Amazon.com : CamelBak Ultra Running Hydration Belt 17oz 。like new adult small /medium : S/M (28–32 in / 71–81 cm) A form-fitting, ultra-lightweight option for trail runners or road runners, the CamelBak Ultra Belt offers streamlined storage, a comfortable bounce-free ride, and easy access to the included 17 oz hydration flask. 17oz Quick Stow flask is lightweight, collapsible and features a one-piece silicone bite valve for rapid hydration 3D Micro Mesh offers multi-directional air flow and cushioning to maximize comfort and breathability Secure phone pocket is conveniently located with a zipper closure for secured quick access Reflectivity for early morning or late-night runs Trekking pole carry lets you easily remove or replace poles while running Ultralight materials provide breathability, comfort, and durability Organize run essentials with pockets specifically designed for Quick Stow flasks