The calculated distance (air line) between Watford and Chalfont St Giles is approximately 8 miles respectively 12 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 Watford and Chalfont St Giles has been calculated here.
Air line (approximately)
12 kilometers
7 nautical miles