Part of HydrafFlasks ultra-light series, the 500 ml is so light, you might forget you brought it. Weighing in at just over an ounce, its optimal hydration, no matter what kind of runner you are. Its pliable material conforms to most pockets, it shrinks as you drink to minimize water movement and it can be used with or without the tube. Topped with a new feature, the SpeedFill cap, makes aid station refills a breeze. Ultra light weight, ultra useful.