Sierra Designs Flex Capacitor 40-60L Rucksack

40-60 Litres / Internal Frame
Weighs 1168g (Sml/Med size)
Code: SS20686
  • Now £152.99
  • (FREE UK & EU Delivery)
+ In Stock Now | Usually dispatched within 24 hours

The Flex Capacitor is a modern classic, renowned for its light weight, class-leading versatility, and innovative features. This is the 40-60L version, the perfect workhorse for everyone from the weekend warrior to the veteran thru-hiker.

Sierra Designs says:

Meet the pack that does it all. The Flex Capacitor family of packs feature adjustable volume to handle both short and long hauls, and a Y flex suspension system that carries above their weight class. With three sizes in a 25-40, 40-60, and 60-75 L capacity, there is a flex pack to fit your needs. A bucket lid with a stash pocket closes everything up, while dual stretch pockets on the shoulder straps and side of pack cover shove it needs.

Sierra Design's Flex Capacitor 40-60L Rucksack is the ultimate in versatile, adaptable load-carrying, and the flexibility offered by the expandable volume make this the one-pack solution for any adventure. Built around a Y-Flex suspension system, the Flex Capacitor is surprisingly burly for its low weight, with a well padded lumbar and a wide hip belt to better handle heavier loads. The pack itself is constructed primarily from 100D fabric, guaranteeing durability and water-resistance while keeping the weight to a minimum. 400D Nylon has been used in the base of the pack, to better protect against rocky campgrounds and long-term use.

The expandable volume of the Flex Capacitor is its greatest strength. Where most expandable packs do so with a longer body, the Flex expands its circumference instead, making it a far more comfortable pack when fully opened. This also provides a far greater range than most of its competitors – while many similar packs only manage 10L of expansion, the 20L offered by the Flex Capacitor make it a pack that can happily accommodate everything from long weekends to weeks on the trail.

The U-shaped lid access to the pack body is a beautifully simple solution to the heavy lids featured on many similar packs, and makes for a rucksack that is refreshingly easy to pack and surprisingly spacious. The Flex Capacitor veritably swallows gear, while still providing dedicated space for a hydration bladder and keeping a small zipped essentials pocket in the roof, ideal for navigation kit and snacks. The finishing touches are all present too – there are two ice axe loops, a pair of roomy hip belt pockets, and four stretchy mesh pockets; two on the sides of the pack body complemented by one on each shoulder strap. All-in-all, the Flex Capacitor 40-60L is the ideal solution to the age-old question of pack size, capable of withstanding the harshest conditions and comfortable under the heaviest loads.

- Reviewed by Ultralight Outdoor Gear

Read reviews here:

Help and advice:Choosing a rucksack at Ultralight Outdoor Gear
  • Expandable Volume
  • 2 stretch mesh side pockets
  • 2 stretch mesh hydration pockets on shoulders
  • Zippered hip belt pockets
  • U-shaped top access zipper
  • Small stash pocket in lid
  • Ultralight weight
  • Lightweight 'Y' Flex DAC suspension

Small/Medium Torso

  • Capacity: 39-56L
  • Weight with S/M waist belt: 1168g
  • Weight with M/L waist belt: 1208g
  • Torso Fit: 16" - 19" / 40.6 - 48.3 cm
  • Hip Fit with S/M waist belt: 29" - 32" / 73.7 - 81.3 cm
  • Hip Fit with M/L waist belt: 32" - 35" / 81.3 - 88.9 cm
  • Dimensions (H x W x D): 27.5" x 11" x 10" / 70 x 28 x 25.4 cm
  • Suspension: Y Flex DAC

Medium/Large Torso

  • Capacity: 42-60L
  • Weight with S/M waist belt: 1186g
  • Weight with M/L waist belt: 1214g
  • Torso Fit: 18" - 21" / 45.7 - 53.3 cm
  • Hip Fit with S/M waist belt: 29" - 32" / 73.7 - 81.3 cm
  • Hip Fit with M/L waist belt: 32" - 35" / 81.3 - 88.9 cm
  • Dimensions (H x W x D): 27.5" x 11" x 10" / 70 x 28 x 25.4 cm
  • Suspension: Y Flex DAC


  • 100D Nylon-Poly Ripstop/420D Nylon Oxford
Showing our 1 review for Sierra Designs Flex Capacitor 40-60L Rucksack. Write a review ›
Review Summary - 5 out of 5 based on 1 rating.
February 25, 2020 | andy mac
Great stuff

Very relaxing bag to use. Usually prefer meshy roll tops but got the gorgeous birch coloured Flex because it looks like something right off the space shuttle. Glad I got it. S/M fits 18" ish torso really well, S/M belt has good tolerance and fits my (obviously fatter than) 32" hips really well. The back pads and arch give a good airy feel but it all sits close enough and moves really well with body. Awesome belt, suspension and load lifters work magic when dropping loads for a loosey-goosey shoulder fit. The expansion and compression straps give you a wrestle-free skinny 30L bag as much as the 39-56 capacity, and that's the thing... the tolerance for both volume and load is incredibly useful. You can't really feel a 6kg load out so guess that can lead to temptation but it's good to know that you can get a weeks worth of food plus tyrannical wet gear in there without grinding against pack load limits. Side pockets are good size and 10/10 for the shoulder pouches that can take a 1L bottle. The hex grid-stop material looks amazing and I am used to the zipper already. Great stuff, cheers UOG!

Ultralight Outdoor Gear Limited
Contact us with any questions you have about this product