Caliber to Millimeter Converter

About this calculator

Firearm caliber traditionally names a bore's diameter in hundredths or thousandths of an inch (e.g. .308, .45), a naming convention inherited from American and British gunmaking, while most of the rest of the world specifies bore diameter directly in millimeters (e.g. 7.62mm, 9mm). The conversion is straightforward unit math: millimeters = caliber (in inches) × 25.4 — so .308 caliber corresponds to 7.82mm, close to (but distinct from) the similarly-named 7.62mm NATO round due to how each naming tradition rounds its numbers.

This converter takes a caliber in inches and returns the equivalent bore diameter in millimeters. Gunsmiths and reloaders use it to cross-reference imperial and metric cartridge and barrel specifications, and firearms hobbyists and collectors use it when comparing rounds named under different regional conventions that may otherwise look unrelated.