How do you enter a custom crop ratio like 21:9, 18:9?

I am trying to crop a extra wide wall paper out of a photo. The widest option is 16:9 while I need 18:9 and 21:9. I know there is a custom option, but it is really hard to determine whether your custom crop is really 21:9 or 18:9, you kind of just have to guess. Is there a way to enter those specific crop dimension?