For E cup, you can only know that the difference between the bust and underbust is 20cm. If you want to know your bust size, you also need to know the size of your underbust.
After knowing the underbust size, use the underbust size plus 20cm to get the E cup bust size.
Cup = Bust - Underbust. The difference between AA cup is 7.5cm; the difference between A cup is 10cm; the difference between B cup is 12.5cm. By analogy, for every 2.5cm difference, add one more cup.
Extended information
Source of cup:
The word cup was determined by the French. Because the bra was invented by the French in the 1920s, its former name was Corset. Bra is simply called bra, and this word comes from French.
When the French invented the bra, they also determined the unit of breast or breast size derived from it. The word "cup" is translated quite vividly by Chinese people, because in addition to the straps, a bra only has two cups that cover the chest. The size of the breast is measured by the size of the cup, which is simple and easy to understand.
Reference materials:
Baidu Encyclopedia - Cup