The calculated distance (air line) between Sint-Pieters-Leeuw and Leuven is approximately 21 miles respectively 34 kilometers.
By car or train, the actual journey to Leuven is certainly longer, as only the direct route (as the crow flies) between Sint-Pieters-Leeuw and Leuven has been calculated here.
Air line (approximately)
34 kilometers
18 nautical miles