Hydrapak Pocketflask 500mlTop to bottom, this is Hydrapak's lowest profile flask. It slips right into the snuggest run belts and leanest vest pockets. The new 28mm locking cap easily opens and closes with a touch of a thumb, preventing leaks and drips while in motion. Made for minimal distraction, the innovative pinch welded bottom gives it a tapered profile and the low diameter cap makes it comfortable to carry no pokes or irritation.