The calculated distance (air line) between Mainz and Nieder-Ingelheim is approximately 9 miles respectively 15 kilometers.
By car or train, the actual journey to Nieder-Ingelheim is certainly longer, as only the direct route (as the crow flies) between Mainz and Nieder-Ingelheim has been calculated here.
Air line (approximately)
15 kilometers
8 nautical miles