The calculated distance (air line) between Heppenheim an der Bergstrasse and Mannheim is approximately 13 miles respectively 21 kilometers.
By car or train, the actual journey to Mannheim is certainly longer, as only the direct route (as the crow flies) between Heppenheim an der Bergstrasse and Mannheim has been calculated here.
Air line (approximately)
21 kilometers
11 nautical miles