Your backpack's size and the number of extra items you carry on your backpack's exterior will determine how big your rain cover should be. Just like with your backpack, rain covers are rated in liters. It's simple to equal the liter size if you don't have any additional equipment connected to the exterior of your bag. You should go a size bigger if you plan to carry items on the outside of your bag. You may have to size up two sizes to achieve the length you need if you tie on sleeping bags. It is important to test the sizes to make sure you get the right fit for your backpack and what you intend to take with you.