Outdoor Vitals CS40 Ultra vs Zpacks Arc Haul Ultra: Choosing an Ultralight Backpack

Introduction

Ultralight backpacking packs make a clear trade. You give up structure, pockets, and reinforcement in exchange for a lighter carry, and that trade only pays off when your base weight is low enough to suit the design.

Two packs sit squarely in that category: the Outdoor Vitals CS40 Ultra and the Zpacks Arc Haul Ultra. Both are built from Ultra 200 fabric, and both are aimed at hikers who count grams. They differ in frame design, in what comes in the box, and in price.

This comparison looks at the parts that matter once you are on trail: how each frame handles load, how the hip belt and pockets behave, and what you actually get for your money.

What to Look For in an Ultralight Backpack

The first question is whether your base weight fits the pack. Designs in this class work best when base weight sits below roughly 20 lb. Above that figure, the shoulder straps start carrying the load, and hikers end up constantly lifting and adjusting them, which leads to sore spots. That makes a brand’s maximum recommended carry load a ceiling rather than a target.

Fabric is the second consideration. Ultra 200 is described as lighter, stronger, and more tear resistant than Dyneema, while keeping the same waterproof properties. A removable frame is another factor, because it lets you choose between the absolute lightest setup and one that carries weight more comfortably.

Pocket access, hip belt width, and drainage round out the practical side. Wide hip belts reduce rubbing under load, angled side pockets make bottles easier to reach while the pack is on your back, and drain ports stop water from pooling in pockets after a wet day.

Outdoor Vitals CS40 Ultra: Overview and Highlights

The CS40 Ultra takes its name from its frame. CS stands for carbon stays, and the internal frame is built from carbon fiber stays that weigh roughly an ounce each, or about two ounces for the pair. In a medium size, the complete pack comes in around 27 oz, and removing the stays brings that down to roughly 25 oz.

Those two ounces are worth keeping. With the stays in place, the frame forms a gentle arc that follows the shape of the back and flexes as you hike, and it spreads weight noticeably better than a frameless setup. The pack body is made from Ultra 200 fabric, which is lighter, stronger, and more tear resistant than Dyneema while retaining the same waterproof properties.

Capacity is listed at 40 L for the main compartment, with the exterior pockets adding usable volume on top of that. Outdoor Vitals lists a maximum recommended carry load of 35 lb. In practice, a pack of this style tends to feel best well below that number, and around 22 lb is roughly where it still carries comfortably. Hikers whose base weight stays under 20 lb will get the most from it.

Zpacks Arc Haul Ultra: Overview and Highlights

The Arc Haul Ultra is an established option in the same fabric class, and its defining trait is the arc built into the frame. That curve holds the pack body away from your back and gives it an edge when loads get heavier, which is why it remains a common reference point for hikers comparing ultralight packs.

The pricing structure is different from the CS40 Ultra. The pack alone is listed at $400, and hip belt pockets are sold separately at about $33 each. Adding a pair brings a comparable setup to roughly $470. The hip belt also uses two adjustment straps rather than one, which some hikers find more fiddly when putting the pack on and taking it off.

For anyone who values the arched frame and is comfortable paying for it, it stays a strong option in this category.

Comfort, Fit, and Load Distribution

Comfort in a frameless-style pack comes down to the frame and the hip belt working together. On the CS40 Ultra, the carbon stays arc with your back and flex while you walk, and the load lifters sit high on the frame, which helps pull weight closer to your body as the load increases. The hip belt is wide, which reduces the chance of rubbing or bruising around the hip bones when you tighten it down for a heavier load, and it uses a single strap that widens as it meets the belt to micro-adjust across both sides.

Sizing deserves attention. A 33 inch waist fit the medium hip belt, but on a 6 ft 2 in, 210 lb frame the medium belt ran a little short, so a large hip belt is the safer choice for a similar build. The belt snaps easily and adjusts quickly. As for ventilation, the back panel is contoured and vented, but a sweaty back is still likely on warm climbs, which is true of almost every pack in this class, including arched designs.

The Arc Haul Ultra’s arched frame is the main reason it holds an advantage with heavier loads, and that difference becomes more noticeable as weight goes up.

Storage, Features, and Small Details

Storage on the CS40 Ultra is generous for its weight. The side water bottle pockets are large enough to hold two one litre bottles in a single pocket, which also means wider bottles will fit, and they are angled and positioned so bottles remain reachable while the pack is worn. Compression straps run down each side and are useful for holding trekking poles or tent poles against the pack instead of letting them swing.

The hip belt pockets open and close easily with one hand and are roomy enough for a phone or snacks. The pocket material appears laminated and water resistant, though the zipper itself is not waterproof. Inside, there is a removable water bladder sleeve with a hose port positioned for easy access while hiking. Drain ports in the bottle pockets and at the base of the pack let water escape, and the stretch mesh front pocket holds a lot while keeping its holes small enough that gear does not snag.

Attachment points include loops for trekking poles or an ice axe, plus small loops underneath for extra cordage if you want to strap something below the pack. The buckles and straps are thin but rigid and well sewn.

A few details are worth knowing before you buy. There are no built in straps on the bottom of the pack, so attaching gear underneath means sourcing your own cord. The top strap is long enough to hold a standard bear canister such as the Bear Vault 450, but only just, so packing height needs some thought. That canister fits easily inside the main compartment instead. Finally, the stretch mesh front pocket showed a small hole after a single trip, which is normal wear for this type of mesh and worth expecting if you are hard on your gear.

Price, Value, and Who Each Pack Is For

The CS40 Ultra is listed at $327.97, with a membership price of around $295 through Outdoor Vitals. Hip belt pockets are included. The Arc Haul Ultra is listed at $400 for the pack alone, with hip belt pockets at about $33 each, so a comparable setup lands near $470. That puts the CS40 Ultra roughly $140 lower once pockets are accounted for on both sides, which is money that can go toward a quilt, a shelter, or a lighter set of the rest of your kit.

The CS40 Ultra suits hikers whose base weight sits under about 20 lb and who want a light, waterproof, well organized pack without paying a premium for it. The included hip belt pockets and the wider belt make it a comfortable choice for that range. It is not the right pack for someone regularly carrying 30 lb or more, since that is where shoulder strain starts to appear.

The Arc Haul Ultra suits hikers who want the arched frame and are willing to pay for it, particularly if heavier loads are part of their normal trips. If your base weight is already low and the arc is not a priority, the price difference is hard to justify.

Conclusion

Both packs are built from the same modern waterproof fabric and both target the same ultralight audience. The decision comes down to frame philosophy and budget. The Outdoor Vitals CS40 Ultra offers carbon stays, a wide hip belt, large angled bottle pockets, drain ports, and included hip belt pockets at a lower price. The Zpacks Arc Haul Ultra offers its arched frame, which gives it an advantage when loads grow heavier, at a higher total cost once accessories are added.

If your base weight is under 20 lb and you want strong value with a comfortable, well equipped carry, the CS40 Ultra is the easier recommendation. If you regularly push toward heavier loads and the arched frame is what you are after, the Arc Haul Ultra earns its place.