You can't. I came in here to face palm this quote.
Note - rifles and pistols are in calibers. Though they aren't always exact, they are generally the measurement of the diameter in inches. So .45cal would be .45 in" in diameter. Again, this isn't an exact science, as there are often bullets of the same diameter, and they take a slightly different number so you can the difference between the two. Or some times they have a different suffix, such as .357 Magum vs a .357 Sig.
There are metric versions that use the mm measurements of the diameter and usually the case length. So you have 9mm Lugar, for example. The 7.62 x 39mm is the round used in the AK-47, while the 7.62 x 51mm is the NATO round used in a variety of guns, and is equivalent to the .308 Winchester. Yes it gets a bit confusing.
Shot guns come in gauges. The size means how many lead spheres the size of the bore one needs to equal a pound.