What does the cm to mm converter measure?

It converts a finger circumference or diameter measured in centimeters into millimeters.

How is the EU size estimated?

The EU ring size is based on the inner circumference in millimeters, rounded to the nearest common size.

Yes. Choose the diameter mode and the calculator will convert that into circumference before estimating the size.

Convert a finger measurement from centimeters to millimeters and see the corresponding EU ring size.

This page is useful when you already have a metric measurement and want the ring size in millimeters.

Measure type

Measurement

Fit allowance

Multiply centimeters by 10 to get millimeters. For ring sizing, the inner circumference in millimeters is also the EU ring size.

Circumference in mm = centimeters x 10

Diameter in mm = circumference in mm / pi

Example 1: 5.6 cm converts to 56.0 mm and about EU 56.

Example 2: A 1.8 cm diameter becomes about 56.5 mm of circumference before allowance.

Example 3: Adding 1 mm of allowance turns a tight measurement into a more comfortable fit.

Output	Meaning	Action
Millimeters	The converted ring measurement.	Use it for EU ring-size matching.
EU ring size	The nearest common size in the European system.	Compare with a jeweler's chart before buying.
Allowance added	The extra room you requested.	Use it when the ring needs a little more comfort.

Millimeters are the standard unit used for EU ring sizes, so the conversion is straightforward once your measurement is in mm.

The millimeter conversion is exact, but the size label is an estimate based on the nearest common ring size.

Only if you want a looser fit. Most users should start with the exact measurement and then compare the result to a size chart.