How to get the summarized sizes of directories and their subdirectories?

This does what you're looking for:

du -sh /*

What this means:

-s to give only the total for each command line argument.
-h for human-readable suffixes like M for megabytes and G for gigabytes (optional).
/* simply expands to all directories (and files) in /.

Note: dotfiles are not included; run shopt -s dotglob to include those too.

Also useful is sorting by size:

du -sh /* | sort -h

Here:

I often need to find the biggest directories, so to get a sorted list containing the 20 biggest dirs I do this:

du -m /some/path | sort -nr | head -n 20

In this case the sizes will be reported in megabytes.

I like to use Ncdu for that, you can use the cursor to navigate and drill down through the directory structure it works really well.

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