To maintain a steady number of ships or vehicles in a military service's inventory; new vehicles or ships must be acquired on a set schedule to replace overage ones.
ServiceLength / FleetSize = IntervalReplacement
ServiceLength = Expected lifetime of the equipment or ship in normal service. For example, Nimitz Class CVNs are expected to have a lifetime of about 50 years.
FleetSize = Equipment pool level that you wish to maintain. For the US CVN fleet, this is 12 ships.
IntervalReplacement = The interval that you need to replace your equipment or ships at. Same timescale as the service length. If you specified service length in years; then the interval of replacement would be in years also.
EXAMPLE: The US Navy commissioned the last of 27 Ticonderoga-class Cruisers in 1994. If we assume a service lifetime of 35 years; what is the interval of replacement?
35 years / 27 Ships = 1.3 years in-between replacements.
Editorial Note: This formula really applies best to large, non-homogenous equipment pools built over long periods of time, such as our aircraft carriers – the eleven CVNs which make up our fleet presently were commissioned between 1961 and 2009, a difference of 47 years; while with the CG-47 class, all the active ships on duty commissioned between September 1986 and July 1994, a span of seven years.