The Ultra-Sil(TM) Pack Cover ends the debate about whether you should carry one or not. It&';s a tiny and compelling solution and the medium size weighs less a mere 4 ounces. Packs into its own small stuff sack - why wouldn&';t you carry one? Features Made of 30D Ultra-Sil(TM) fabric constructed from CORDURAA(R) yarns for durable, waterproof performance. This pack cover does not use seam construction to ensure that it is completely waterproof Packaged in its own mini siliconized Nylon stuff sack that remains attached to the cover so it can&';t get lost Keeper strap (sizes S-L) pulls the pack cover tight across the pack harness Sold in four sizes to fit daypacks up to large backpacks Tech Specs Best Use: Ultralight Backpacking Material(s): 30 denier Ultra-Sil(TM) nylon Waterproof: Yes Size Guide X-Small: Fits a 15 to 30-liter pack Small: Fits a 30 to 50-liter pack Medium: Fits a 50 to 70-liter pack Large: Fits a 70 to 95-liter pack Pro Tip: If you stow any gear externally on your pack, we would recommend buying an Ultra-Sil Pack Cover a size larger than normal.