The calculated distance (air line) between Watermael-Boitsfort and Leuven is approximately 14 miles respectively 22 kilometers.
By car or train, the actual journey to Leuven is certainly longer, as only the direct route (as the crow flies) between Watermael-Boitsfort and Leuven has been calculated here.
Air line (approximately)
22 kilometers
12 nautical miles