Paris → Leuven

Start with the 07:00 FlixBus bus from Paris to Antwerp Rooseveltplaats, about 4h 20m. Transfer in Antwerp Rooseveltplaats, then take the 13:00 FlixBus bus on to Brussels Boudewijn / Baudouin metro (40 min). Transfer in Brussels Boudewijn / Baudouin metro, then take the 15:30 FlixBus bus on to Brussels-North (20 min). Transfer in Brussels-North, then take the 17:45 FlixBus bus on to Leuven (1h). You arrive in Leuven around 18:45. In total it's roughly EUR 53 over 11h 45m, 3 transfers.