How to calculate the size of the blockchain?
du rounds up file sizes to the next multiple of your filesystem's block size.  It also includes the sizes of directories.  So this will give you a (slight) overestimate.

You really just want to add up the total sizes of all the blk*.dat files.  You can get that with

du -c --apparent-size /home/me/.bitcoin/blocks/blk*.dat


You can divide this number by 1000000000 to get GB, or by 1024*1024*1024 to get GiB (note the distinction).

Note that this will still include "orphan" blocks that you downloaded and stored, but that are not part of the main block chain.  There isn't any easy way to exclude those, as far as I know.