The calculated distance (air line) between Zürich (Kreis 9) / Altstetten and Rapperswil is approximately 19 miles respectively 31 kilometers.
By car or train, the actual journey to Rapperswil is certainly longer, as only the direct route (as the crow flies) between Zürich (Kreis 9) / Altstetten and Rapperswil has been calculated here.
Air line (approximately)
31 kilometers
17 nautical miles