Caliber to MM Conversion for Rifle Cartridges

Susan Fernandez November 11 2021

Have you ever wondered what the terms ‘caliber’ and ‘mm’ mean when it comes to rifle cartridge caliber? This is a very confusing topic, and we will try our best to make everything as clear as possible.

What is caliber

The internal diameter of the barrel of any firearm or the diameter of a bullet measured in inches and millimeters is called caliber. In simple words, it can be said that the caliber of a rifle is the diameter of its barrel bore.

Caliber is one of the most important specifications when it comes to choosing a new firearm. A firearm with a larger diameter barrel has more room for free spinning and faster movement of the bullet inside it. Handguns usually have calibers ranging from .22 up to 9mm while some can reach even 10mm and 12mm. The barrels of rifles generally range from 7mm to 14.5mm and sometimes even 21mm or 24 mm (for shotguns).

Different calibers are used for different types of hunting rifles. For example, 7mm Remington Magnum is used for big game hunting. Calibers that are larger than .308 Winchester are mostly used for long-range shooting. Some of the most common calibers for rifles include .223 Remington, .30-06 Springfield, 30 Carbine, and 7.62x39 mm Russian.

Types of caliber measuring systems

There are two types of measuring systems.

  1. First is the metric system that uses millimeters as scale
  2. The second is the imperial system that uses inches as the scale.

7mm Remington Magnum, 5.56x45 mm, and 6.5 PRC rounds are examples of the metric system, whereas .458 Winchester magnum, .300 Winchester Magnum, and .270 Winchester are named using the imperial system.

This clarifies that when someone talks about cartridges using the word calibers such as 22 and 30 caliber cartridge, he refers to the imperial system. Here we can also say that 22 caliber means .22 inch diameter and 30 caliber means .30 inch diameter.

How to convert inches (caliber) to millimeters

Calibers are not converted to millimeters when they are too close to one another. For example, there is no difference between 5.56mm NATO and 5.56x45mm, but 7X57 R is different from 7mm Remington Magnum because of their numbers being far apart, that's why they do not share the same dimensions in millimeters which would be 7X57 mm and 7mm RM respectively. That said, how can you convert the caliber of a rifle cartridge to millimeters? There are a few ways to do it:

  • Height/width method using geometry A=p*r² (A = Cross-sectional area; p = width; r= height)

The first method - Using geometry formula A=p*r². To measure caliber using this method, follow these simple steps: Get a ruler Measure the height (h) of the bullet using calipers. It does not have to be an accurate measurement. Measure the width of the bullet (p) using calipers or micrometer. Using a calculator, calculate the area of your cartridge's cross-section by multiplying p by r squared and dividing it by 4π (pi).

  • Caliper method using the cartridge box. Sometimes it can be difficult to find out the caliber of a rifle cartridge, so if someone asks you what is your 7mm rifle's caliber, you will have two fast ways to find out its real size in millimeters.
    The first way is by measuring the width and height of the bullet with a caliper or micrometer. You know that just having these devices are not enough, but knowing how to use them helps you understand the concept better. So let's see how you can do it!

The second method - Using a cartridge box. A useful tip is that you can find caliber information on almost any rifle cartridge on its box or ammunition pack. You don't need to do all those complicated calculations every time! Some manufacturers also write caliber measurements in inches on their boxes, but some use millimeters instead while others provide both imperial and metric readings.

Converting from inches to millimeters is quite straightforward. 10 mm equals 1 cm, and 1 inch equals 2.54 cm, so if we want to convert from inches to mm, we need to multiply the number by 25.4 (10x2.54=25.4). Similarly, if we want to convert mm to inches, we must divide the number by 25.4. To make it easier to understand, let’s see an example.

7.62 mm and 0.3 inches are equivalent to each other how? If we multiply 0.3 inches with 25.4, we get 7.62mm, and if we divide  7.62 by 25.4, we get 0.3 inches.

This method will show that .30 caliber and .22 calibers are 7.62mm and 5.56 mm cartridges, respectively. We can convert many rifle cartridge names to metric names while comparing their bullet diameters using this method. Let’s take a look

Note: The first value shows caliber, 2nd shows metric measurement, and 3rd is the bullet diameter or bore.

  • .22 caliber equals 5.56mm with a bullet diameter of .224”.
  • .270 caliber equals 6.8mm with a bullet diameter of 277”.
  • .30 = 7.62 mm = .308”
  • .366 = 9.3 mm = .366”
  • .416 = 10.6 mm = .416”
  • .50 = 13 mm = .51”

Looking at these values will make your head spin because the names of some popular cartridges don’t seem right. The truth is manufacturers bend the rules to give their cartridges nice-sounding names, which causes all this confusion.

For example, the .308 Winchester is named so because of its bullet diameter that is .308”.  When we convert it to metric, it becomes 7.82mm (.308 x 25.4). However, everyone still calls it by the name 7.62 because it sounds cool. Also if you look closely the .308” is referred to as .30 caliber which is not correct at all.


All of this is the cause of so much confusion because there is no proper rule manufacturers follow when naming the cartridges. All they care about is a memorable and cool name for their product.

This is why it’s always better to buy your ammo and try to find out the caliber of the cartridge you want. At least, by knowing its caliber you will be able to compare it with other cartridges and look up their measurements in meters and millimeters instead of inches.

So we hope this article answered your question and gave you the information you needed. People often get confused when measuring caliber cartridges because they don’t know how to relate their measurements to their size in millimeters.