Best Soft Flasks for Trail Running: 10 Options for Race Vests
Buy the HydraPak UltraFlask Speed 500 unless your vest is a Salomon, then buy Salomon’s own. Harrier for spares. How to match a flask to a pocket, and seven more considered.
Affiliate disclosure: links to Amazon in this guide are affiliate links. If you buy through one we earn a small commission at no extra cost to you. The link to Harrier is not an affiliate link; it's there because that's where you buy it. None of it makes any difference to which flask we tell you to buy.
The short version
The best soft flask is the one that fits your vest pocket. If your vest came with flasks and they're not leaking, replace them like for like and stop reading. If you need to choose, three flasks cover almost everyone.
Buy this: HydraPak UltraFlask Speed 500 — for most vests that aren't Salomon. HydraPak make the flasks inside a lot of other brands' packaging, so this is the shape most front pockets were designed around. The structured top slides into a pocket without a fight, the wide SpeedFill opening takes powder and ice, and the high-flow valve means you don't have to suck hard at mile forty. Check the height against your pocket before you buy two.
Salomon vest: Salomon Soft Flask 500/42 — if you run an ADV Skin or Active Skin, buy the flask shaped for the pocket rather than something that nearly fits. The 42mm neck is wide enough for powder and cleaning, which the older narrow-neck version wasn't. Fit in other brands' pockets is a gamble, particularly on height.
Spend less, or need spares: Harrier 500ml Soft Flask — a standard soft flask from a UK trail-running company at a price that makes carrying a third one for a hot day reasonable. Fine for training, race backups and anything with a Harrier vest. Check the cap diameter before mixing it with other brands' lids.
Still not sure? Running without a vest: HydraPak SkyFlask 500 with the hand strap. Measuring carbs or sodium into a bottle for a race plan: Precision Fuel Soft Flask 500, which has markings. Tall flasks push the valve into your chin: Montane SoftFlask Short 500. All three are further down.
We haven't run with all ten of these. They're here because they're what UK runners actually buy and the specifications hold up against the manufacturer's own, not against other people's lists. Cap threads and valve designs change between generations, so check the listing says which one you're getting.
The three picks
HydraPak UltraFlask Speed 500
500ml · high-flow bite valve · wide SpeedFill cap · structured top
HydraPak supply flask technology to many of the running-vest brands, which is why this shape fits so many pockets: it is, more or less, the reference. The stiffened top is what makes the difference in use. A floppy flask fights you every time you push it back into a pocket with cold hands; this one goes in first time. The wide opening takes a scoop of powder and a handful of ice, and the valve flows freely enough that you actually drink.
Watch out for: height. A 500ml flask that's a centimetre too tall for the pocket bounces, or sits with the valve in your face. Measure the pocket before buying two, and check whether you want the straw version for a low pocket.
Salomon Soft Flask 500/42
500ml · high-flow valve · 42mm wide opening
Shaped around Salomon's own pockets, which are shaped around it. If your vest is an ADV Skin or an Active Skin, nothing else sits as cleanly or shrinks as neatly as you drink. The 42mm neck is the current version's improvement over the old one: powder goes in without a funnel and a bottle brush goes in afterwards.
Watch out for: it's a Salomon flask for a Salomon pocket. In other brands' vests the fit varies, mostly on height, and there's no reason to prefer it over the HydraPak.
Harrier 500ml Soft Flask
500ml · bite valve · standard soft flask
A plain soft flask from a UK company that also makes vests, at a price where buying a spare isn't a decision. It does what the expensive ones do without the structured top or the wide neck, and for training and as the third flask on a hot day that's enough.
Watch out for: check the cap diameter against your other flasks before you assume the lids swap. And it's a softer body than the HydraPak, so it needs a bit more pushing into a tight pocket.
Also considered
Seven more that are good flasks with a narrower case for buying them.
HydraPak SkyFlask 500
The handheld: a soft flask with an adjustable hand strap and a locking valve, for runs where you want water and not a vest. The flask collapses as it empties, so it gets less noticeable rather than more. One hand is occupied the whole way. Amazon
Precision Fuel Soft Flask 500
A standard flask with clear volume markings, for people who measure a known amount of carbohydrate or sodium into a bottle and want to repeat the same mix at every checkpoint. The markings don't replace weighing the powder before the race. XMiles
Montane SoftFlask Short 500
A shorter, wider 500ml for vests where a tall flask pushes the valve into your chin, and for Montane's own Gecko vests. The width means it won't slide into a narrow pocket as easily. Amazon
CamelBak Quick Stow 500
CamelBak's Big Bite valve on a soft flask, with a wide cap and a hanging loop that helps with drying. The pick if you already like CamelBak mouthpieces. The valve shape sits differently against the chest than a straight HydraPak nozzle. Amazon
Osprey Hydraulics Soft Flask 500
Designed for Osprey's Duro and Dyna vests; a matched fit and Osprey's warranty. Listings mix generations, so confirm which one you're getting. Amazon
Ultimate Direction Body Bottle 500
Made for Ultimate Direction's front pockets, with a shaped upper and a comfort bite valve. Cap threads changed between generations, so buy replacement parts for the version you have. Amazon
AONIJIE 500ml Soft Flask
The cheapest flask here, and fine for testing whether a 500ml fits your pocket before buying the real thing, or as an emergency spare. Valve quality, taste and cap compatibility are less consistent than the established brands. Amazon
All ten at a glance
| Soft flask | Best for | Capacity | Valve | The point of it |
|---|---|---|---|---|
| HydraPak UltraFlask Speed 500 | Most vests | 500ml | High-flow bite | Structured top, wide SpeedFill cap |
| Salomon Soft Flask 500/42 | Salomon vests | 500ml | High-flow | 42mm opening |
| Harrier 500ml Soft Flask | Spares, value | 500ml | Bite | UK company, sensible price |
| HydraPak SkyFlask 500 | Running without a vest | 500ml | Locking bite | Hand strap |
| Precision Fuel Soft Flask 500 | Measured fuel mixes | 500ml | Bite | Volume markings |
| Montane SoftFlask Short 500 | Low or short pockets | 500ml | Bite | Short, wide body |
| CamelBak Quick Stow 500 | CamelBak valve fans | 500ml | Big Bite | Wide cap, hanging loop |
| Osprey Hydraulics Soft Flask 500 | Osprey vests | 500ml | Bite with extension | Matched fit |
| Ultimate Direction Body Bottle 500 | Ultimate Direction vests | 500ml | Comfort bite | Shaped upper |
| AONIJIE 500ml Soft Flask | Cheapest spare | 500ml | Bite | Price |
What actually matters
The pocket. Measure its height and check the cap diameter. A good flask in the wrong pocket bounces, sinks out of reach, or holds the valve against your chin for six hours. This is the whole decision and everything else is detail.
Straw or not. A straw helps when the flask sits low in the pocket. It also flaps, catches on things and is harder to clean. If the flask sits high enough to drink from directly, skip it.
Neck width. Wide openings take powder, ice and a bottle brush and refill faster at a checkpoint. Narrow ones look tidier and are slower at everything.
Two of them. Two 500ml flasks give a litre, which is what most race vests are built around and what most UK routes need between refills. A third for hot days or dry ridges is cheap insurance, and it's why the value pick exists.
Cleaning. Rinse straight after a run, wash in warm soapy water, dry fully with the cap off. Energy drink is far harder to shift than water once it's dried in, and a flask that tastes of last month's orange is a flask you won't drink from.
Frequently asked questions
What is the best soft flask for trail running?
The HydraPak UltraFlask Speed 500 for most vests, or Salomon's own 500/42 if your vest is a Salomon. The best flask is the one shaped for your pocket, which is why the answer depends on the vest.
Do Salomon flasks fit other running vests?
Sometimes. Height and width vary between brands, so test the exact flask in the pocket before you race with it.
Are soft flasks better than hard bottles?
For a vest, yes: they bounce less and shrink as you drink. Hard bottles are easier to refill and stand up on their own, which matters more at a table than on a hill.
How do I stop a soft flask leaking?
Tighten the cap on clean threads, check the valve for grit, and replace a damaged seal rather than living with it. Fill it, shake it upside down over the sink, then trust it.
Can I put energy drink in a soft flask?
Yes. Clean it as soon as you're home, because sugar and flavour set in a way water doesn't.
How many soft flasks do I need?
Two 500ml flasks give a litre and suit most race vests. Long dry sections or hot weather want a third, or a bladder as well.