Salomon Soft Flask Review

Overview

The Salomon Soft Flask is one of the most ubiquitous pieces of hydration gear in trail running — and increasingly, in ultralight backpacking circles too. It’s a collapsible, BPA-free flask built around two core ideas: minimal weight and zero water bounce. Salomon offers the line in a handful of confirmed sizes — 250ml with a 28mm cap, and 500ml (plus a Speed and Ultra variant) with a wider 42mm cap, as well as an insulated 400ml option — so there’s a configuration for almost every carry style. Note: A 450ml variant has been listed in some product data, but we were unable to confirm it exists in Salomon’s current lineup; the known sizes are as listed above.

Key Specs

Spec	250ml / 28mm	500ml / 42mm	400ml Insulated / 42mm
Capacity	250ml / 8 oz	500ml / 17 oz	400ml / 13 oz
Cap Diameter	28mm	42mm	42mm
Weight	~28g (1 oz)	~30g (just over 1 oz)	~57g (2 oz)
Material	56% TPU, 23% PP, 16% Silicone, 5% POM	55% TPU, 35% POM, 10% Silicone	55% TPU, 35% POM, 10% Silicone
BPA / PVC Free	Yes	Yes	Yes
Insulation	No	No	Yes (dual-skin)
Price (MSRP)	~$15	~$25	~$30
Comparison	See how Salomon Soft Flask compares to similar gear

Performance

Collapsibility and Water Bounce

This is where the Soft Flask earns its keep. The TPU body compresses progressively as you drink, which means the sloshing that plagues hard-sided bottles in vest pockets simply doesn’t happen here. That might sound minor on a short day hike, but carry a half-full rigid bottle in a chest pocket for six hours and you’ll understand why this matters. The flask also shows you exactly how much you have left at a glance — the volume markings on the side are clear enough to read on the move.

Flow Rate and Valve

The high-flow bite valve on the 42mm version genuinely delivers. Squeeze and drink is easy even when your hands are tired — no fussing, no dry-mouthing it. The 28mm version uses Salomon’s updated Rocket Valve, which is similarly fast for a smaller opening. A few users at REI reported rare valve failures mid-use, and there’s at least one account of a valve detaching on only the second use — so it’s not a universal experience, but it’s worth knowing. The majority of long-term users report no issues, with some noting consistent performance after years of hard use.

Fill and Cleaning

Here’s where size really matters. The 42mm opening is wide enough to add ice cubes, drop in an electrolyte tablet, or pour in a powder mix without decorating your kit. It also means cleaning with a brush is actually feasible. The 28mm opening on the 250ml model is a tighter squeeze — literally. That narrow neck can be slow to fill from a water source and harder to clean thoroughly. A few users recommend running Bottle Bright tablets overnight before first use to knock out any initial plastic taste, which fades on its own after a wash or two.

Compatibility

The 42mm versions unlock the flask’s most compelling backpacking use case: swap the standard bite-valve cap for the Salomon XA Filter lid or a Katadyn BeFree filter cap, and you have a squeeze-filter system that front-mounts cleanly in a vest or shoulder strap pocket. That’s a meaningful upgrade for anyone doing source-to-source travel on a trail. The 28mm models don’t share this compatibility, so they’re more purpose-limited.

Insulated Version

The 400ml Insulated 42 adds a dual-skin construction that slows warming in summer and — more uniquely — resists freezing in winter, with a bite valve designed to evacuate ice buildup with a squeeze. At 2 oz, it’s obviously heavier than the standard flask, but for winter use or genuinely hot desert conditions, the trade-off can be worth it.

Durability

The long-term track record is generally solid. Multiple users report getting four-plus years of hard use out of a single flask, and accounts of the Soft Flask surviving Grand Canyon crossings and mountain ultras are easy to find. That said, the TPU body is not puncture-resistant — a sharp rock or an unlucky seam stress point can end a flask’s life early. One reviewer noted a split developing on an older flask after sustained use. It’s softer and lighter than alternatives like Hydrapak’s UltraFlask, which means it’s trading some structural resilience for packability.

Pros & Cons

Pros

• Genuinely lightweight — the 500ml comes in just over 1 oz
• Shrinks progressively as you drink; water bounce is a non-issue
• 42mm opening is easy to fill, add ice, and clean
• Compatible with Salomon XA and Katadyn BeFree filter lids (42mm only)
• High-flow valve performs well even when fatigued
• Insulated variant extends functionality into cold and hot weather extremes
• Pairs seamlessly with Salomon ADV Skin and Sense Pro vests

Cons

• Initial plastic taste — fades after cleaning, but it’s there out of the box
• 28mm (250ml) opening is too narrow for ice, tablets, and most cleaning brushes
• Not insulated in standard versions; water warms quickly in summer heat
• Occasional valve failure reports in newer models
• Premium price for what is, functionally, a simple piece of kit
• The XA filter variant is slightly tall for some vest pockets

Who Should Buy This

The Salomon Soft Flask 500ml / 42mm is the easy recommendation for anyone already running a Salomon vest or any vest with front flask pockets. Pair two of them for a full liter up front, or run one with a filter cap for lightweight backcountry travel. If you want the smallest possible option for short efforts, the 250ml works well handheld or in a belt. The insulated 400ml earns its place in winter racing or summer desert conditions where temperature management actually matters. Anyone without a vest-based carry system, or who needs a filter-compatible flask, may find better value in Hydrapak’s UltraFlask — which offers back-flush cleaning and a slightly more durable feel.

Verdict

The Salomon Soft Flask is popular for the right reasons: it’s light, it handles well in motion, and the 42mm version’s filter lid compatibility gives it genuine utility beyond just storing water. It’s not the most durable soft flask out there, and the 28mm variant’s narrow neck is a genuine daily-use annoyance. For Salomon vest users, it’s still the most natural fit in the lineup. I’d rate it 7.5 / 10 — a well-executed product that earns its category dominance, with a few rough edges that stop it from being the definitive choice for every hiker.