ConverterHub

Caliber to Millimeter Converter

Enter a caliber in inches to convert it to millimeters.

How it is calculated

The tool treats the caliber as a diameter in inches and multiplies it by 25.4, the exact number of millimetres in an inch: mm = caliber × 25.4. Enter the caliber as a decimal fraction of an inch, so .45 is typed 0.45 (or .45) and .308 is typed 0.308. The result is shown in millimetres with two decimals, and your value is echoed back in inches with three decimals. The number must be above zero. The tool does not guess hundredths: typing 45 for a .45 caliber is read as 45 inches and returns 1,143.00 mm. It is a pure unit conversion of the number you give. Cartridge names do not always match the real measured dimension, so the result is the arithmetic conversion of the inch name, not a guarantee of the metric name of a cartridge.

Worked example

A .308 caliber is entered as 0.308 inch. Multiply by 25.4: 0.308 × 25.4 = 7.8232, which the tool shows as 7.82 mm. The inch value is echoed as 0.308 in. A .45 caliber (0.45 in) converts the same way to 11.43 mm, and a .223 caliber to 5.66 mm.

Try this example →

Common mistakes

  • Typing the caliber without the decimal point (308 or 45). The field is in inches, so 308 means 308 inches and the result is hundreds of times too large.
  • Expecting the result to equal the metric name of the cartridge. A cartridge called 7.62 mm and one called .308 are close in class, but the names come from different naming traditions, so the converted figure of 7.82 mm will not match 7.62.
  • Using the tool for shotgun gauge (12 gauge, 20 gauge). Gauge is a different system based on the weight of a lead ball, not a diameter in inches.
  • Confusing bore diameter with bullet diameter. Names such as .308 are nominal, and the diameter actually measured on the bullet or in the barrel can differ slightly.
  • Rounding to one decimal in a tolerance-sensitive task such as reloading or machining. Use the full conversion and measured values for actual work.

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, yet the 7.62mm NATO round fires essentially the same bullet: the NATO name gives the bore's land diameter, while .308 gives the groove (bullet) diameter.

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.

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