If you have a sleeping bag that by default does not come with a compression sack (for example, our Jazz™ Synthetic Sleeping Bag) or you want to upgrade to a dry bag, it's easy to figure out what size you need!
The rule of thumb is: Take the compressed volume of your sleeping bag and add 5 liters. For instance, our Forte™ Men's Endless Promise® Synthetic Sleeping Bag in 20° Regular lists a compressed volume of 5.3 liters. Add 5 liters to that and a compression sack size of at least 10.3 liters is needed. This size estimate will allow enough room in the bag’s main compartment (without it being too tight) so you can successfully get the end cap on. Note that many compression sacks on the market do not come in half liter sizes;in this case, we'd round up to 11 or 12 liters.
NOTE: If you are MISSING the compression sack for your newly purchased bag, please reach out to customer support so we can help you with a spare part, no need to buy a new one!