Ring Size Converter
Convert between common international ring-size systems, or find the closest size from an inside diameter or circumference.
Conversions are approximate. For a ring being made or altered, use a professional bench measurement.
Which sizing system was this?
The same number can describe different physical sizes. Choose the system the original size was given in.
Choose a system and enter the size you have. The result appears here with nearby supported sizes and measurement guidance.
View the full conversion chart
| ISO circ. (mm) | Diameter (mm) | US / Canada | UK letter | Japan | EU |
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Methodology & data
A ring size is defined by ISO 8653 as the inside circumference of the band in millimetres. Inside diameter is circumference ÷ π. This tool matches your input to the nearest supported entry in a fixed reference dataset and reports the closest size in the target system; it never interpolates a value it was not given, and never fills a blank mapping.
Where each system is used
Before relying on this conversion
- Sizing systems do not align perfectly; letters, numbers and half sizes differ by system.
- Conversions may be rounded to the nearest supported size.
- Finger size varies with temperature and time of day.
- Wide bands fit differently from narrow bands.
- Full-eternity, tension and some patterned bands may be difficult or impossible to resize.
- Measure an existing ring across its inside diameter, never the outer edge.
- Final manufacture or alteration should use a professional measurement.