This ring-size converter accepts an inside diameter or circumference in millimetres, centimetres, or inches and maps it to captured international size thresholds.
Everyday life
mm to Ring Size Converter
Convert an inside diameter or circumference in millimetres, centimetres, or inches to international ring sizes.
Calculator
About this calculator
Formula notes
circumference = diameter x pi; diameter = circumference / pi. The normalized millimetre diameter is matched upward to the first supported table row.
Worked examples
- Example: a 19.84 mm inside diameter has a circumference near 62.33 mm and maps to US 10, UK T 1/2, Germany 20, and Japan 20.
- An inside diameter of 19.84 mm corresponds to a circumference near 62.33 mm and maps to the captured US size 10 row.
How to use it
- Choose inside diameter or inside circumference.
- Enter the measurement and confirm its unit.
- Compare the returned regional sizes with the seller's sizing chart.
Frequently asked questions
Should I measure the outside diameter?
No. Outside diameter includes band thickness; use the inside diameter or the finger circumference.
Why can a nearby measurement return another size?
The lookup uses discrete size rows, so a small measurement change near a boundary can move to the next row.
Limits and interpretation
- The lookup uses discrete captured thresholds instead of interpolation, so a small measurement error near a boundary can move the result to the next size.
- Finger size varies with temperature, swelling, time of day, band width, and fit preference; confirm the result with a jeweler or the seller's chart.