The hydration backpack included 2 Liter hydration bladder for men women kids. The perfect size offer a nice balance of reasonable weight and bulk while providing a sufficient quantity of water. This hydration pack is a versatile adventure life style gear for adventurers everywhere. The Hydration packs are designed specifically for road cycling /running/ Hiking ,So that's Compact and low-profile so they feel light and stable on your back and won’t create a ton of wind resistance ,keep your body functioning at peak performance whether you're training for a marathon or cycling.