The calculated distance (air line) between Radlett and Chalfont St Giles is approximately 11 miles respectively 18 kilometers.
By car or train, the actual journey to Chalfont St Giles is certainly longer, as only the direct route (as the crow flies) between Radlett and Chalfont St Giles has been calculated here.
Air line (approximately)
18 kilometers
10 nautical miles