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If you're about to buy a ring online and it's only listed in millimeters or only in centimeters, then this cm to mm ring size converter will come in handy! 💍

This ring size conversion tool will make it easy for you to convert between the two units, determine a ring's diameter from its inner circumference and vice versa, as well as get the standardized ring sizes for different regions.

To help you understand more about ring sizing, you should keep reading to learn:

  • How to convert from centimeters to millimeters for a ring size;

  • The difference between ring diameter vs. circumference; and

  • How to measure the ring size in cm.

How do I convert cm to mm for a ring size?

To convert cm to mm for a ring size:

  1. Measure the inner diameter or inside circumference of the ring in centimeters. For instance, let's say we measure an inside circumference of 6.0 cm.

  2. Multiply this value by the conversion factor 10 mm / 1 cm. In our example:

    6.0 cm × 10 mm / 1 cm = 60.0 mm

  3. You're done! You now have your measurement in millimeters.

You can apply the same steps to convert the inner diameter from centimeters to millimeters.

Ring diameter vs. ring circumference

Maybe you're looking to buy a ring for yourself, someone special, or even your mom, and the store has a table of sizes labeled with measurements like internal diameter and inner circumference, but you're not sure what's the difference and what each measure represents. Don’t worry! We'll quickly explain everything:

Ring diameter or ring circumference refers to the total length of the inner circumference of the ring, the side that goes in direct contact with your finger. If you were to wrap your finger with a string or ribbon, the total length of the string would equal the length of the circumference.

Whereas the internal diameter refers to the diameter of the ring's inner circumference. This is the straight line that runs side to side through the center of the ring.

You can easily convert from diameter to circumference by applying a simple geometry formula that relates both the diameter and the circumference or perimeter:

Perimeter=Diameter×π\rm{Perimeter = Diameter \times \pi}

This way, if you know the diameter of the ring you can find the perimeter by simply multiplying by π (3.1415)\rm{\pi \ (\sim 3.1415)}.

To find the diameter, just divide the perimeter by π\pi:

Diameter=Perimiterπ\rm{Diameter = \cfrac{Perimiter}{\pi}}

How do I measure my ring size in cm?

To measure your ring size in centimeters, follow these steps:

  1. Use a piece of string, ribbon, or strip of paper and wrap it around the thickest part of your finger.

  2. Mark the point where the string overlaps.

  3. Using a centimeter ruler, measure the length of the string.

  4. This is your ring's inner circumference measured in centimeters.

Illustration of how to measure a finger using a strip of paper and a ruler.

How to use the cm to mm ring size converter

With this cm to mm ring size converter, you can quickly determine the equivalence between centimeters and millimeters for ring measurements. Plus, you can also find out the ring size for a given measurement. Here's how to use the tool:

  1. In the Measurements section of the calculator, select either the Inside diameter or Inside circumference in centimeters (cm).

  2. Immediately, the calculator will display the measurements in millimeters (mm).

  3. In the section below, Ring size equivalents, you'll find the standardized size for different regions according to your chosen measurements.

  4. Yes, that's it! Could it get any easier? 😉

🙋 Click on the Advanced mode of the calculator to see the results in inches.

More ring size tools!

Did you enjoy using this cm-to-mm ring size converter? Then why not keep the fun going with even more of our other ring size conversion tools 💍 You might find your perfect fit:


What's the ring diameter for a 65.5 mm circumference ring?

It's 20.85 mm or 2.08 cm. Let's see how we got these values:

  1. To calculate the ring's diameter, we use the circumference or perimeter formula in terms of the inner diameter D:

    Perimeter = Dπ

  2. Let's solve for the diameter:

    D = Perimeter / π

  3. Substitute our circumference value:

    D = 65.5 mm / π
    D = 20.85 mm

  4. That's the ring's circumference in millimeters.

  5. To convert to centimeters, multiply the above by the mm-to-cm conversion factor:

    20.85 mm × 1 cm / 10 mm = 2.08 cm

Gabriela Diaz
Inside diameter
Inside circumference
Ring size equivalents
US & Canada
UK & Australia
France & Russia
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