ProView – Sierra Designs Flex Capacitor 25-40L

Founded in 1965, Sierra Designs is an extraordinary company with a history rooted on a solid foundation. Founders Bob Swanson and George Marks believed that all people deserve to have good gear, regardless of their experience level or background. We are thankful to have this great company here in our backyard in Broomfield, Colorado.

For this product review, I tested the Sierra Designs Flex Capacitor 25-40L backpack which is described in outdoor circles as the “pack that does it all”. This pack should not be confused with the time traveling device called the “Flux Capacitor” from the 1985 movie, Back To The Future! Did you know that some auto parts stores in the United States advertise a Flux Capacitor module as an aftermarket add-on for cars? It’s pretty comical.

I was skeptical about this backpack at first, but after putting it to the test on our local front range trails, it seems to live up to its reputation. Read on to find out more about this comfortable pack that has a simple, yet flexible design.

Sierra Designs Flex Capacitor 40-60

Product Description: The Flex Capacitor features a unique patent pending gusset system that allows the pack to quickly and easily expand from 25-40 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.

Offer price: MSRP: $179.95

  • Comfort & Frame
  • Versatility
  • Fit
  • Weight
  • Value


A highly capable and flexible pack that delivers comfort in the outdoors. Sierra Designs was clever in naming this pack, but they should add flashing lights and the ability for it to enable time travel. Seriously though, you will love this backpack when you take it on your next adventure!



  • Internal frame is well designed
  • Great comfort for the price


  • Top zipper gets caught often during normal use
  • Rain cover is not included

Our experience using the Flex Capacitor was a good one. ☝️

The weight:price ratio seems to make sense compared with other packs on the market today and it can be used for day trips or short, 2-3 day thru-hiking adventures.

The Back To The Future movie inspired, internal frame shape is comfortable and provides just the right amount of support whether used with a light or heavy load.

In the next pack revision, I would like to see a larger top pocket with dividers for essential gear as well as a panel access zipper from the inside of the pack.

We tested this pack out on a photography shoot and day hike in our local area during the COVID-19 pandemic. Although we were not out in the wilderness or in one of the national parks, we were able to test it just the same by loading the Flex Capacitor with my typical adventure and camera gear. For a sense of scale, I am 5’7” 140 lbs. and the model pictured is 5’1” 120 lbs.


The “Y” shaped, internal frame is uniquely designed to position the weight of the pack at the hips. It is complemented by large, padded areas both just above the tailbone and at the shoulder blades. This provides a hiker with better stability on uneven terrain. Another benefit that we discovered with the frame is that it gets the middle of the pack away from the body and creates increased airflow in the middle of the back. This is a big benefit that helps a hiker stay cool out on the trail during the summer months.

The padded shoulder straps are very comfortable and have two strap adjustments. In addition, the padding on the replaceable hip belt is lightweight and comfortably supportive.


Many people traveling in the outdoors today bring cameras to document their adventures and want quick access to the internal camera units that protect their gear. The simple, all in one design of the Flex Capacitor causes one problem for outdoor enthusiasts that bring along their DSLR or mirrorless cameras. It is difficult to quickly access heavy camera gear inside of the pack unless that heavy load is placed at the top of the pack. This is not an ideal way to pack for longer excursions and can become uncomfortable. Ultimately, it would be ideal to have a panel access zipper for this use case.

The Flex Capacitor has a number of utilitarian pockets – one zipper pocket on each side of the hip belt, one elasticized pocket on each shoulder strap, and one elasticized pocket on each side of the lower pack. Additionally, there is one small, zipper pocket on the top of the pack along with a top-loading zipper directly underneath it that allows access to the main compartment. I found the small, top pocket to be fairly useless. With a bit more volume, it could be way more functional. I also discovered that the zipper on the main compartment tends to get caught on the concealing flap quite often. This could also use a re-design in the next iteration of this pack.

Sierra Designs offers optional replacement hip belts in two different sizes, so depending upon your body type, they have got you covered. The adjustable straps on the front of the belt are quite long though. They could use some elastic straps around the belt so that a hiker could neatly tuck-in the excess length. This would be a simple, yet refreshing improvement.

Inside the pack, there is a hydration sleeve that can hold a hydration reservoir and there is a hose portal at the top for easy installation. 


At 2lbs 7oz, the 25-40L Flex Capacitor is comparable to other backpacks in this category and price range. If you are looking for a super ultra-light pack, this would not be the best choice on the market. However, for the average day or short-trip, thru-hiker that wants a solid weight:price ratio, you cannot go wrong with this pack.


At $179.95 MSRP, the Flex Capacitor 25-40L is comparable in price to its industry competitors. The adjustable size also gives it a competitive advantage that offers more flexibility to the consumer.


When packing up for your next adventure, be sure to completely loosen all of the straps on the Flex Capacitor as your first step. Then load the pack with your perfect amount of gear and cinch down the straps to adjust the volume of the pack. This will reduce bulk and make it a much more enjoyable experience when packing.


Here’s a quick list of features direct from the Sierra Designs website:

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


100D Nylon-Poly Ripstop/420D Nylon Oxford

Tech Specs

Capacity: 25-40L / 1525-2440
Weight with S/M waist belt: 2 lbs 7 oz  /1.1 kg
Weight with M/L waist belt: 2 lbs 8 oz / 1.13 kg
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): 23″ x 11″ x 8.5″ / 58.4 x 28 x 21.6 cm
Suspension: Y Flex DAC

The Final Word

A highly capable and flexible pack that delivers comfort in the outdoors. Sierra Designs was clever in naming this pack, but they should add flashing lights and the ability for it to enable time travel. Seriously though, you will love this backpack when you take it on your next adventure!

➳ Outdoor Prolink provided equipment and associated materials for this review. All photos are a courtesy of LaGuardia Adventure Photography.

Find Sierra Designs Flex Capacitor 25-40L on Outdoor Prolink. Not a member? Apply today!

About the Gear Tester

Outdoor Prolink Pro
John LaGuardia
Adventure Photographer

John LaGuardia is a Colorado-born, American adventure photographer who has worked in outdoor industry retail for many years. It all began back in the late 80’s when he started up one of the first Colorado snowboard shops. Since then, he has worked with outdoor brands associated with snowsports, watersports, mountain biking, backpacking, trail running, and photography. His current work includes commercial photography and films that are related to outdoor active lifestyles. You can often find John out photographing nature and participating in the same activities as his subjects. You can connect with him on Instagram at @johnlagphoto


  1. Robin

    Hello and thanks for the review. Do you know if the 25-40 l is just the mainpocket or the complete pack including all sidepockets etc?



  2. John LaGuardia

    Hi Robin,
    Great question! The 25-40 L measurement applies to the main pocket area. The side pockets are additional for storing water bottles on the sides and small snacks on the belt. Enjoy your adventures with this great pack!

    Thank you,


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