels in the most important aspect for a backpack - carrying capability with no pain on shoulders or hips/waist. I do not consider lack of front mesh "stuff it" pocket as deal breaker. I can easily unzip the top and remove the items I need that I would normally keep in the mesh pocket. You could look at it as an advantage in the sense the stuff in mesh pocket can get wet while it will not in this backpack:-).

      The key is to be open minded when trying this bag as lack of mesh front pocket, torso adjustments did turn me away from this backpack. But, am I glad that I took a chance and try it! Because, this is the bag I am going to carry with me on the JMT - not the Divide or the Arc Haul. You just cannot beat the load carrying capability of this bag with no shoulder/hip/waist pain even from 5 days of food in Bearikade Blazer and 4.75 liters of water (37+ lbs). Of course on the JMT, I will not be carrying 4.75 liters of water - like to train with more weight.

      I guess one does not think of Sierra Designs as a backpack powerhouse and would have expected something like this to come from Osprey/Gregory/Zpacks/Seek Outside/ULA etc.
      on Jul 17th 2018

    5. 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

    6. 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

    7. 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

    8. 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

    9. 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

    10. 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