Travelling from Dubrovnik to Frankfurt by bus is possible, but the bus is not an “officially” scheduled direct bus. What does that mean? That means you have to first take a bus from Dubrovnik to Split (
Schedule here) and then catch the
bus to Frankfurt from there. If you want more travel options you should
take a bus from Dubrovnik to Zagreb and continue to Frankfurt from there one, schedule for the route
you can see here. Total travel time by bus is about 24 hours.
By car: the travel time for the 1500 km is about 15-16 hours, keep in mind that you have toll cost on the Croatian highway, and cost for a vignette both in Slovenia and Austria.
By Plane: There is direct plane most of the year
By train: no trains from Dubrovnik
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