Salomon Soft Flask Review

Overview

The Salomon Soft Flask is the bottle that trail runners clip into a vest pocket and forget about, and it has been migrating steadily into ultralight backpacking kits. It is a collapsible, BPA-free TPU flask with a high-flow bite valve, built to shrink as it empties so water never sloshes against the ribs. The trade-off is durability: it is among the lightest ways to carry water, and it is also softer and easier to puncture than most hiking bottles.

Key Specs

Spec	Value
Weight	33–38 g depending on version
Capacity	Up to 500 ml / 17 oz (Ultra 42)
Cap opening	42 mm on the larger flasks; 28 mm on the 250 ml size
Body material	TPU with RF-welded seams
Cap and valve	High-flow bite valve, angled cap
BPA / PVC	Free of both
Collapsible	Yes
Filter compatibility	Salomon XA Filter and Katadyn BeFree caps (42 mm flasks)
Typical price	About $22–25

Performance

Bounce and bulk. The flask’s main job is to stop being a bottle as soon as it stops being full. The TPU body collapses progressively with each sip, so a half-empty flask in a chest pocket stays put rather than sloshing. The translucent walls and volume markings also give a quick read on what is left, which matters more at mile 18 than it sounds. Empty, it folds down to roughly the size of a wallet.

Flow and fill. Owners on retail sites repeatedly single out two things: the valve delivers a strong stream with a light squeeze, and the wide 42 mm mouth fills quickly from a stream or tap. That opening also accepts ice cubes, electrolyte tablets and drink powder without a funnel, and it is wide enough for a bottle brush. One retail reviewer who ran two flasks through a first ultra called them reliable. The 28 mm neck on the 250 ml size is a different story: slower to fill, and harder to scrub.

Leaks and seams. Complaints are scattered rather than systemic. Some buyers report occasional leaks from the cap when the flask is squeezed hard, and a few say seams eventually weep after long service. Others describe four-plus years of hard use, including Grand Canyon crossings and mountain ultras. The pattern is a flask that lasts if it is kept away from sharp rock and abuse, and that has no real defense against a stray thorn or a pack full of tent stakes.

Filter use. For backpackers, the 42 mm flasks are the real draw. Swap the bite-valve cap for Salomon’s XA Filter lid or a Katadyn BeFree filter and the flask becomes a squeeze filter that rides up front in a vest or strap pocket, ready for source-to-source water. The 28 mm flasks do not take these caps.

Taste and cleaning. New flasks can carry a faint plastic taste. Users generally report it fading after a wash or two, with overnight soaking in cleaning tablets as a faster fix. The wide mouth makes routine cleaning easy on the 42 mm models.

Fit. The flasks are designed around Salomon’s own vests, including the ADV Skin and Sense Pro lines, and slide in and out of front pockets one-handed. They also work in running belts or in hand. The Ultra’s angled, anatomical shape sits flatter against the chest than a straight-sided flask, which owners say reduces the jutting-bottle feel on long days.

Insulated option. Salomon also sells a 400 ml insulated 42 mm flask with dual-skin walls, which slows warming in heat and resists freezing in the cold. At about 2 oz, it weighs noticeably more than the standard flasks. It is a niche pick for winter racing or desert heat.

Choosing a Version

Standard (33 g). The conventional soft flask, and the one most hikers will want. It is the lightest of the two listed here, fills and cleans easily through the wide opening, and is the straightforward choice for a vest-pocket or belt carry. It takes the same 42 mm caps as the larger models, so the filter-lid setup works the same way.

500ml/17oz Ultra 42 (38 g). The 5 g penalty buys a different shape. The Ultra is built with an anatomical tilt and a conical profile that sits close to the body and slips into deep vest pockets without catching. It holds a full 500 ml, reads well in the pocket, and uses the same 42 mm high-flow cap. Runners and ultra racers who drink on the move from a front pocket benefit most. Hikers who just want to carry and filter water gain little from the extra shaping and can save the grams.

Pros & Cons

Pros

• 33–38 g, light enough to carry as a spare without noticing
• Collapses as it empties, so water bounce is minimal
• 42 mm opening takes ice, tablets and powder, and can be brushed clean
• Accepts Salomon XA and Katadyn BeFree filter caps
• High-flow valve is easy to drink from while moving
• Fits Salomon vests and most front-pocket designs

Cons

• TPU body is not puncture-resistant
• Occasional reports of cap leaks under pressure and seams that fail over time
• Plastic taste out of the box
• The 28 mm size is slow to fill and cannot take filter caps
• Standard versions are uninsulated, so water warms quickly in the sun
• Pricey for a simple bottle

Who Should Buy This

Anyone running a vest with front flask pockets, and ultralight hikers who want a featherweight bottle that doubles as a squeeze-filter reservoir. Pair two for a liter up front, or carry one as a dedicated clean-water flask. Hikers who stuff bottles into abrasive, overloaded packs, or who want a bottle that tolerates being dropped and stepped on, should look at a rigid or heavier-duty option instead. Those who need back-flush cleaning or tougher walls often turn to Hydrapak’s flasks.

Verdict

The Soft Flask earns its popularity: it is light, quick to fill and drink from, and the 42 mm models gain real backcountry utility from filter-cap compatibility. Take the Standard at 33 g unless the Ultra 42’s tilted fit suits a vest pocket you actually use. Neither is the toughest soft flask around, but for vest and ultralight carry, both are easy to recommend.