Time-of-flight is the period of time between a gun's discharge and the impact of the projectile against land, sea, or other object.
Generally, time-of-flight is considered as that when a shell would strike the water, as in all but the shortest range firings the abbreviation of a projectile's flight by hitting a vessel before it reaches the water is quite small.
Due to aerodynamic drag, the time-of-flight increases more and more quickly as range increases. For instance, a 1,250 pound British 13.5-in shell with a 4 CRH head would take 14.28 seconds to reach 10,000 yards range, 23.8 seconds to reach 15,000 yards and 34.98 seconds to reach 20,000 yards.
By 1908, the Royal Navy was trialling Time-of-Flight Watches to assist spotters in identifying their own shell fall. The watch would be set to the given gun range, started when the guns fired, and would ring a bell at the moment the shells should fall.
- Range Tables for His Majesty's Fleet. Volume I. 1918. Table 55.
- Fire Control, 1908. p. 5.