cm to Ring Size Converter

Enter your finger's measured circumference or diameter in centimeters to get an estimated US and EU/French ring size, using the standard sizing chart formula.

Quick Facts

Formula
US size ≈ (diameter mm − 11.67) ÷ 0.81
Diameter = circumference ÷ π; a linear fit to the standard US ring sizing chart.
EU / French sizing
Ring size = circumference in mm
Used across France, Germany, and much of continental Europe — e.g. EU 52 ≈ US 6.

Your Results

Calculated
US ring size
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Estimated size on the US / Canada scale
EU / French ring size
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Circumference in mm, rounded
Diameter
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Inner diameter in millimeters
Circumference
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Inner circumference in millimeters

Ready

Enter your finger measurement in centimeters, then press Calculate.

About the cm to Ring Size Converter

This tool turns a simple centimeter measurement of your finger into an estimated ring size. Wrap a strip of string, paper, or a fabric measuring tape snugly around the base of your finger, mark where it overlaps, then measure that length with a ruler in centimeters. Enter it below as a circumference — or, if you already own a ring that fits, lay it flat and measure the inside edge-to-edge diameter instead.

The formula

Ring sizing standards are built on the finger's inner diameter and circumference, which are related by the basic geometry of a circle: circumference = diameter × π. Across the standard US sizing chart, ring size increases in close to a straight line against diameter, giving the approximation US size ≈ (diameter in mm − 11.67) ÷ 0.81. Diameter and circumference are computed directly from your cm measurement, and the US size is derived from diameter using that formula.

US vs EU / French sizing

The EU scale (also used in France, Germany, and much of continental Europe) is simpler: the ring size is just the inner circumference in millimeters, rounded to the nearest whole number. That is why a EU size 52 and a US size 6 describe roughly the same finger — 52mm of circumference converts to about 16.6mm of diameter, which lands near US size 6 on the formula above.

Reading a result between sizes

Because real rings are sold in fixed steps (usually whole or half sizes), your calculated size will often fall between two standard options. Round up rather than down for a wider band — wide bands fit more snugly than thin ones — and consider going a half size up if the ring will be worn in warm weather or over thick gloves, since fingers swell slightly with heat.

Frequently Asked Questions

How do I measure my finger for ring size in cm?
Wrap a strip of string or paper around the base of the finger, mark where it overlaps, then lay it flat and measure that length with a ruler in centimeters — that is your circumference. Alternatively, measure the inside diameter of a ring that already fits by laying it flat on a ruler and switch the calculator to diameter mode.
What formula converts cm to ring size?
The tool first converts your measurement to millimeters, then uses circumference = diameter × π to get whichever value you didn't enter. The US size is estimated as (diameter in mm − 11.67) ÷ 0.81, a linear approximation of the standard US sizing chart. The EU/French size is simply the circumference in millimeters, rounded.
Why are US and EU ring sizes such different numbers?
The two scales are built differently. US sizes follow a chart that increases in small steps starting near size 0, while EU/French sizes are literally the inner circumference in millimeters. The numbers look unrelated, but they describe the same finger — for example, EU 52 and US 6 are the same size.
Should I round up or down if my size falls between two sizes?
Round up. A slightly loose ring is more comfortable and easier for a jeweler to resize than one that is too tight, and wider bands in particular need extra room since they can't flex around the knuckle as easily as thin bands.

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