What is a Soft Flask? The Complete Beginner's Guide for Runners

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Disclaimer: This guide is for educational purposes only. Individual hydration needs vary based on factors including body weight, climate, exercise intensity, and personal health conditions. The information provided here should not replace professional medical or nutritional advice. Always consult with a healthcare professional or registered dietitian for personalised hydration recommendations, especially if you have any underlying health conditions.

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What Exactly is a Soft Flask?

A soft flask is a flexible, collapsible water bottle designed specifically for runners, hikers, and endurance athletes. Unlike rigid plastic bottles that maintain their shape whether full or empty, soft flasks are made from a pliable material that compresses as you drink.

Think of it like a high-tech version of a water balloon – but engineered for performance.

The magic happens because of what soft flasks are made from: TPU (thermoplastic polyurethane). This food-grade material is the same stuff used in medical equipment like IV bags and catheters. It's completely BPA-free, won't leach chemicals into your drinks, and can handle temperatures from freezing to around 40°C.

Here's how one runner on Geeks On Feet describes the appeal: soft flasks are "small, flexible bottles made of TPU that collapse as you drink, so they take up less space and don't slosh around."

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Why Runners Are Switching to Soft Flasks

If you've ever run with a half-empty hard bottle, you know the frustration: the constant slosh-slosh-slosh with every stride. It throws off your rhythm, it's annoying, and it can even affect your form on longer runs.

Soft flasks solve this problem elegantly. As you drink, the flask compresses, eliminating the air pocket that causes sloshing in traditional bottles.

The 6 Key Benefits

1. Anti-Slosh Design

The collapsible walls mean water stays pressed against the valve, not bouncing around inside an air pocket. Bite down, squeeze, and the water flows smoothly without any internal tsunami.

2. Incredibly Lightweight

A typical 500ml soft flask weighs around 30-40 grams, barely noticeable even when full. Compare that to a standard plastic bottle at 100+ grams empty.

3. Compresses When Empty

Once you've drained your flask, you can fold it up and stuff it in your pocket. Try doing that with a rigid Nalgene.

4. Vest-Compatible Design

Soft flasks are designed to fit the front pockets of running vests perfectly. Many vests actually come with soft flasks included because they work so well together.

5. Easy Drinking On The Move

Most soft flasks feature a bite valve that you simply bite down lightly and squeeze to drink. No need to unscrew caps, tilt your head back, or break stride. As one trail runner notes, "You barely need to tilt the flask; just squeeze and sip."

6. Versatile Use

Beyond water, soft flasks excel at carrying electrolyte mixes, sports drinks, and even energy gels (more on this later).

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The Honest Downsides

No piece of running gear is perfect, and soft flasks have their limitations:

Harder to Clean

The flexible material and often narrow openings make thorough cleaning more challenging than a wide-mouth hard bottle. You'll need to be diligent about rinsing after every use and doing regular deep cleans.

Can Be Tricky to Refill

Getting a floppy, empty soft flask back into a tight vest pocket after refilling at an aid station takes some practice. Many ultra runners cite this as their biggest frustration.

Durability Concerns

Over time, soft flasks can develop small punctures or splits, especially at stress points near the neck. Most last 1-2 years with regular use before needing replacement.

Flavour Retention

If you use sports drinks or electrolytes regularly, the TPU can absorb some flavour that's difficult to remove completely.

Not Ideal for Waist Belts

Because they lose their shape as they empty, soft flasks can be harder to remove and replace from waist belt sleeves compared to rigid bottles.

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Soft Flask Sizes: Which One Do You Need?

Soft flasks typically come in three main sizes, each designed for different purposes:

150ml: The Gel Flask

This tiny flask is designed specifically for carrying energy gels. You can squeeze 4-5 gel servings into one flask, add a splash of water to thin the mixture, and sip throughout your run instead of fumbling with individual packets. The HydraPak SoftFlask 150ml holds about 4 gel servings and fits easily in shorts pockets.

Best for: Ultra runners, marathon fueling, anyone tired of sticky gel packets

250ml: The Minimalist

Perfect for shorter runs where you just need a bit of hydration without the bulk. These compact flasks fit in most shorts pockets, waist belts, or can be carried handheld with a strap.

Best for: Runs under 10km, parkruns, speed sessions, minimalist carriers

500ml: The Workhorse

The most common size and the standard for hydration vests. Two 500ml flasks give you a litre of carrying capacity, enough for most training runs and races with aid stations.

Best for: Half marathon+ distances, trail running, vest-based hydration systems

Size Guide by Activity

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Anatomy of a Soft Flask

Understanding the parts helps you choose the right flask and use it properly:

The Body

Made from TPU, the flexible body is the defining feature. Higher-quality flasks use thicker, more durable TPU that resists punctures while remaining pliable.

The Mouth Opening

Comes in two standard sizes:

• 28mm: Narrower opening, lighter weight, can be harder to fill and clean
• 42mm: Wide mouth, easier to add ice, electrolyte tablets, or powder; easier to clean

If you plan to add anything other than plain water, strongly consider the 42mm option. As one frustrated runner discovered, "The narrow necks are an absolute pain to try and either fit big tablets into, or pour sachets of powder into."

The Valve

Most soft flasks use a bite valve (also called a nipple valve). You bite down lightly to open the valve, then squeeze the flask to drink. Some benefits:

• Leak-proof when not in use
• Squeeze out air to eliminate sloshing
• Hands-free drinking in vest pockets

Some flasks offer a straw attachment that positions the drinking point closer to your mouth when the flask is in a vest pocket. This saves you from the "awkward chin-tuck" movement, though straws can bounce around annoyingly.

The Cap

Usually includes:

• A dust cover to keep the valve clean
• An easy-open mechanism for one-handed operation
• A wide opening on the opposite end for filling

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How to Use a Soft Flask: Step-by-Step for Beginners

First Use Setup

Before your first run with a new soft flask:

1. Wash thoroughly with warm water and mild dish soap
2. Remove any plastic taste by soaking overnight with 1-2 tablespoons of baking soda dissolved in warm water
3. Rinse multiple times until no residue remains
4. Air dry completely with the cap off

Filling Your Flask

1. Remove the cap/valve assembly
2. Hold the flask upright or place it in a cup for stability (empty flasks don't stand on their own)
3. Fill to just below the neck, don't overfill
4. If adding electrolytes: pour powder first, then add water, shake to mix
5. Replace the cap securely
6. Squeeze out excess air. This is the key anti-slosh trick

Drinking On The Run

1. Grab the flask (or lower your chin if it's in a vest pocket)
2. Bite down gently on the valve to open it
3. Squeeze the flask body to push water out
4. Release the bite to close the valve
5. Optional: Give a small squeeze after drinking to keep air out

Pro tip from experienced runners: "Fill the bottle with air after each sip to keep the bottle compressed against your hand" if using a handheld setup.

Reinserting Into Vest Pockets

This is the tricky part that takes practice:

1. Pinch the top of the flask to create a rigid point
2. Guide that point into the pocket opening
3. Use your thumb to push the body in, section by section
4. Don't rush, forcing a full flask can stretch vest pockets

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Soft Flask vs. Hard Bottle: When to Use Each

When Soft Flasks Win

• Running with a hydration vest
• Trail running where weight matters
• Races where you need to stash empty bottles
• Ultra distances with frequent sipping
• Energy gel storage

When Hard Bottles Win

• Handheld running (easier to grip consistently)
• Gym or everyday use
• When insulation matters (hot days)
• Quick refills at aid stations
• Durability is the priority

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Soft Flask vs. Hydration Bladder

Many runners debate between soft flasks in vest pockets versus a bladder reservoir on their back:

The verdict: Many experienced trail runners use both a bladder on the back for water backup, soft flasks in front for primary hydration and electrolyte drinks.

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The Gel Flask Hack: Why Ultra Runners Swear By Small Soft Flasks

One of the most popular uses for 150ml-250ml soft flasks is storing energy gels. Instead of carrying individual gel packets that are sticky, fiddly, and create litter, you can:

1. Squeeze 5-6 individual gel packets into a 250ml flask
2. Add 20-50ml of water to thin the mixture
3. Shake well to combine
4. Sip throughout your run as needed

Benefits of the gel flask method:

• No more fumbling with packets at race pace
• Zero trail litter
• Better absorption (gels are already diluted)
• Easier to track how much fuel you've consumed
• Mix different flavours for variety

This technique is popular among elite ultra runners and is recommended by brands like GU Energy for long-distance events.

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How to Clean Your Soft Flask Properly

Proper cleaning is essential, as soft flasks can develop mould and funky smells quickly if neglected.

After Every Use

1. Empty any remaining liquid immediately
2. Rinse thoroughly with warm water
3. Squeeze water through the valve to clean it
4. Shake out excess water
5. Leave open and upside-down to air dry completely

Weekly Deep Clean (or After Electrolyte Use)

Baking Soda Method:

1. Add 1-2 tablespoons of baking soda to warm water
2. Fill the flask and shake vigorously
3. Let sit for 30 minutes to several hours
4. Rinse thoroughly multiple times
5. Air dry completely

Vinegar Method:

1. Fill flask with equal parts white vinegar and warm water
2. Let sit for 15-30 minutes
3. Rinse thoroughly (vinegar smell will dissipate)
4. Air dry completely

Denture Tablet Hack: The running community discovered that denture cleaning tablets (like Polident or Steradent) work just as well as expensive bottle-specific tablets at a fraction of the cost. Both contain similar active ingredients, including sodium bicarbonate and citric acid. Use unflavoured tablets to avoid minty-tasting water on your next run.

The Freezer Storage Trick

Here's a game-changing tip that many runners swear by: store your clean, dry soft flask in the freezer between uses. Freezing temperatures halt bacterial and mould growth completely, keeping your flask fresh even if it wasn't perfectly dry.

Signs It's Time to Replace

• Visible cracks, punctures, or splits
• Persistent bad taste or smell despite cleaning
• Mould that won't wash away
• Valve no longer seals properly
• Material becoming brittle or discoloured

Most soft flasks last 1-2 years with regular use and proper care.

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Choosing Your First Soft Flask: What to Look For

Material Quality

• TPU (thermoplastic polyurethane): The gold standard. BPA-free, food-safe, durable
• Avoid cheap PVC: Can impart a plastic taste, be less durable, and potential health concerns
• Look for: "Food-grade TPU," "BPA-free," "PVC-free" on packaging

Mouth Size

• 42mm recommended for versatility (ice, powders, easier cleaning)
• 28mm is acceptable if you only use plain water

Valve Type

• Bite valve: Most common, proven reliable
• High-flow design: Important for quick hydration without excessive squeezing
• Leak-proof when closed: Test before your first run

Compatibility

• Check your vest: Different vests have different pocket shapes
• Salomon flasks tend to be taller and narrower
• HydraPak flasks are often shorter and wider
• Match flask shape to your vest pockets for best fit

Brand Reputation

Established brands with proven track records:

• HydraPak: Industry leader, lifetime warranty
• Salomon: Comes standard with many vests
• Nathan Sports: Known for ExoSpine stability feature
• CamelBak: Quick Stow design for easy filling

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Common Beginner Mistakes to Avoid

Mistake 1: Not Squeezing Out the Air

The whole point of a soft flask is that it collapses. After filling and sealing, squeeze the flask to push out excess air before your run. This is what eliminates sloshing.

Mistake 2: Leaving Wet Flasks Sealed

This is how mould happens. Always store your flask open, upside-down, until completely dry.

Mistake 3: Overfilling

Soft flasks are designed to expand slightly, but overfilling creates pressure that can cause leaks. Fill to just below the neck.

Mistake 4: Using the Wrong Size for Your Needs

A 500ml flask is overkill for a 5K. A 250ml flask won't cut it for a long trail run. Match your flask size to your typical run distance and conditions.

Mistake 5: Neglecting the Valve

The valve is a prime spot for bacterial buildup. Make sure to clean it specifically, squeeze water through it during cleaning and let it dry open.

Mistake 6: Not Testing Before Race Day

Never use new gear on race day. Do at least 2-3 training runs with your soft flask to get comfortable with filling, drinking, and reinserting.

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Soft Flasks for South African Conditions

Running in South Africa presents unique challenges that soft flasks handle well:

Heat Management

Summer runs in the Western Cape, Gauteng, and KZN can push 30°C+. Soft flasks don't insulate well, so your water will warm up, but that's often fine for runs under 2 hours. For longer hot runs, freeze your flask overnight (TPU handles this fine) and let it thaw during your run.

Dust and Trail Conditions

The Cape mountains and Drakensberg throw dust, dirt, and debris at your gear. The dust cap on your bite valve is essential to keep it on when not drinking.

Event Compatibility

Major South African events like Two Oceans and Comrades have specific hydration rules. Soft flasks in vest pockets are generally allowed, but always check event regulations. Aid station cups can be used to refill your flask, though this takes practice with a floppy soft flask.

Availability in South Africa

Soft flasks are available from:

• Specialty running stores in Cape Town, Johannesburg, and Durban
• Decathlon: Budget-friendly options
• Online retailers: Wider selection
• SoftFlask.co.za: Local options with nationwide shipping

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Frequently Asked Questions

Can I put hot drinks in a soft flask?

Most soft flasks are rated for temperatures up to 40-60°C, warm but not boiling. Check your specific flask's specifications. Hot liquids can also make the TPU material more pliable and prone to damage.

Will electrolytes damage my soft flask?

No, but they can leave flavour residue. Clean more frequently when using electrolytes, and consider dedicating one flask to plain water and another to sports drinks.

Can I put my soft flask in the dishwasher?

Many are technically dishwasher-safe (top rack only), but hand washing is gentler and extends flask life. The heat and detergents in dishwashers can degrade TPU over time.

How do I get rid of the new plastic smell?

Soak overnight in baking soda solution (2 tablespoons per flask of warm water). Some runners also use lemon juice. Rinse thoroughly and air dry.

Are soft flasks better than handheld bottles?

Different tools for different jobs. Soft flasks excel in vests and for collapsible storage. Handheld bottles with straps offer easier refilling and more consistent grip. Many runners own both.

Can I use a soft flask for anything besides running?

Absolutely. Soft flasks work great for hiking, cycling, gym workouts, travel, and any activity where light weight and packability matter.

How many soft flasks do I need?

Most runners start with two 500ml flasks (for vest pockets) and add a 250ml or 150ml gel flask later as needed.

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The Bottom Line

A soft flask is one of those pieces of running gear that seems simple but can genuinely improve your running experience. No more sloshing, no more bulky empties, no more awkward drinking.

Start here:

• For your first soft flask, get a 500ml with a 42mm wide mouth
• Practice filling, drinking, and cleaning before race day
• Store dry and open to prevent mould
• Replace every 1-2 years or when you notice wear

Once you've run with a soft flask that fits your vest perfectly, compresses as you drink, and lets you hydrate without breaking stride, you'll wonder why you ever tolerated that sloshing hard bottle.

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Recommended Gear

Mini T SoftFlask (250ml)

Perfect for short runs, gel storage, or minimalist hydration. Wide mouth for easy filling, BPA-free TPU construction, and a bite valve for easy drinking.

Shop the Mini T SoftFlask →

OG SoftFlask (500ml)

The workhorse for longer runs and vest-based hydration. 42mm wide mouth fits ice cubes and electrolyte tablets, collapsible design eliminates sloshing, fits most hydration vest pockets.

Shop the OG SoftFlask →

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References & Further Reading

• Trail & Kale: "How To Carry Water When Running: Using Soft Flasks"
• Outside Online: "The Best Ways to Carry Fluids on a Run"
• iRunFar: "Best Water Bottles for Running in 2026"
• HydraPak: "Hydration Quick Guide - Trail Running"
• Geeks On Feet: "Runner's Guide to Soft Flasks"
• Running Warehouse: "Four Reasons You Should Use a Soft Flask for Your Energy Gel"
• Runner's World: "Collapsible Water Bottles for Running"
• Orange Mud: "Soft Flask Care Guide"
• GearLab: "How to Choose a Hydration Pack for Running"
• TrailRunnersDog: "Bladder, Softflask or Hard Bottle? Pros and Cons"