Sea to Summit Ultra-Sil Pack Cover (Medium / Gray) Made of 30D Diamond Ripstop silicone impregnated Cordura, so its waterproof finish cant peel or wear off over time Siliconized Cordura is about one third the weight of traditional coated nylon This pack cover uses a no seam construction for complete waterproofness Has a mid-back keeper strap that helps secure it to the pack and keep the sides from catching wind (except size XS) XS: 15-30L/900-1800 cu in, 2.0 oz; SM: 30-50L/1800-3000 cu in, 3.0 oz; MD: 50-70L/3000-4300 cu in, 4.0oz; LG: 70-95L/4300-5800 cu in, 4.6oz