The caliber of a weapon ("calibre" in British English) is the internal diameter of the barrel (the bore). In firearm texts, it usually appears with relation to Imperial caliber measurements.

In traditional systems, the caliber of a gun was expressed in inches, with all figures after hundredths disregarded. For example: a .308 inch diameter bullet is referred to as 30 caliber or a .458 in inch diameter bullet is referred to as 45 caliber. In some cases for the purposes of advertising, a caliber is round to some number that sounds good; a .429 inch diameter bullet is referred to as 44 caliber not 42 or 43.

Caliber can refer to:

• The diameter of a bullet or other projectile, in any system of measurement.
• The diameter of the bore of a gun usually expressed in hundredths or thousandths of an inch and typically written as a decimal fraction
• The length of a gun barrel expressed as the number of times longer than the diameter of its bore it is. For example, the "16 inch 50 caliber" gun of an Iowa-class battleship is 50 times longer than its 16 inch bore, ie 800 inches or 66ft 8in.

Military Usage

The American Military uses SI (metric system) to classify weapon cartridges. For example: 7.62x51mm NATO and 5.56x45mm NATO.