REI Co-op Flash 55 vs Mountainsmith Zerk 40
Quick take: The REI Co-op Flash 55 costs $1 less; the REI Co-op Flash 55 carries more (55 vs 40 liters); the Mountainsmith Zerk 40 is lighter (2.13 vs 2.81 lbs).
Which one to buy
Don't let the badges mislead you. One review measured the Zerk 40 at about 51 L of real volume, so it may hold close to what the Flash 55 claims, and the two are a dollar apart. The choice is between two ways of being versatile: the Flash strips down with REI's Packmod system, losing its lid, pockets and straps to reach about 2.35 lbs from 2.81, while the Zerk pulls its frame sheet so you can run it framed or frameless from one pack.
Load is where the Flash pulls ahead. Reviewers say it shines under 30 lbs, with a ceiling that arrives abruptly as the hipbelt wings flex and the load sags onto thin shoulder straps. Mountainsmith suggests up to 30 lbs for the Zerk, but reviewers found it most comfortable at 20-25 lbs. Leave the sheet in for loads in the mid-20s, though a sheet transfers load less effectively than real stays. A BV500 fits the Flash horizontally, which is a big reason canister-country hikers buy it.
Fabric and fit favor different buyers. The Zerk's Extreema ripstop is tough, and woven fabric takes trailside tape repairs well, though it isn't waterproof. The Flash's 100D nylon wets through in sustained rain, so use a compactor bag liner. The Zerk is one size for 16-21 in torsos; the Flash comes in S-L covering roughly 17-21 in, with about 2 in of adjustment in each size, and REI's in-store fitting and return window take the risk out of sizing. The Flash also has a foam back that sweats in summer and pieces that are easy to lose. Picking the Zerk means a lower comfortable ceiling in exchange for a pack you can run frameless and patch on trail; picking the Flash means a better carry near 30 lbs and a canister that fits sideways.
REI Co-op Flash 55
A good match for first-time ultralight hikers carrying up to 30 lbs or a BV500, who want to try sizes in a store.
Mountainsmith Zerk 40
For hikers whose loads stay around 20-25 lbs and who want one pack that runs framed or frameless, with a fabric that patches easily.
| Spec | REI Co-op Flash 55 | Mountainsmith Zerk 40 |
|---|---|---|
| Price | $229 | $230 |
| Rating | 4.2★ (98) | — |
| Type | Ultralight Pack | Ultralight Pack |
| Frame Type | Internal Frame | Internal Frame |
| Capacity | 55 L | 40 L |
| Weight | 2.81 lbs | 2.13 lbs |
| Max Load | 30 lbs | 30 lbs suggested (up to 45 lbs) |
| Torso Fit | 4 sizes (S, M, L, L torso with S hipbelt), each adjustable: S 17-19 in, M 18-20 in, L 19-21 in | One size, 16-21 in torso |
| Material | 100D ripstop nylon | Robic nylon X-Grid Extreema ripstop with UTS coating |
| Back Panel | Framed foam with ventilation channel | Removable frame sheet |
| Adjustable torso | Yes | No |
| Ventilated back | No | No |
| Raincover included | No | No |
| Hipbelt pockets | Yes | Yes |
| Removable lid | Yes | No |
| Hydration sleeve | Yes | Yes |
| Fits BV500 bear can | Yes | Yes |
| Warranty | REI satisfaction guarantee: returns within 1 year for members (90 days for non-members); defects in materials or workmanship returnable at any time | Mountainsmith lifetime warranty against material and workmanship defects; repair or replacement at Mountainsmith's discretion |
Pros & cons
REI Co-op Flash 55
- ✓Often named the best-value ultralight pack; many ultralight hikers rank it the cheapest pack that doesn't compromise the essentials.
- ✓The Packmod system is genuinely clever: strip the lid, pockets, and straps to drop from 2.8 to about 2.35 lbs.
- ✓REI sales periodically cut the $229 list price, at times to near $150.
- ✓The roll-top plus side zip gives both compression and mid-pack access, a combo rare at any price.
- ✓Hipbelt pockets come standard and fit modern phones — a dig at the Exos in every comparison thread.
- ✓A BV500 fits horizontally, which canister-country hikers specifically buy it for.
- ✓The 100D recycled ripstop wears better than its weight suggests; multi-thousand-mile PCT reports exist.
- ✓REI's return window makes it the lowest-risk way to test whether ultralight works for you.
- ✗The 30 lb comfort ceiling is real and arrives abruptly — reviewers agree it sours above that line faster than the Exos.
- ✗The foam back panel ventilates poorly compared to trampoline designs; sweat is the common summer gripe.
- ✗Torso adjustment spans only about 2 in within each size, so picking the right size still matters.
- ✗The hipbelt wings flex under load, letting heavy carries sag onto shoulders.
- ✗Shoulder-strap padding is thin, a frequent complaint from higher-load users.
- ✗The bottle pockets angle awkwardly; retrieving a bottle one-handed takes practice.
- ✗Removable pieces are easy to lose and cost real money to replace.
- ✗Its 4.4-star REI average hides a bimodal split: light packers love it, heavy packers don't.
Mountainsmith Zerk 40
- ✓The removable frame sheet means one pack covers both framed and frameless use.
- ✓One review measured about 51 L against the 40 L badge, where most packs here measure under.
- ✓Robic X-Grid Extreema ripstop (100D body, 210D base and back) is a tough woven fabric that repairs easily.
- ✓$230 is mid-range for the segment and below most cottage packs of similar volume.
- ✓Mountainsmith is a mainstream brand, so returns and support are simpler than cottage brands'.
- ✓1 lb 14 oz stripped (2 lb 2 oz full) is competitive with dedicated ultralight packs.
- ✓Hipbelt pockets included.
- ✓Widely available rather than made to order.
- ✗A removable frame sheet is less effective at load transfer than stays or a hoop.
- ✗No adjustable torso.
- ✗Not waterproof, unlike the laminate packs at this price.
- ✗Mountainsmith has less ultralight credibility than the cottage brands it competes with.
- ✗The 40 L badge understates the pack, which makes cross-shopping confusing.
- ✗Fewer configuration options than a made-to-order pack.
- ✗Limited presence on long trails, so fewer real-world reports.
REI Co-op Flash 55 vs Mountainsmith Zerk 40: common questions
Does the Zerk 40 really hold as much as the Flash 55?▾
Possibly. One review measured about 51 L of real volume despite the 40 L badge (Mountainsmith lists 44 L with the roll top extended). That makes it far closer to the Flash 55 than the names suggest, so compare by your kit rather than the labels.
Which one carries heavier loads?▾
The Flash 55. Reviewers say it carries well up to 30 lbs before comfort drops off sharply. Mountainsmith suggests up to 30 lbs for the Zerk, but reviewers put its comfortable range at 20-25 lbs, and its frame sheet transfers load less effectively than stays.
Can I run either one without a frame?▾
The Zerk 40, yes: its frame sheet is removable, so you can pull it once your base weight is dialed. The Flash 55 strips down differently, by removing its lid, pockets and straps to reach about 2.35 lbs.