The distance between two cities is calculated with an online distance calculator. Timeanddate.com offers a distance calculator that calculates the greatest theoretical air distance between any two cities in kilometers, miles and nautical miles.
In addition to the air distance, Timeanddate.com provides a map of the shortest path between cities, latitude and longitude of the cities, and local time comparisons.
Distancetofrom.net provides less overall detail than Timeanddate.com, but allows users to select driving or air distance. Driving distance uses mapped roads to calculate distance, instead of theoretical air distance, and is only available for two cities that are connected by land.