The sizes given in the ad didn't give out a 20L capacity either, but I thought the backpack material was flexible so a 20L was possible. The given sizes are close to the real ones, but instead of the calculated 13L, the size of the backpack is around 12L in reality. (measured from seam to seam: 42 cm * 24 cm * 12 cm)
The shipment of item was very fast. The product is as described and is a good one. Thanks!!