Nathan 18oz Soft Flask — 18 oz bite-valve collapsible hydration bottle for running and vests

Nathan's 18oz Soft Flask with a bite-activated valve is a pliable, refillable water bottle designed to tuck into front hydration-vest pockets for easy drinking on the move.

This soft flask provides 18 ounces (535 mL) of capacity in a flexible, pack-friendly shape that suits running, hiking, and other endurance activities. Built to work with Nathan hydration vests but versatile enough for many pack brands and stash pockets, the flask focuses on low weight, easy access, and quick drinking through a bite top that prevents spills when not in use. It folds flat as you drink to save space and is straightforward to rinse or clean between outings.

How does the Nathan 18oz Soft Flask fit into my hydration vest?

The flask's slim, compressible profile is sized to slide into front vest pockets or a back stash compartment without bulking up. Its flexible walls conform to pocket shapes, making it comfortable against your chest while running or moving quickly. It also works as a supplemental bottle if your vest includes a larger reservoir.

• 18 oz / 535 mL capacity tailored for mid-run refills
• Slim, soft-sided design fits most front vest pockets
• Collapses as liquid is consumed to reduce bounce
• Compatible with Nathan hydration vests and many other brands

What makes the bite top valve practical on trails and routes?

The bite valve opens only when compressed by your lips or teeth, which helps stop leaks and minimizes accidental flow when the bottle is jostled. It’s an intuitive on-demand drinking method so you can sip while moving without fiddling with caps. The valve also keeps dirt and debris out of the drinking tip when stowed.

• Bite-activated valve prevents unwanted leakage
• Quick sip access without removing the flask from a pocket
• Protects the drinking surface when tucked away
• Simple one-piece valve reduces moving parts to maintain or replace

How do I clean and maintain the soft flask?

Cleaning is straightforward because the soft material rinses out easily and the wide opening permits access to the interior for scrubbing or drying. For deeper cleaning, use warm soapy water and allow the flask to air dry fully with the cap removed. Periodically check the bite valve for wear and replace if it shows cracking or reduced function.

• Rinse with warm water after use; soap for odor removal
• Open completely to air dry and prevent mildew
• Inspect bite valve and replace if deteriorated
• Lightweight construction tolerates routine field cleaning

Will this flask leak in my pack or on hot days?

The bite valve design greatly reduces leakage when the bottle is closed and stowed, but proper seating of the bite top and avoiding excessive squeezing help minimize any risk. Because the flask is soft-sided, it should be kept away from sharp objects inside a pack; heat can expand liquids, so leave some headspace when filling on hot days.

• Bite-top stays closed when not engaged, limiting drips
• Do not overfill—leave slight headspace for thermal expansion
• Store away from sharp gear to avoid punctures
• Check cap seating before placing inside pockets or packs

Who is this for? The Nathan 18oz Soft Flask with Bite Top is aimed at trail runners, fastpackers, long-distance walkers, and anyone who needs a lightweight, compact secondary water source that fits in vest pockets. If you want an easy-to-access sip while moving, prefer a collapsible bottle to a rigid bottle, and value minimal added weight in your loadout, this flask is a practical choice. It’s also useful as an extra reserve for hikers and cyclists who use packs without fixed reservoirs.