How To Convert Caliber To MM (And Vice Versa)

Ever wonder how to convert caliber to mm and vice versa?

Caliber (or calibre in British English) is the approximate internal diameter of the barrel on a particular firearm or the diameter of a bullet and is usually expressed in inches or millimeters. In addition to using different units of measurement, different countries sometimes use different methods of measuring caliber. Because of these factors, deciphering the actual diameter of a bullet or the bore size of a firearm can be bewildering.

However, I’m attempting to clear up some of the confusion in this article by explaining exactly how to convert caliber to mm and mm to caliber.

Note: the system described below applies to small arms cartridges. Big naval guns and other artillery pieces use a different naming system where caliber denotes the length of the gun barrel (which directly affects muzzle velocity) instead of the bore diameter. Scroll down to the comments for a more detailed description of that system.

Keep reading to learn more about caliber vs mm as it relates to rifle, handgun, and rimfire cartridges.

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Cartridge sizes are described using either the metric (millimeter) or imperial (inch) systems. For instance, the 5.56x45mm cartridge uses the metric system of measurement while the .300 Winchester Magnum uses the imperial system. When a person is talking about a twenty two caliber or a thirty caliber rifle, they are referring to a cartridge using the imperial system. In this case, it is also correct to write the cartridge caliber out as “.22 caliber” or “.30 caliber”, which is the approximate diameter of the cartridge in inches.

Switching between millimeters and inches is relatively straightforward. There are 10 millimeters (mm) in 1 centimeter (cm) and 2.54 centimeters in 1 inch (in). Therefore, multiply a bullet or bore caliber given in inches by 25.4 in order to convert it to millimeters. The opposite is also true: divide a given caliber expressed in millimeters by 25.4 in order to convert it to inches.

For example, .30″ and 7.62mm are equivalent (.3 multiplied by 25.4 is 7.62).

Using this simple method, you’ll see that a 7.62mm and a 5.56mm cartridge are approximately .30 caliber and .22 caliber respectively. If you are looking for an approximation of the bore size or bullet diameter, that is all you need to know.

However, if you want more specific information on the exact size of the bullet or bore, then you’ll need to dig a little deeper.

Unfortunately, it seems like the deeper you dig, the more complicated and confusing things seem to get. One thing that makes this exercise even more complicated is the seemingly arbitrary naming conventions that many manufacturers use for their cartridges.

Sometimes a cartridge fires bullets pretty close to what you’d think. For instance, the .45 ACP fires bullets .451″ in diameter, the .45 Colt fires bullets .452″ in diameter, the .40 Smith and Wesson fires bullets .40″ in diameter, the 9mm Luger (also known as the 9mm Parabellum) fires bullets .355″ in diameter, and the .357 Magnum fires bullets .357″ in diameter.

However, that’s not always the case. For example, a .44 Remington Magnum (also known as the .44 Magnum) handgun actually fires bullets .429″ in diameter, the .38 Special handgun cartridge fires bullets .357″ in diameter, and the .380 ACP fires bullets .355″ in diameter.

If that’s not enough to make your head spin, then consider this: a rifle chambered in .30-06 Springfield (7.62x63mm) has a .30 caliber (7.62mm) bore diameter. However, it actually shoots .308″ (7.82mm) bullets.

Why the difference?

The caliber of a rifled barrel is measured one of two ways: either by measuring the distance between opposing lands (high points in the rifling) or opposing grooves (low points in the rifling) in the barrel. As you can see in the photo below, in a given barrel, the distance between the grooves will be larger than the distance between the lands.

How To Convert Caliber To MM bore
Hunter-Ed.com

The .308 Winchester is an American designed rifle cartridge and most (but not all) centerfire cartridges originating in the United States use groove measurements when measuring bullet diameter. So, even though the bore of the rifle measured across the lands is .30″, the actual bullet diameter is .308″ because the bullet must closely match the groove diameter (.308″ in the .308 Winchester as well as the .30 Carbine, .30-06 Springfield, and .300 Win Mag) in order to form a good seal in the barrel.

Additionally, older cartridges from the black powder era have a slightly different naming convention. These cartridges were named for the caliber  was also known as the 45-70-405 got its name from the fact that that it fired a 405 grain .45 caliber bullet propelled by 70 grains of black powder.

To make things even more confusing, most cartridges that were designed in other countries use land measurements.

Take the 7.62x39mm cartridge used by the AK-47 as an example. At first glance, this cartridge would appear to use a bullet with the same diameter as the .30-06 Springfield since they are both 7.62mm cartridges.

However, the Soviet-designed 7.62x39mm cartridge measures bullet diameter using land measurements instead of groove measurements. Because of this, the groove measurement (and the actual diameter of the bullet) is .312″ (7.92mm) instead of .308″ (7.62mm), a difference of .004″.

For some applications, such as purchasing a bore snake for your gun cleaning kit, that difference is not significant at all. Since they are usually sold to fit a range of bore sizes, a .30 caliber bore snake will work equally well on rifles chambered in .30-06 Springfield, .303 British, and 7.62x39mm (among others).

However, a few hundredths or thousands of an inch is a significant difference when it comes to actually shooting a bullet. Attempting to shoot a bullet just a few thousandths of an inch too large for the barrel can be extremely dangerous. By the same token, shooting a bullet a few thousands of an inch too small for a barrel isn’t dangerous, but it probably won’t shoot very accurately.

While it may seem obvious, the way to avoid problems with improperly sized bullets is to shoot the right ammunition in your guns. If you’re hand loading, you should use a reputable reloading manual to ensure you’re 100% certain you’re using the right sized bullet when hand loading for a particular cartridge. It should also go without saying, but I’ll say it anyway: you should only shoot ammo in the caliber marked on the receiver or barrel of the firearm you’re using. Don’t ever try to mix and match different cartridges, even if you think that they are actually the same size.

Finally, what about metric cartridges like the 7.62x51mm NATO? When you see that nomenclature, the first number describes the diameter of the bullet and the second number describes the length of the cartridge case used (also known as case length). So, the 7.62x51mm NATO has a 51mm long case length and fires a 7.62 mm caliber bullet.

So, what the heck does all this mean to the average shooter?

For a pretty good estimate of the size of a given cartridge, you can use the multiply/divide by 25.4 method to convert between inches and millimeters. You’ll get a rough approximation of the size of the cartridge, which should be enough to make a generalized comparison between it and another cartridge as well as for applications that do not need to be extremely precise, like purchasing a bore snake. When you need the exact size of the bore or bullet diameter though, the only guaranteed ways of finding that information out are to either measure it yourself or to consult a trustworthy reloading manual.

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NEXT: 223/5.56 vs 7.62×39: WHAT YOU NEED TO KNOW

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8 thoughts on “How To Convert Caliber To MM (And Vice Versa)”

  1. That is very interesting to someone new to the gun world. Although it does not completely answer my question, it does bring light on the confusion with weapon sizes. Thanks

    Reply
  2. This may be slightly off topic , but it’s this question that got me to this site .
    A 5” Naval gun is a 125mm . How do you determine its caliber ? They do range .

    Reply
    • Al,
      Artillery pieces like the 5″ naval gun use their own naming system that’s pretty different from what you see with small arms cartridges. Big guns like that normally have a designation of 5″/38 or 5″/L38. This means that the gun has a bore diameter (caliber) of 5″ (127mm) measured from land to land in the rifling. The second number denotes the length of the barrel from breech to muzzle measured in calibers. So, a 5″/38 gun has a 190″ long barrel (5×38=190) and a 5″/54 gun would have a 270″ barrel (5×54=270). Both guns have the same bore diameter of 5″.
      Hope this helps,
      John

      Reply
  3. Could you do more examples? And if I understand what you’re saying then there is a small difference between a.22 long rifle round and a 5.56?? Can that be? Sorry I’ve never seen a 5.56 round.

    Reply
    • The .22 Long Rifle and 5.56×45 both use the same diameter bullet. However, there is a big difference between those two cartridges otherwise. Check out this article to see a photo of a .22 Long Rifle cartridge compared to a .223 Remington (which is very similar to a 5.56x45mm). Hope this helps!
      John

      Reply

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