Stoves and water

Soft flask

Also called collapsible flask, soft bottle

What is a soft flask?

A soft flask is a collapsible bottle, usually 250 to 750 ml, carried in the front pockets of a hydration vest. It shrinks as you drink so it doesn't slosh, and you can see exactly how much water you have left.

Common sizes: 250 to 750 ml
Carried: Vest front pockets
Strength: No slosh, visible level

Why collapsing matters

The appeal is that it collapses. A half-empty rigid bottle sloshes and bounces; a soft flask simply shrinks, so the weight stays quiet against your chest as you drink it down. In a vest's front pockets that's the difference between a comfortable run and one where you're aware of your water every step.

Being able to see your water

The bigger practical advantage is that you can see it. A bladder in your back is invisible, so you either guess or stop and check, and the classic hydration bladder failure is arriving at camp having drunk far less than you thought. A flask in front of you gives you a running count without breaking stride, and refilling one takes seconds rather than requiring the pack to come off.

How long they last

They wear out faster than hard bottles. The bite valves fail, the seams eventually leak, and they hold flavours from anything you mix in them. Rinse and dry them after every use, ideally inside out, and expect to replace them periodically rather than treat them as permanent. Most people who run with flasks also carry a bladder for longer efforts and use both.

What this means when you're choosing

A bite valve you can operate one-handed at pace
Whether the neck takes a filter you already own
How it packs when empty, which is the point

Soft flask vs Hydration bladder

	Soft flask	Hydration bladder
Seeing what is left	Yes, it is in front of you	No, it is behind your back
Drinking on the move	Reach forward, one hand	Hose over the shoulder, hands free
Capacity	250 to 750 ml each	1.5 to 3 L in one reservoir
Sloshing	None. It collapses as you drink	Sloshes until it is nearly empty
Lifespan	A season or two	Years with cleaning

Which to pick: Flasks for running and fast hiking, where seeing your water and no slosh matter. A bladder for long carries where the volume is the point.

The common mistake

Expecting hard-bottle lifespan

Bite valves fail, seams eventually weep, and flavours from drink mix stay in the plastic. They are consumables with a season or two in them. That is the price of the thing collapsing as you drink.

Common questions

Are soft flasks better than a hydration bladder?

For running and fast hiking, most people prefer flasks because they're visible, refill quickly and don't slosh. For long days where you want to carry 2 liters without stopping, a bladder is simpler. Plenty of hikers carry both.

How do I clean a soft flask?

Rinse straight after use, turn it inside out if the design allows, and let it dry fully. A little bicarbonate soda and warm water clears lingering flavours, and drying is what actually stops mould.

Where this comes up

Guides on Tom's Hiking where soft flask changes which product is right for you.

Going deeper

More in stoves and water

Canister stove

A canister stove screws directly onto a sealed cartridge of pressurised isobutane-propane. It lights instantly, simmers well and needs no priming, which is why it's the default backpacking stove almost everywhere fuel canisters are sold.

Integrated stove system

An integrated stove system pairs a burner with a matched pot that locks on, usually with a heat exchanger ring on the base. It boils water faster and uses less fuel than a separate stove and pot, at the cost of versatility.

Regulated valve

A regulated valve holds a stove's fuel pressure steady as the canister empties or cools. Without one, output drops off through the canister's life and falls sharply in the cold; with one, the flame stays consistent much further down.

Isobutane-propane blend

Isobutane-propane is the fuel blend inside standard threaded backpacking canisters, usually around 80 percent isobutane and 20 percent propane. Propane provides cold-weather pressure, isobutane provides most of the burn time.

Hollow fiber filter

A hollow fiber filter passes water through a bundle of tiny straw-like tubes whose walls are perforated with pores around 0.1 to 0.2 microns. It's the technology in almost every modern backpacking filter, and a single freeze can destroy it invisibly.

Micron rating

A micron rating states the largest particle a filter lets through, in microns. Backpacking filters are typically rated 0.1 or 0.2 microns, which stops bacteria and protozoa. Viruses are around 0.02 microns and pass straight through.

Flow rate

Flow rate is how fast a filter delivers clean water, usually quoted in liters per minute. The advertised figure is measured on a brand-new filter with clean water, and real flow drops as the element clogs with sediment.

Backflushing

Backflushing pushes clean water backwards through a filter to dislodge the sediment clogging its pores. It's routine maintenance on a hollow fiber filter, and it's the difference between one that lasts a season and one that lasts years.

Purifier vs filter

A filter physically strains out bacteria and protozoa. A purifier also deals with viruses, which are too small to strain, using chemistry, UV light or a much finer element. Most backcountry hiking needs a filter; international travel often needs a purifier.

Hydration bladder

A hydration bladder is a flat water reservoir carried inside a pack with a drinking hose routed over the shoulder. It holds 1.5 to 3 liters, packs flat against your back and lets you drink without stopping or taking the pack off.

Bite valve

A bite valve is the silicone mouthpiece on the end of a hydration reservoir's drink tube. It stays shut until your teeth compress a slit in it, then self-seals when you let go, so water only flows while you are drinking.

Quick disconnect

A quick disconnect is the two-part coupling in a hydration hose that lets you unclip the tube from the reservoir without threading it back through your pack. Both halves usually self-seal, so you can pull the bag out to refill and neither end drips.

Hard-anodized aluminum

Hard-anodized aluminum is aluminum whose surface has been grown into a thick, very hard oxide layer by an electrochemical bath. In cookware it gives a pot that spreads heat far more evenly than titanium and resists scratching far better than bare aluminum.

Last reviewed August 13, 2026. Back to the glossary