Bullet Caliber Converter
Convert between bullet caliber measurements in inches, millimeters, and common cartridge designations used in firearms.
Type a caliber in inches or millimeters — the other updates instantly. Or select a common cartridge below.
.45 ACP = 11.48 mm
.308 Win = 7.82 mm
Understanding Bullet Caliber
Caliber refers to the internal diameter of a gun barrel, and by extension, the diameter of the projectile (bullet) it fires. Caliber can be expressed in inches (common in the United States) or millimeters (common in Europe and in military designations). Understanding the relationship between these measurements helps when comparing ammunition types across different naming conventions.
Basic Conversion Formulas:
Millimeters = Inches × 25.4Inches = Millimeters / 25.4Caliber (hundredths of an inch) = Inches × 100
For example, a .45 caliber bullet has a diameter of 0.45 inches, which equals 11.43 mm.
Common Caliber Cross-Reference Table:
| Designation | Diameter (in) | Diameter (mm) | Typical Use |
|---|---|---|---|
| .22 LR | 0.223 | 5.66 | Target shooting, small game |
| .223 Rem / 5.56 NATO | 0.224 | 5.69 | Sporting rifles, military |
| 9mm Luger | 0.355 | 9.01 | Handguns, most popular worldwide |
| .357 Magnum | 0.357 | 9.07 | Revolvers |
| .40 S&W | 0.400 | 10.16 | Law enforcement handguns |
| .45 ACP | 0.452 | 11.48 | Handguns |
| 7.62×39mm | 0.312 | 7.92 | AK-pattern rifles |
| .308 Win / 7.62 NATO | 0.308 | 7.82 | Hunting, military rifles |
| .30-06 Springfield | 0.308 | 7.82 | Hunting, military (historic) |
| .50 BMG | 0.510 | 12.95 | Heavy machine guns |
| 6.5 Creedmoor | 0.264 | 6.71 | Long-range precision |
| .300 Win Mag | 0.308 | 7.82 | Long-range hunting |
Important Notes About Caliber Naming:
The name of a cartridge does not always match its exact bullet diameter. For example, the .38 Special actually uses a bullet that measures 0.357 inches in diameter, not 0.380 inches. Similarly, the 7.62×51mm NATO and .308 Winchester use nearly identical bullet diameters but have slightly different chamber pressures and case dimensions.
The second number in metric designations like “7.62×39mm” refers to the cartridge case length (39mm), not the bullet diameter. This is a common source of confusion for newcomers.
Practical Tips:
- Always verify the exact cartridge designation, not just the caliber, before purchasing ammunition.
- Metric and imperial designations are sometimes used interchangeably for the same cartridge (for example, 9mm and .355).
- Bullet weight is measured in grains (1 grain = 0.0648 grams), which is a separate measurement from the caliber diameter.