An Ubuntu installation may vary hugely in size, depending on the installed packages. Is there any quick way to check how mutch disk the current installation uses.

I want to know, because I plan to build a new PC, with a smaller and fast SSD for / and a large mechanical drive for /home (I have need for a large home directory), to decresase boot time. To solve this problem I would like to know the size of my current installation, so I know how large the system file system needs to be for my use.