Calculate how much disk-space would have been used

Taken from https://stackoverflow.com/questions/13418688/use-pipe-with-du-to-compute-size-of-stdin

You can pipe it to wc -c to count the number of bytes that goes through the pipeline.

Of course, this is just the raw bytes, and have nothing to do with sector size etc, so take it with a grain of salt...

The command pv is perfect for this.

mysqldump | pv -b > /dev/null

I think the above will give you the right command you want, it may need some adjusting such as pv -b | > /dev/null as I can't test right now

-b gives you a value in bytes.