Choosing the Right Replacement Cap for Your HydraPak Soft Flask
By HydraPak | Performance Hydration Reservoirs, Soft Flasks ... | Published: 2026-10-19
Category: Buying Guides
Learn how to pick between the 42mm Drink Tube Cap and the 28mm SoftFlask Cap based on your bottle and activity.
Quick answer
Choose the 42mm Drink Tube Cap if you want a drink tube for hands-free hydration on bottles with a 42mm opening. Choose the 28mm SoftFlask Cap for a compact, bite-valve replacement on smaller soft flasks.
Why cap size matters
HydraPak uses two common thread sizes for their soft flasks: 28mm and 42mm. Picking the wrong cap means it won't thread on, so you need to match the cap to your bottle's opening. The cap size is often printed on the bottle or listed in the product specs.
Drink tube vs. bite valve trade-offs
The 42mm Drink Tube Cap adds a flexible drink tube that lets you sip without tilting your bottle—ideal for running or biking. The trade-off is a slightly bulkier setup and an extra tube to manage. The 28mm SoftFlask Cap uses a direct soft bite valve, which is more compact and easier to stow but requires you to tilt the flask to drink.
How to choose: decision rules
- Check your bottle opening: Look for the thread size printed on your soft flask or in its product manual. If it says 42mm, you need the Drink Tube Cap. If it says 28mm, you need the SoftFlask Cap.
- Match the cap to your activity: For running or cycling where you want to drink without stopping, choose the 42mm Drink Tube Cap (if your bottle fits). For hiking or casual use where compactness matters more, choose the 28mm SoftFlask Cap.
- Consider the drinking style: If you prefer a straw-like tube, go with the 42mm Drink Tube Cap. If you prefer a direct bite valve that's simpler to clean, go with the 28mm SoftFlask Cap. If your bottle has a 42mm opening but you want a bite valve instead of a tube, check if a flat cap is sold separately for that size.
Cap options at a glance
- Drink Tube Cap - 42mm: Fits all HydraPak products with a 42mm opening including the SoftFlask 500ml and 750ml, Stash 750ml bottle, UltraFlasks, and Seeker water storage containers. If you own a 42mm bottle, this cap gives you a drink tube for hands-free sipping without needing a new bottle.
Drink Tube Cap - 42mm - SoftFlask™ Cap - 28mm: Fits HydraPak SoftFlasks with a 28mm opening and features a soft bite valve for drinking on the move. This is the direct replacement for smaller soft flasks, offering a lightweight, compact drinking solution.
SoftFlask™ Cap - 28mm
Which cap fits you?
Best for
- Runners and cyclists who want hands-free hydration with a drink tube
- Owners of 42mm HydraPak bottles like the SoftFlask 500ml/750ml, Stash, or UltraFlask
Not for
- Owners of 28mm soft flasks (the 42mm cap will not thread on)
- Users who want the most compact, no-tube setup (choose the 28mm cap instead)
