Note that caliber is only a very rough approximation of the power of a cartridge. Other variables that come into play include bullet weight & shape, powder capacity of the cartridge casing, and peak operating pressures.

In large naval guns (cannons), caliber refers to the length of the barrel, expressed as a multiple of the bore diameter. So the great guns of the Iowa class battleships are properly referred to as 16"/50 caliber as they have a 16 inch diameter bore and are 800 inches (approx 20 meters) long.