Paris → Leuven

Start with the 07:00 FlixBus bus from Paris to Brussels Zaventem, about 5h 10m. Transfer in Brussels Zaventem, then take the 13:00 FlixBus bus on to Brussels South (30 min). Transfer in Brussels South, then take the 15:30 FlixBus bus on to Antwerp Rooseveltplaats (1h 20m). Transfer in Antwerp Rooseveltplaats, then take the 17:45 FlixBus bus on to Brussels-North (45 min). Transfer in Brussels-North, then take the 20:15 FlixBus bus on to Leuven (1h). You arrive in Leuven around 21:15. In total it's roughly EUR 67 over 14h 15m, 4 transfers.