This ring-size converter accepts inside diameter or inside circumference in millimeters, centimeters, or inches and matches it to the nearest supported international size row.
Everyday life
cm to mm Ring Size Converter
Convert ring diameter or circumference and look up international ring sizes from the captured reference table.
Calculator
About this calculator
Formula notes
circumference = diameter × pi; diameter = circumference / pi. The normalized diameter is matched upward to the first supported reference-table threshold.
Worked examples
- Example: an inside diameter of 17.35 mm has a circumference of about 54.51 mm and maps to US 7, UK O, France 55 1/4, and Japan 14.
- An inside circumference of 54.51 mm corresponds to a diameter near 17.35 mm and returns the reference row for US size 7.
How to use it
- Choose whether you measured inside diameter or circumference.
- Enter the value and its unit.
- Compare the returned regional sizes with the jeweler's own chart.
Frequently asked questions
Should I measure the outside of the ring?
No. Use the inside diameter or the finger circumference; outside diameter includes the band thickness.
Why can two charts disagree?
Regional labels, half-size practices, manufacturing tolerance, and comfort-fit bands can differ between sellers.
Limits and interpretation
- The lookup uses discrete captured thresholds rather than interpolation, so a measurement near a boundary can move to the next size after a small measuring error.
- Finger size changes with temperature, swelling, time of day, band width, and fit preference; confirm the result with the seller or a physical sizing ring.