DISCLAIMER - by now the conversion to Nashville Number System is implemented roughly but there are still some things missing or incomplete.
If you are new to the Nashville Number System you can find detailed information at wikipedia or have a look some of the videos at YouTube.
See these examples:
Within Chord Sheet Maker Online you do not actually write the numbers, but can transform the resulting chart to show numbers instead of regular letters.
There are three steps required to make a Nashville number chart ...
See some more nns examples.