REASONS TO BUY
Comfortable and supportive
Ventilated back panel
Highly durable
Removable hip belt
REASONS TO AVOID
Relatively small internal volume
Side pockets are small
Minimal external lash points
Outdoor Vitals CS40 Ultra Review
This ultralight pack features first-rate materials and an internal frame for additional support
"Load up and go!" is what the CS40 Ultra urges you to do.
Credit: Justin Simoni
By Justin Simoni ⋅ Review Editor ⋅ June 10, 2026
RELATED: The Best Ultralight Backpack of 2026
Our Verdict
The Outdoor Vitals CS40 Ultra keeps your pack weight low without sacrificing comfort. Unlike many other ultralight backpacks of its size, this one features breathable back padding, carbon fiber stays for added stiffness, and an integrated hip belt to help transfer weight from the shoulders to the hips. Made from very tough, very light, very water-resistant Challenge UltraWeave 200, the CS40 Ultra proves you can have a rough-and-tumble, supportive pack at an attractive weight.
Compare to Similar Products
 This Product
Outdoor Vitals CS40 Ultra | |||||
|---|---|---|---|---|---|
| Awards |  Best Overall Ultralight Backpack |  Best Bang For Your Buck |  Best Full-Featured UL Pack | Best Pack for Expert Ultralighters | |
| Price | $330 List | $315 List | $279 List | $330 List Check Price at REI | $350 List |
Overall Score  |
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| Star Rating | |||||
| Bottom Line | This ultralight pack features first-rate materials and an internal frame for additional support | This pack wowed us with its perfect set of features, comfortable design, and impressively large carrying capacity | A tough, voluminous, and reasonably priced ultralight backpack that's perfectly designed for long trips | An excellent pack for those who want to move up from a conventional backpack, but don't want to lose out on creature comforts | When it comes to frameless ultralight packs, this one leads the way in weight, design, and innovation |
| Rating Categories | Outdoor Vitals CS40... | Gossamer Gear Maripo... | Durston Kakwa 55 | Osprey Exos Pro 55 | Nashville Pack The C... |
| Comfort (40%) | |||||
| Ease of Use (25%) | |||||
| Weight-to-Volume Ratio (20%) | |||||
| Adjustability (15%) | |||||
| Specifications | Outdoor Vitals CS40... | Gossamer Gear Maripo... | Durston Kakwa 55 | Osprey Exos Pro 55 | Nashville Pack The C... |
| Measured Weight | 28.6 oz | 34.3 oz | 31.0 oz | 33.2 oz | 22.0 oz |
| Stripped Weight | 25.0 oz | 18.0 oz | 27.5 oz | 30.4 oz | 17.3 oz |
| Advertised Volume | 53 L | 60 L | 70 L | 55 L | 40 L |
| Measured Volume (Main Compartment) | 40 L | 54 L | 55 L | 46 L | 43 L |
| Weight per Liter (Full Pack) | 0.54 oz/L | 0.57 oz/L | 0.44 oz/L | 0.60 oz/L | 0.40 oz/L |
| Sizes Available | Torso: Short, Medium, Large Hip Belt: XS, S, M, L, XL |
Torso: S, M, L Hip Belt: S/M Curved (w/ S torso), M Straight (w/ M and L torso); other hip belts sold separately |
Torso: S, M, L Hip Belt S/M, M/L |
Torso: S/M, L/XL | Torso: 16', 18", 20" Hip Belt: S (28"-38"), M (32"-42"), L (26"-48") Should Straps: 16", 17.5", 19", 20.5" |
| Organization Compartments | Side pockets, center pocket, hip belt pockets, main compartment | Top flat zippered pocket, three total side pockets, center pocket, dual zippered pockets on hip belt, main compartment | Side pockets, side zipper pocket, center pocket, hip belt pockets, strap pockets, main compartment | Lid, mesh side pockets, center pocket, dual zippered pockets on hip belt, main compartment | Main compartment, center pocket, running vest-style pockets, back-entry bottom stash pocket |
| Hydration Compatible | Yes | Yes | No | Yes | No |
| Side Pocket 700ml Smartwater Capacity | 2 | 3 | 3 | 5 | Continuous wraparound pocket |
| Single Hip Belt Pocket Capacity | 6 Clif Bars | 6 Clif Bars | 10 Cliff Bars | 4 Clif Bars | N/A |
| Removable Frame and Hip Belt | Yes | Yes | No | No | Yes |
| Sternum Strap Whistle | No | Yes | No | Yes | No |
| BearVault BV500 Compatibility | OK | Good | Excellent | Good | OK |
| Frame Type | Carbon fiber frame stays and EVA foam back pad | Removable SitLight foam pad / removable stay | Removable foam pad / removable stay | 3.5 mm powder-coated LightWire frame | None |
| Fabric | Challenge UltraWeave 200 | 100D & 210D recycled Robic nylon | Challenge Ultra 200x | NanoFly: 100D nylon x 200D UHMWPE ripstop | 210D Venom Gridstop ECO |
Our Analysis and Test Results
With an industry-leading internal framed system, the Outdoor Vitals CS40 is one of the more comfortable packs we tested. Credit: Justin Simoni
Likes
The CS40 Ultra features an honest-to-goodness hip belt, back padding, internal stays, and functional load lifters usually found only on much larger packs. If good back support in a pack – even in an ultralight pack – is a non-negotiable, then this pack was made especially for you.
The back padding in the CS50 Ultra is intelligently shaped with cutouts to both save weight and encourage airflow. The internal carbon fiber also means you don't have to jerry-rig additional padding or devise a framing scheme. A bonus is that you can load your gear a bit haphazardly without worrying about anything poking you in the back.
From pre-dawn packing to high-mountain passes, the CS40 Ultra is a get-up-and-go type of backpack.
The presence of a bona fide hip belt means much of the weight is shifted to your hips, making overloaded carries – like when you're first starting out on a weeklong trip – less soul (and shoulder) crushing. The hip belt is also removable and comes in a range of sizes, from XS to XL.
The back padding has a "U"-shaped cutout to help promote air circulation. Credit: Justin Simoni
Load lifters that attach high above the shoulders offer an extensive amount of adjustability to help hug the packs load closer to your body. Credit: Justin Simoni
Dislikes
The CS40 Ultra is sewn using 132 g/sm Challenge UltraWeave 200, one of the lightest and most durable fabrics used in ultralight packs. But by adding many of the comfort features, like back padding, frame stays, and a supportive hip belt, the weight of those features adds up.
At 28.6 ounces (810 grams), this pack isn't the lightest for its 40-liter internal volume. Though we'll admit it's still pretty impressive when we have a relatively heavy loadout.
We measured the internal volume at approximately 40 liters. Credit: Justin Simoni
We wish the hip belt pockets were a little wider and a bit longer. When we pack them to the gills with snacks, they stick out and sometimes graze our arms while we're hiking. The side pockets are also on the smaller side, only big enough to hold one Smartwater bottle each.
Hip belt pockets are somewhat small and stick out when packed with snacks. Credit: Justin Simoni
Side pockets are smaller than those of other packs that we've tested. Credit: Justin Simoni
Should You Buy the Outdoor Vitals CS40 Ultra?
The Outdoor Vitals CS40 Ultra strikes a delicate balance between gram counting and built-in comfort. It's an easy recommendation for anyone who wants an ultralight backpack but needs one with a bit more padding and structure for better back support.
What Other Ultralight Backpacks Should You Consider?
Like many of their popular, conventional backpacks, the Osprey Exos Pro 55 features a trampoline back with an internal frame, which rivals the CS40 Ultra for overall support. The Zpacks Arc Haul Ultra isn't much different, yet despite all of the features, it still manages to be one of the lightest packs available based on carrying capacity.
—Justin Simoni
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