| What it is | Collapsible soft running bottle |
| Material | Soft TPU |
| Benefit | Collapses as you drink (no slosh); packable |
| Carried in | Hydration vest front pockets |
A soft flask is a collapsible, flexible water bottle — usually made of soft TPU — that fits in the front pockets of a hydration vest and collapses as you drink, which reduces sloshing and saves space. Lightweight and packable, soft flasks (often with bite valves or wide caps for mixing) are the standard way trail runners carry water and electrolyte drinks up front.
Slosh-free hydration
Rides in a hydration vest and is ideal for mixing electrolytes; an alternative to a reservoir-based hydration pack.
Frequently asked questions
What is a soft flask?
A soft flask is a flexible, collapsible bottle made of soft plastic (typically TPU) used by runners to carry water or sports drink, usually in the front pockets of a hydration vest. Unlike a rigid bottle, it squishes down as you empty it, so it collapses to take less space and reduces the sloshing of liquid while running.
Why use a soft flask instead of a hard bottle?
Soft flasks collapse as you drink, eliminating the slosh and bounce of a half-full rigid bottle and packing down small when empty. They sit comfortably in vest pockets close to your chest for easy sipping. Hard bottles are more durable and easier to clean, but soft flasks are favored in trail running for comfort and packability.
How do you care for soft flasks?
Rinse them after each use, especially after using drink mixes, and clean them periodically with warm soapy water and a bottle brush or specialized brush, then air dry fully (often inverted or propped open) to prevent mold. Avoid hot liquids unless the flask is rated for them, and replace flasks if they develop leaks or persistent odor.
Sources
- Running hydration — American Trail Running Association
- Hydration gear — The Mountaineers