The calculated distance (air line) between Kassel and Bad Sooden-Allendorf is approximately 21 miles respectively 33 kilometers.
By car or train, the actual journey to Bad Sooden-Allendorf is certainly longer, as only the direct route (as the crow flies) between Kassel and Bad Sooden-Allendorf has been calculated here.
Air line (approximately)
33 kilometers
18 nautical miles