Sierra Designs Flex Capacitor 40-60L backpack review: an impressively light, versatile and streamlined gear hauler

Sierra Designs has thought of everything with this expandable backpack, from extra pockets that hold blankets and water bottles plus loads of padding and airflow for long and short treks

Sierra Designs Capacitator Flex backpack in the woods
(Image: © Julia Clarke)

Advnture Verdict

This robust and roomy backpack can be adjusted to cinch snugly around your best gear and is light and comfortable on your back, no matter the weather what distance you’re going

Pros

  • +

    Padded back, hip belt and shoulder straps

  • +

    Expandable volume for short and long hauls

  • +

    Six compression straps

  • +

    Ultralight

  • +

    Loads of useable pockets

  • +

    Water resistant

  • +

    Two sizes available

Cons

  • -

    Top lid makes it harder to access gear at the bottom

  • -

    Not waterproof

  • -

    Pricey

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Sierra Designs Flex Capacitor 40-60L backpack: first impressions 

Touting this as the versatile backpack that hauls it all, Sierra Designs have really outdone themselves with this impressive gear hauler. This backpack has six compression straps so you can cinch it tightly around your gear for a short backpacking trip, making it a 40L bag, but for longer treks and thru-hikes when you’ve just refueled, you can expand it into a roomy 60L backpack. In addition to a spacious main compartment, you’ll find roomy pockets in the top flap, hip belt and shoulder straps as well as deep side pockets that can handle everything from your GPS device or phone to rolled up camping blankets and water bottles. The top flap access point means it’s a little harder to access gear that’s buried deep, but with all those pockets there is no reason not to have everything you need close to hand. 

Specifications

 • RRP: $219.95
Weight: 2 lbs 10 oz / 1.19 kg (M/L)
Capacity: 39 - 56L
Sizes available: S/M, M/L
Dimensions: 27.5" x 11" x 10" / 70 x 28 x 25.4 cm
Materials:100D Nylon-Poly Ripstop/420D Nylon Oxford 

When you’ve got all that gear on your back and a long way to go, the last thing you want to deal with is any uncomfortable rubbing, and this backpack is loaded with plush padding in the back and straps which, in addition to its light weight, make it supremely comfortable. Two sizes of pack and hip belt mean you can really tailor yours to suit your anatomy, too. 

Though it’s not waterproof, it’s water resistant and while it’s priced on the higher end of backpacks, you’re essentially getting two bags for the price of one.

Sierra Designs Flex Capacitor 40-60L backpack: in the field 

Staff writer Julia Clarke wearing the Sierra Designs Flex Capacitor backpack

Me and my forever pack, gazing into the sunset (Image credit: Julia Clarke)

I actually feel like I’ve discovered my forever backpack with this new hauler from Sierra Designs. I’ve been using the same old backpack for well over ten years now, even though it’s on the verge of being too big for me and doesn’t do a great job of holding all my gear close to my body and helping me stay balanced. For this reason, I’ve long coveted the more streamlined packs that cinch the gear into a more cylindrical shape, making it easier to carry, and I’ve finally got my hands on one with this backpack.

I love how roomy it is, easily fitting all my gear for a few nights on the West Highland Way, while the six compression straps can really compress the load and help with balance. I almost can’t believe how many spacious, usable pockets it has too. The ones on the hip belt are easily big enough for phones, cameras and snacks while the shoulder strap pockets fit my water bottles nicely and the side pockets actually carried my camping blanket on my last trip. It even has a handy mesh pocket inside that can fit my hydration pack or carry any wet items.

Sierra Designs Capacitator Flex backpack in the woods

At just over two pounds for the larger size, this barely adds any weight to your load, but it fits a ton of gear with an expandable design and millions of pockets (Image credit: Julia Clarke)

For all that weight, it’s amazingly comfortable to carry, thanks to generous padding and the different sizing available. I got the smaller version with the smaller hip belt which means I can finally make sure that I’m carrying everything properly on my hips and not taking some of the weight on my shoulders. The lightweight design also allows for a little airflow between me and the pack, which has been ideal for this sweaty summer we’ve been having.

It isn’t waterproof, but it’s as water resistant as any other backpack I own and here in Scotland, I’d be using a waterproof cover in the rain anyway. I can see how the top loading flap would make it difficult to reach gear at the bottom, but since I always pack everything I need in the pockets, it hasn’t mattered at all. I know that it’s priced on the higher end of backpacks in this size range, but for the quality I wouldn't quibble at the cash register.

Here’s how it performed:

Comfort and breathability

Loads of padding, light weight and airflow between you and the backpack make this super comfortable even for heavy loads on hot days. 

Sierra Designs Capacitator Flex backpack in the woods

Loads of padding, light weight and airflow between you and the backpack make this super comfortable even for heavy loads on hot days (Image credit: Julia Clarke)

Sizing

Two sizes of pack and hip belt mean you can really adjust this to your needs. 

Weight and packability

At just over two pounds for the larger size, this barely adds any weight to your load, but it fits a ton of gear with an expandable design and millions of pockets. 

Durability 

Sturdy nylon and quality design makes this a forever pack. 

Julia Clarke is a staff writer for Advnture.com and the author of the book Restorative Yoga for Beginners. She loves to explore mountains on foot, bike, skis and belay and then recover on the the yoga mat. Julia graduated with a degree in journalism in 2004 and spent eight years working as a radio presenter in Kansas City, Vermont, Boston and New York City before discovering the joys of the Rocky Mountains. She then detoured west to Colorado and enjoyed 11 years teaching yoga in Vail before returning to her hometown of Glasgow, Scotland in 2020 to focus on family and writing.