REI Flash 45 Backpack Review

The REI Flash 45 Backpack is an innovative lightweight backpack ideal for weekend trips
The REI Flash 45 Backpack is an innovative lightweight backpack ideal for weekend trips.
REI has a new version of the Flash 45 and Flash 55 available. Read our review of these exciting new Ultralight backpacks.

The REI Flash 45 is a lightweight backpack with an adjustable, ventilated frame that’s perfect for weekend backpacking trips or more technical day hikes. Weighing just 2 lbs 14 oz in a size L, (a women’s specific version is also available) it’s a surprisingly comfortable and functional backpack capable of carrying 30 pound loads. If you’re in the market for a lightweight pack, I’d definitely give the REI Flash 45 a look. It has several innovative design features that enhance its comfort and utility well beyond similarly sized top loading backpacks from Osprey, Gregory, and Deuter, but at a lower price.

Specs at a Glance

  • Frame: Internal frame
  • Adjustable length: Yes
  • Capacity: M (45L), L (47L)
  • Weights: M (2 lb 12 oz), L (2 lb 14 oz)
  • Torso Range: M (18″-20″), L (19″-21″)
  • Hip Belt Length: M (32″-42″), L(34″-44″)
  • Fabric: 420 denier ripstop nylon
  • Gender: Mens (Women’s-specific, also available)
  • Click for complete specs

Internal Storage and Organization

The REI Flash 45 is a top loading backpack with a floating top lid that can be raised higher if you need to overload the pack. The floating lid has an external zippered pocket with a key fob and is generously sized to store maps, hats or gloves. The pack has two mesh side water bottle pockets and a rear mesh stuff pocket for storing loose layers or wet gear that you want to segregate from dry gear inside your pack. It is hydration compatible with an internal hydration pocket and a central hang loop for hanging a bladder, with dual hydration ports above the shoulders. Two large mesh hip belt pockets complement the storage available.

The Flash is a top loader with a back stretch mesh pocket that can hold extra layers or wet gear
The Flash is a top loader with a back stretch mesh pocket that can hold extra layers or wet gear.

If you don’t want to hike with the top lid, it is removable, which can help streamline the pack. In this configuration, you’d close the top of the pack using the draw string closure at the top of the main compartment, while tucking the rear lid straps down into the rear mesh pocket to keep them out-of-the-way.

Side Water Bottle Pockets

The Flash 45 has unusual side pockets that are designed for to hold water bottles. Unlike other backpacks, they’re not on the sides of the main compartment, but closer forward, over the wings of the hip belt where they wrap around your hips.  This makes it easier to pull them out and replace if you have limited shoulder mobility or shorter arms. It also places your water squarely on your hips for optimal carrying efficiency, the guiding principle that motivates the overall design of the pack.

The bottle pockets work well with tall 1 liter plastic bottles and Nalgenes, but are harder to use with soft bottles that doesn’t have a rigid or round shape like the Platypus Duolock or a 1 liter SoftBottle. An additional snap is provided to help hold bottles in place and is easy to re-snap while wearing the backpack.

The REI Flash 45 has specialized water bottle pockets that are located closer to the hip belt for ease of access and better load transfer
The REI Flash 45 has specialized water bottle pockets that are located closer to the hip belt for ease of access and better load transfer.

Despite their convenience, these dedicated side bottle pockets can’t be used to hold cylindrical objects, like tent poles or a tightly rolled tent, that are lashed to the side of the pack. With this pack, you’ll want to pack most of your gear inside the main pack bag: something to consider if you like to pack extra stuff into side pack pockets.

Compression and External Attachment System

The REI Flash 45 has two tiers of compression, both diagonal to help pull the load closer to your center of gravity. The bottom tier has an extra pulley-like strap system which  helps prevent heavy loads from pulling you backwards. Combined with the pack’s load lifter straps, it helps align the pack angle closer to your natural, slightly forward-leaning stance.

The Flash 45 has two tiers of compression, both diagonal to help pull the load closer to your center of gravity
The Flash 45 has two tiers of compression, both diagonal to help pull the load closer to your center of gravity.

Called the “Uplift”system (note orange buckle above) it helps prevent the heaviest items in your pack from drooping behind your butt and below your waist where they can throw you off-balance. It’s also quite simple and lightweight. While it’s eye-catching, I don’t think it’s really that necessary on a low volume backpack like the Flash 45, which has a fairly shallow pack bag, and would be more suitable on a 60+ liter pack which has a much deeper main compartment.

As noted previously, the lack of full width side pockets makes it difficult to securely last tent poles along the sides of the pack. There are however, 4 gear loops (two front and two rear) along the bottom of the pack that you can hang gear from, say a sleeping bag, pad, or tent, using cord loops or webbing to secure it in place. I’m not a huge fan of carrying heavy gear below the bottom of a pack bag like this because it can throw you off-balance, but the capability is there if you need it.

The torso length can be adjusted using a rip-and-stick velcro system to raise or lower the shoulder pads of the Flash 45
The torso length can be lengthened or shortened using a rip-and-stick (velcro) system to raise or lower the shoulder pads. Simple, easy to use, and lightweight.

Frame and Suspension

The Flash 45 has an adjustable frame so you can dial in a custom fit that matches your torso length. The adjustment system uses a rip-and-stick style velcro system that let’s you easily raise or lower the shoulder strap height. It’s a very lightweight but effective fit system, similar to one used by Osprey Packs.

A wrap-around balloon style frame gives the Flash 45 pack stiffness, while maintaining torsional flexibility. In addition to two vertical segments, there are three cross pieces at the bottom, middle, and top of the back panel that keep the pack bag from collapsing, while driving the load down into the hip belt.

The Flash 45 frame is covered with a ventilated mesh to help wick perspiration away from your back with a wide air channel over your spine and convoluted foam padding along the sides to improve air flow. There’s a thin lumbar pad at the base of the back panel, which is also ventilated for enhanced comfort.

The REI Flash 45 has a ventilated backpanel to help keep you cool and dry perspiration quickly
The REI Flash 45 has a ventilated backpanel, lumbar pad, hip belt, and shoulder straps to help keep you cool and dry perspiration quickly

The wings of the hip belt are aggressively pre-curved and wrap around your hips to enable effective load-to hip-transfer. The fit is really great and there’s no slip at all, even on squarish man hips like mine. I think the fit of the hip belt rivals that found on Osprey’s AG backpacks, but at a fraction of the weight and cost. Beefy seatbelt-sized webbing and a large-sized plastic buckle make it easy to adjust the fit of the hip belt, while providing a secure fit.

The REI Flash 45 has a body hugging fit that makes it easy to carry 30 pounds loads with ease
The REI Flash 45 has a body hugging fit that makes it easy to carry 30 pounds loads.


The REI Flash 45 is an innovative lightweight backpack with an adjustable length frame that weighs under three pounds. It’s also a remarkably comfortable backpack that’s designed to pull loads close to your center of gravity so they can be carried more efficiently and with less effort. An aggressively pre-curved hip belt locks the hip belt in place so it can’t slip down your thighs, while ventilated mesh keeps it cool and comfortable. Specialized side water bottle pockets make it easy to reach and replace water bottles while you’re still wearing the pack, a highly desirable feature for fast paced hikes and backpacking trips.

I’ve reviewed many iterations of the REI’s Flash Backpack product line over the years, but I’m really quite impressed with the Flash 45 and think it’s an excellent value. If you’re looking for an affordable lightweight backpack that can carry up to 30 pounds, try on the Flash 45 and take it for a spin. The fit and carry of the REI Flash 45 Pack will help you understand what a great fitting backpack should feel like.


  • Adjustable ventilated frame
  • Side water bottle pockets put the load squarely on your hips for maximum carry efficiency.
  • Side water bottle pockets are every easy to reach.
  • Rear “uplift” straps help pull bulky loads closer to your hips unlike conventional compression straps or load lifters.
  • Pre-curved hip belt wraps securely around the hips
  • Wire balloon frame provides excellent stiffness and load transfer without being heavy or uncomfortable


  • Placement of side water bottle pockets makes them impossible to use for carrying tent poles or other cylindrical objects.
  • Mesh water bottle pockets, hip belt pockets, and rear stretch pockets are relatively fragile mesh and likely to rip.

Written 2017.

Disclosure: The author purchased this product with his own funds.SectionHiker is reader-supported. We independently research, test, and rate the best products. We only make money if you purchase a product through our affiliate links. Help us continue to test and write unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.


  1. After 20 miles, the rip-and-stick (velcro) system fails on bouncy trails such as jogging downhill.
    This becomes the single point of failure, when the Velcro design slips.

    Love reading your write-up. Thank you.

    Me: SoCal middle aged short fat bald guy (like Wally from Dilbert) former reader of BPL.

  2. Rear mounted water bottles are impossible, side mount just get slapped with my swinging arms. I prefer my water bottles mounted on my shoulder straps. Up front and easy access.

  3. How does this compare with Granite Gear Lut 45? I walked across Spain for 26 days last September and ended up hating the Granite Gear which I purchased after reading your review. It kept sagging no matter how well I adjusted it. Only carried 22-24 lbs including pack weight and 2 lt h2o

  4. I have the older version of the Flash 45 and I can’t say I like it. The rear mesh of the newer version seems way better. On my pack, the entire side and rear mesh is just one large net that just droops down and I have lost a few items while using this as a a drying net. I really hate the shoulder straps on my Flash 45 which I find way to thin to be comfortable. I even had to add some fleece tubing to make them thicker. Does the new Flash 45 have decent shoulder straps?

  5. Hey Philip,

    I am interested in hearing your thoughts on the load lifters on this pack in comparison to those on the ULA circuit. I have also read your review on the circuit which I enjoyed. All and all I am curious to hear your thoughts on comparing the circuit to the flash.


    • They’re really different backpacks. Can you give me an idea what you want to do with them or where you want to go, so I can couch my response in those terms?

      • David Beroza-Torren

        Hey Philip,

        Sure thing. My intended purpose for this pack is to use it for solo trips as a 3 season bag for trips that are 3-6 days in length. This is the same intent with the circuit as well. I’m hoping to slim my pack weight more which is why I like the flash because it’s smaller but still provides ample space. But I do really like the roll top feature to really pack the pack down nice and tight. I also am suspicious of the uplift mechanism on the flash because it seems like a gimmick. What do you make of that system coupled with the load lifters? Thoughts? Rei can be hit or miss with durability too, but from seeing this in the store it seems to hold true. I have really liked the circuit, I’m sure partly this is because of brand, durability, adjustability with load lifters, roll top, and nice sized pockets. But not a huge fan of the limited return, price, and the idea that I’d just be essentially buying sort of a duplicate to my current main pack. I also appreciate how the flash seems to provide more of a suspension and comfort than the circuit. Hoping for total pack weight between 20-30ish at the most. I have been using a 2015-2015? Atmos osprey. Sorry for blowing this page up but I do very much enjoy and appreciate your reviews.

  6. It looks like REI has updated this pack by adding side pockets, improving the compression straps, and shedding a few ounces. Have you seen or tried the new version? I’m comparing this to the Granite Gear Crown2 38.

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