When you discuss a bullet's caliber, you're referring to its size -- specifically the diameter of the slug, the portion of the bullet that leaves the gun when you fire it. A bullet's caliber is an expression of how large its diameter is. The larger the caliber, the larger the diameter of the slug. In the U.S., caliber is expressed in inches, but caliber can also be expressed in millimeters. Converting one to the other is not hard.
Making the Conversion
Caliber in the U.S. measures the diameter of a bullet in fractions of an inch. A .22 caliber bullet is 0.22 inch in diameter and a .45 caliber bullet is 0.45 inch in diameter. The diameter of a slug can also be expressed in metric measurements -- 9 mm is a common bullet size, for example. One inch equals 25.4 mm, so to convert from caliber measured in inches to millimeters, multiply by 25.4. For example, a .44 caliber bullet is .44 X 25.4 = 11.18 mm. A quick way to estimate the caliber in inches of a metric caliber is to multiply by four. Thus, a 9 mm bullet is approximately .36 caliber. Dividing the inch caliper by 4 and tossing out two decimal places will yield you a quick approximation of the bullet's caliper in millimeters, so .36 caliber divided by 4 = .09; toss out the decimal places = 9 mm.