Add filename to title

I have filenames like 'Allegro' and titles like 'Symphony No. 1'.
I want add the filename to the title so that the title becomes 'Symphony No. 1, Allegro' but can't figure out how.

Use Action menu.
Format Value
tag: %title%
expression: %_filename%, %title%

Super! Thanks a lot.

I just tried this out and it didn't work. I want to be able to tag the title field of a group of files with their respective filename. Is this still a functionality in mp3tag?

Yes, there is.
Do you want to tag the TITLE with the complete filename or just part of the filename? Better you give an example.

Not my post but same problem. Example. Filename is 25 Or 6 To One.mp3. Nothing in Title tag so I want to add this ( without the mp3) to Title. Thanks for any help you can give

No, it is not.
see the FAQs: