Ultimate Direction SJ Ultra Running Hydration Vest Cuben Fiber, Hex Mesh and Power Mesh Imported Designed and endorsed by Scott Jurek and updated for 2014 The best weight to volume ratio of any pack of any kind Weight: 11 oz. (16.5 oz. with bottles),7L (427 sq. Inch) Total Volume Comes with two 20 oz Ultimate Direction water bottles Sizing At Chest (Unisex): SM: 23 to 33 in. / 58 to 84 cm. Measure wearing the clothes you intend to wear. A vest full of gear will fit smaller.