Use a flexible tape measure to wrap around the outside of your pack a few inches above the base—typically just above the hipbelt or taper. Make sure exterior pockets are flattened and straps or cords are out of the way for an accurate measurement. It can be easier to pre-stuff the pack and measure the outside of your pack, or you can measure the inside of an empty pack.

36" or under? Medium
Over 36" diameter? Large