Ring Size Calculator

Choose what you can measure

An existing ring that fits the intended finger is a useful starting point. Measure its inside diameter straight across the center, from inner edge to inner edge. Do not include the metal band. Use a round, undamaged ring and enter the measurement in millimeters, centimeters or inches.

Alternatively, measure the finger with a narrow strip or flexible tape. Wrap it snugly without tightening, mark where the ends meet, and measure that length. Choose Finger / inside circumference. The calculator treats this as an approximate internal circumference; it does not add a hidden fitting allowance.

Converting a known size

Choose the exact system for a known size. Europe uses whole circumference sizes 44–71. The United Kingdom uses the letters and fractions listed in Tiffany’s table. Its Italian and Spanish tables contain selected decimal sizes, not every possible increment. Brazil uses whole sizes 11–35 from H.Stern; the United States, Japan and South Korea retain their separately identified charts.

All eight sizing systems appear together in equally sized result cards. After conversion, changing the input type or unit updates the result. Changing the input into a known size selects the nearest listed row and may round the measurement. Editing the value clears results until Convert is pressed again. Clear keeps the selected input type and measurement unit.

How the calculation works

For a circular ring, circumference C = π × diameter d, and d = C / π. One inch is exactly 25.4 mm and one centimeter is 10 mm. The calculator keeps unrounded values internally and displays dimensions with at most two decimal places.

Each country's result is the row with the nearest inside diameter. A midpoint tie chooses the larger size. If the diameter lies beyond a chart's first or last row, that country shows a dash instead of silently clamping to an endpoint. Overall, measured diameters from 13 to 23.3 mm are supported.

Two worked examples

An inside diameter of 17 mm gives about 53.41 mm circumference. The nearest European circumference size is 53; Tiffany gives United States 6.5 and United Kingdom N. The Brazilian H.Stern size is 16. Japan and South Korea both return 13. The dimensional result still shows the measured 17 mm.

An inside diameter of 18.5 mm gives about 58.12 mm circumference: Europe 58, United States 8.5, United Kingdom R and Brazil 21. Japan returns 18 because the diameter falls halfway between JCS 17 and 18 and ties go larger. Each country is matched independently by its documented inside diameter.

Which sizing systems are used?

European circumference sizes express internal circumference in millimeters; the 44–71 range follows Cartier’s published guide. The United States, United Kingdom, Italy and Spain use Tiffany’s country tables. Brazil uses H.Stern’s published diameter table. These retailer references are identified explicitly and do not claim to cover every jeweler or every intermediate size.

Japan uses Platinum Guild International’s JCS table with exact one-third-millimeter diameter steps. South Korea uses 12pearl’s KS gauge chart, which lists decimal diameters. JCS numbering differs from circumference numbering. Italy, Spain and Brazil are also kept separate: a similar-looking number is not proof that the underlying sizes are identical.

Source-specific size tables

Open a table to see the inside diameters used for conversion. These are separate references, not one universal international standard. Circumference is calculated from diameter, rather than copied from a separately rounded circumference column.

Frequently asked questions

Sources

• ISO 8653 — Inside circumference
• Cartier — Europe · circumference (44–71)
• Tiffany — United States
• Tiffany — United Kingdom
• Tiffany — Italy
• Tiffany — Spain
• H.Stern — Brazil
• Platinum Guild International — Japan (JCS)
• 12pearl — South Korea (KS)