How to determine directory size of macOS with apfs?

I have a situation on my Mac, where I would like to know the real size of all files combined of a given directory. Below is a screenshot of the Info panel for a selected directory with almost 4TB, which can’t be correct, since I only have a 512 GB drive.

Of course this is caused by the Copy-on-Write feature, so I understand that this is supposed to be 3.7 TB, but it says on disk which is not correct. Is there any way to calculate the real size of a directory?