Apple - How can I convert a DMG to ISO on Mac OS X (preferably for free)?

You can use hdiutil to convert from .dmg to .iso directly:

$ hdiutil makehybrid -iso -joliet -o [filename].iso [filename].dmg

Renaming .cdr to .iso is bad advice! An .iso has a slightly different format that allows PC users to play the DVD as well. A renamed file will work on macOS, but NOT on a PC.

If you have a .cdr and want to convert it to an .iso, execute:

$ hdiutil makehybrid -iso -joliet -o [filename].iso [filename].cdr

Yes, Disk Utility can do this. Use Convert then select your dmg file. In the Save As dialog that follows, select DVD/CD master. Disk Utility will insist on saving the new ISO as a .cdr file, but it is really an ISO. You can rename it to .iso in the Finder, if you like.

At http://osxdaily.com/2012/07/18/convert-dmg-to-cdr-or-iso-with-disk-utility/ there are screen shots that walk through this process.

Would be the same:

hdiutil makehybrid -iso -joliet -o [filename].iso [filename].dmg

For example:

hdiutil makehybrid -iso -joliet -o output_file.iso file_to_convert.dmg